Study Matereal 2

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STUDYMATEREAL

FOR

10th

(SCIENCE)

-BY

RAJPREET SINGH


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INDEX

1. Carbon compounds

2.Periodic classification of elements

3.Reproduction

4.Heridity and evolution

5.Reflection and refraction of light

6. Human Eye

7. Management of natural resources

8.Our environment


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Carbon Compounds

1. Carbons Capacity to Share Electrons:

Carbon has four electrons in its outermost shell and needs to gain or lose fourelectrons to attain noble gas configuration. If it were to gain or lose electrons

(i) It could gain four electrons forming C4 anion. But it would be difficult for thenucleus with six protons to hold on to ten electrons, that is, four extra electrons.

(ii) It could lose four electrons forming C4+ cation. But it would require a large amountof energy to remove four electrons leaving behind a carbon cation with six protons inits nucleus holding on to just two electrons.

Carbon overcomes this problem by sharing its valence electrons with other atoms of

carbon or with atoms of other elements. Not just carbon, but many other elementsform molecules by sharing electrons in this manner. The shared electrons belong tothe outer shells of both the atoms and lead to both atoms attaining the noble gasconfiguration.

This type of bond formed by sharing of electrons is called covalent bond. Covalentlybonded molecules are seen to have strong bonds within the molecule, butintermolecular forces are small. This gives rise to the low melting and boiling pointsof these compounds. Since the electrons are shared between atoms and no chargedparticles are formed, such covalent compounds are generally poor conductors ofelectricity.

Let us now take a look at methane, which is a compound of carbon. Methane iswidely used as a fuel and is a major component of bio-gas and Compressed NaturalGas (CNG). It is also one of the simplest compounds formed by carbon. Methanehas a formula CH4. Hydrogen, as you know, has a valency of 1. Carbon is tetravalentbecause it has four valence electrons. In order to achieve noble gas configuration,carbon shares these electrons with four atoms of hydrogen as shown below:

Allotropes of carbon: The element carbon occurs in different forms in nature withwidely varying physical properties. Both diamond and graphite are formed by carbonatoms, the difference lies in the manner in which the carbon atoms are bonded toone another. In diamond, each carbon atom is bonded to four other carbon atoms


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forming a rigid three-dimensional structure. In graphite, each carbon atom is bondedto three other carbon atoms in the same plane giving a hexagonal array. One ofthese bonds is a double-bond, and thus the valency of carbon is satisfied. Graphitestructure is formed by the hexagonal arrays being placed in layers one above theother.

VERSATILE NATURE OF CARBON

The numbers of carbon compounds whose formulae are known to chemists wasrecently estimated to be about three million! This outnumbers by a large margin thecompounds formed by all the other elements put together. The nature of the covalentbond enables carbon to form a large number of compounds. Two factors noticed inthe case of carbon are

(i) Carbon has the unique ability to form bonds with other atoms of carbon, givingrise to large molecules. This property is called catenation. These compounds may

have long chains of carbon, branched chains of carbon or even carbon atomsarranged in rings. In addition, carbon atoms may be linked by single, double or triplebonds. Compounds of carbon, which are linked by only single bonds between thecarbon atoms are called saturated compounds. Compounds of carbon having doubleor triple bonds between their carbon atoms are called unsaturated compounds. Noother element exhibits the property of catenation to the extent seen in carboncompounds. Silicon forms compounds with hydrogen which have chains of uptoseven or eight atoms, but these compounds are very reactive. The carbon-carbonbond is very strong and hence stable. This gives us the large number of compoundswith many carbon atoms linked to each other.

(ii) Since carbon has a valency of four, it is capable of bonding with four other atomsof carbon or atoms of some other mono-valent element. Compounds of carbon areformed with oxygen, hydrogen, nitrogen, sulphur, chlorine and many other elementsgiving rise to compounds with specific properties which depend on the elementsother than carbon present in the molecule.

Again the bonds that carbon forms with most other elements are very strong makingthese compounds exceptionally stable. One reason for the formation of strong bondsby carbon is its small size. This enables the nucleus to hold on to the shared pairs ofelectrons strongly. The bonds formed by elements having larger atoms are much

weaker.

Organic compounds

The two characteristic features seen in carbon, that is, tetravalency and catenation,put together give rise to a large number of compounds. Many have the same non-carbon atom or group of atoms attached to different carbon chains. Thesecompounds were initially extracted from natural substances and it was thought thatthese carbon compounds or organic compounds could only be formed within a livingsystem. That is, it was postulated that a vital force was necessary for theirsynthesis. Friedrich Whler disproved this in 1828 by preparing urea from

ammonium cyanate. But carbon compounds, except for oxides of carbon, carbonateand hydrogencarbonate salts continue to be studied under organic chemistry.


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Saturated and Unsaturated Carbon Compounds

In order to arrive at the structure of simple carbon compounds, the first step is to linkthe carbon atoms together with a single bond and then use the hydrogen atoms tosatisfy the remaining valencies of carbon. For example, the structure of ethane is

arrived in the following steps

C2H6 Ethane

C2H4 Ethene

In the first example of ethane all valency of carbon atoms is satisfied, hence this iscalled as saturated carbon compound or saturated organic compound.

In the second example of ethane valency of carbon atom is not fully satisfied,hence this is called as unsaturated organic compound.

Compounds formed by carbon and hydrogen only are called hydrocarbons.

Those hydrocarbons which have single bonds in molecular structure are calledalkanes. Generic formula for alkanes is CnH(2n+2)

Hydrocarbons with double bonds are called alkenes. Generic formula for alkenes isCnH2n

Hydrocarbons with triple bonds are called alkynes. Generic formula for alkynes isCnHn

Carbon seems to be a very friendly element. So far we have been looking atcompounds of carbon and hydrogen. But carbon also forms bonds with otherelements such as halogens, oxygen, nitrogen and sulphur. In a hydrocarbon chain,one or more hydrogens can be replaced by these elements, such that the valency of

carbon remains satisfied. In such compounds, the element replacing hydrogen isreferred to as a heteroatom. These heteroatoms confer specific properties to the


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compound, regardless of the length and nature of the carbon chain and hence arecalled functional groups. The functional group is attached to the carbon chainthrough this valency by replacing one hydrogen atom or atoms.

Homologous Series

The presence of a functional group such as alcohol dictates the properties of the

carbon compound, regardless of the length of the carbon chain. For example, thechemical properties of CH3OH, C2H5OH, C3H7OH and C4H9OH are all very similar.Hence, such a series of compounds in which the same functional group substitutesfor hydrogen in a carbon chain is called a homologous series.

As the molecular mass increases in any homologous series, a gradation in physicalproperties is seen. This is because the melting points and boiling points increasewith increasing molecular mass. Other physical properties such as solubility in aparticular solvent also show a similar gradation. But the chemical properties, whichare determined solely by the functional group, remain similar in a homologous series.

Nomenclature of Carbon Compounds

The names of compounds in a homologous series are based on the name of thebasic carbon chain modified by a prefix phrase before or suffix phrase afterindicating the nature of the functional group.

Naming a carbon compound can be done by the following method

(i) Identify the number of carbon atoms in the compound. A compound having threecarbon atoms would have the name propane.

(ii) In case a functional group is present, it is indicated in the name of the compoundwith either a prefix or a suffix.


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(iii) If the name of the functional group is to be given as a suffix, the name of thecarbon chain is modified by deleting the final e and adding the appropriate suffix.

For example, a three-carbon chain with a ketone group would be named in thefollowing manner Propanee = propan + one = propanone.

(iv) If the carbon chain is unsaturated, then the final ane in the name of the carbonchain is substituted by ene or yne. For example, a three-carbon chain with adouble bond would be called propene and if it has a triple bond, it would be calledpropyne.

CHEMICAL PROPERTIES OF CARBON COMPOUNDS

Combustion: Carbon, in all its allotropic forms, burns in oxygen to give carbondioxide along with the release of heat and light. Most carbon compounds alsorelease a large amount of heat and light on burning.

Saturated hydrocarbons will generally give a clean flame while unsaturated carboncompounds will give a yellow flame with lots of black smoke. However, limiting thesupply of air results in incomplete combustion of even saturated hydrocarbons,giving a sooty flame. The gas/kerosene stove used at home has inlets for air so thata sufficiently oxygen-rich mixture is burnt to give a clean blue flame. If you observethe bottoms of cooking vessels getting blackened, it means that the air holes areblocked and fuel is getting wasted. Fuels such as coal and petroleum have someamount of nitrogen and sulphur in them. Their combustion results in the formation ofoxides of sulphur and nitrogen which are major pollutants in the environment.

Oxidation: Carbon compounds can be easily oxidised on combustion. In addition tothis complete oxidation, we have reactions in which alcohols are converted tocarboxylic acids

Addition Reaction: Unsaturated hydrocarbons add hydrogen in the presence ofcatalysts such as palladium or nickel to give saturated hydrocarbons. Catalysts are

substances that cause a reaction to occur or proceed at a different rate without the


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reaction itself being affected. This reaction is commonly used in the hydrogenation ofvegetable oils using a nickel catalyst.

Substitution Reaction: Saturated hydrocarbons are fairly unreactive and are inert inthe presence of most reagents. However, in the presence of sunlight, chlorine is

added to hydrocarbons in a very fast reaction. Chlorine can replace the hydrogenatoms one by one. It is called a substitution reaction because one type of atom or agroup of atoms takes the place of another. A number of products are usually formedwith the higher homologues of alkanes.

SOME IMPORTANT CARBON COMPOUNDS ETHANOL AND ETHANOIC ACID

Properties of Ethanol: Ethanol is a liquid at room temperature. Ethanol iscommonly called alcohol and is the active ingredient of all alcoholic drinks. Inaddition, because it is a good solvent, it is also used in medicines such as tinctureiodine, cough syrups, and many tonics. Ethanol is also soluble in water in all

proportions. Consumption of small quantities of dilute ethanol causes drunkenness.Even though this practice is condemned, it is a socially widespread practice.However, intake of even a small quantity of pure ethanol (called absolute alcohol)can be lethal. Also, long-term consumption of alcohol leads to many healthproblems.

Reactions of Ethanol

(i) Reaction with sodium Alcohols react with sodium leading to the evolution ofhydrogen. With ethanol, the other product is sodium ethoxide.

(ii) Reaction to give unsaturated hydrocarbon: Heating ethanol at 443 K withexcess concentrated sulphuric acid results in the dehydration of ethanol to giveethene The concentrated sulphuric acid can be regarded as a dehydrating agentwhich removes water from ethanol.

Effects of Alcohol: When large quantities of ethanol are consumed, it tends to slowmetabolic processes and to depress the central nervous system. This results in lackof coordination, mental confusion, drowsiness, lowering of the normal inhibitions, andfinally stupour. The individual may feel relaxed but does not realise that his sense ofjudgement, sense of timing, and muscular coordination have been seriouslyimpaired. Unlike ethanol, intake of methanol in very small quantities can causedeath. Methanol is oxidised to methanal in the liver. Methanal reacts rapidly with thecomponents of cells. It causes the protoplasm to get coagulated, in much the sameway an egg is coagulated by cooking. Methanol also affects the optic nerve, causing

blindness. Ethanol is an important industrial solvent. To prevent the misuse ofethanol produced for industrial use, it is made unfit for drinking by adding poisonous


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Periodic classification of elements

Elements are classified on the basis of similarities in their properties.

Dbereiner grouped the elements into triads and Newlands gave the Law ofOctaves.

Mendelev arranged the elements in increasing order of their atomic masses andaccording to their chemical properties.

Mendelev even predicted the existence of some yet to be discovered elements onthe basis of gaps in his Periodic Table.

Anomalies in arrangement of elements based on increasing atomic mass could beremoved when the elements were arranged in order of increasing atomic number, a

fundamental property of the element discovered by Moseley.

Elements in the Modern Periodic Table are arranged in 18 vertical columns calledgroups and 7 horizontal rows called periods.

Elements thus arranged show periodicity of properties including atomic size,valency or combining capacity and metallic and non-metallic character.

Periodic Law: Properties of elements are a periodic function of their atomicnumber.

Atomic number gives us the number of protons in the nucleus of an atom and thisnumber increases by one in going from one element to the next. Elements, whenarranged in order of increasing atomic number Z, lead us to the classification knownas the Modern Periodic Table. Prediction of properties of elements could be madewith more precision when elements were arranged on the basis of increasing atomicnumber.

Position of Elements in the Modern Periodic Table :

The Modern Periodic Table has 18 vertical columns known as groups and 7

horizontal rows known as periods. Let us see what decides the placing of anelement in a certain group and period. All elements of a group contain same numberof valence electrons, which justifies similar chemical properties.

The atomic radius decreases in moving from left to right along a period. This is dueto an increase in nuclear charge which tends to pull the electrons closer to thenucleus and reduces the size of the atom.

Atoms of different elements with the same number of occupied shells are placed inthe same period. Na, Mg, Al, Si, P, S, Cl and Ar belong to the third period of theModern Periodic Table, since the electrons in the atoms of these elements are filled

in K, L and M shells.


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Metallic & Non-metallic Properties:

Metals like Na and Mg are towards the left-hand side of the Periodic Table while thenon-metals like sulphur and chlorine are found on the right-hand side. In the middle,we have silicon, which is classified as a semi-metal or metalloid because it exhibits

some properties of both metals and non-metals.

In the Modern Periodic Table, a zig-zag line separates metals from non-metals. Theborderline elements boron, silicon, germanium, arsenic, antimony, tellurium andpolonium are intermediate in properties and are called metalloids or semi-metals.Metals tend to lose electrons while forming bonds, that is, they are electropositive innature.

As the effective nuclear charge acting on the valence shell electrons increasesacross a period, the tendency to lose electrons will decrease. Down the group, theeffective nuclear charge experienced by valence electrons is decreasing because

the outermost electrons are farther away from the nucleus. Therefore, these can belost easily. Hence metallic character decreases across a period and increases downa group.

As the trends in the electronegativity show, non-metals are found on the right-handside of the Periodic Table towards the top. These trends also help us to predict thenature of oxides formed by the elements because it is know that the oxides of metalsare basic and that of non-metals are acidic in general.


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Reproduction

Importance of Reproduction:

1. To create next generation. Unique property of a particular organism istransferred from one generation to the next generation through genes, which aresituated in the DNA (Deoxyribonucleic Acid).

2. During Meiosis number of chromosomes become half of that in the parent cells.As a result when both male and female gametes fuse to form zygote, the number ofchromosomes becomes adequate for the species involved.

3. Every animal cell has fixed number of chromosomes. Human cell contain 23 pairs

or 46 chromosomes. After meiosis egg and sperm cells contain 23 chromosomes.After zygote formation the number of chromosomes is once again 23 pairs. This isnecessary to maintain the unique identity of a species.

4. To create variations in species. As no two individuals are same, so geneticcharacters from both parents will help make a slightly different copy of themselves.These small variations accumulate over hundreds of years resulting in formation ofnew species.

5. New species facilitate evolution of organisms. Evolution is necessary for survivalas environmental conditions keep on changing from time to time. As per Darwinnature has a method of selecting the best fit species for survival.

Types of Reproduction:

1. Asexual Reproduction

2. Sexual Reproduction

Asexual Reproduction:

In unicellular plants and animals and some multicellular organisms as well the modeof reproduction is asexual. In this case the organism doesn't make zygote. There arefollowing types of asexual reproduction:

1. Binary Fission: As the name suggests, the organism breaks into two parts by celldivision. Unicellular organism like Amoeba and bacteria reproduce in this way.

2. Budding: Some multicellular organisms like Hydra and Yeast make a bud outsidetheir body. The bud, after growing to certain extent detaches from the parent bodyand goes on living like an independent organism.

3. Vegetative Reproduction in Plants: Certain plants have capacity to make a newplant from their vegetative parts. For example if you plant a stem of rose it will


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develop root and ultimately a new plant is born. Leaves of Bryophyta grow roots atthe margins of their leaves, which ultimates gives birth to a separate plant.

4. Reproduction in Virus: Virus enters the nucleus of the host cells. After that itmanipulates the DNA of the host to reproduce a new virus.

5. Parthenogenesis: Some lower plants and animals, like some bees and waspsreproduce in this way. An organism develops embryo without fertilization. Theembryo ultimately gives birth to a new generation.

6. Spore Formation: Some fungi and algae make spores. You must have noticedwhite cotton like growth on stale bread and food. These are spores of fungi. Thesespores, during favourable environmental conditions give birth to the new generation.

Sexual Reproduction:

Sexual reproduction involves formation of zygote or embryo to facilitate transfer ofgenetic information from both parents, and development of embryo in a developedoffspring.

Sexual Reproduction in Plants:

Flower can be termed as the sexual organ of a plant. All the parts of a flower arearranged around an axis. These are as follows:

1. Sepals: Green leaf like structure.

2. Petals: Colourful structures, which add attraction to a flower. This attraction is notonly having ornamental value, but a more important role in facilitating reproduction.Insects and birds, attracted by the colour, help transfer pollen grains or malegametes from male flower to female flower. This helps in pollination.

3. Androecium.

4. Gynoecium.

Male Reproductive Organ of Plant: Androecium


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The flower of a plant contains tube like structures called stamen. At the top ofstamen is a chambered structure called Androecium. Androecium is responsible forthe production of male gamete also called pollen grains.

Female Reproductive Organ of Plant: Gynoecium

Usually at the centre of a flower you can notice Gynoecium. Gynoecium is pitchershaped structure with a long tube protruding out of it. The gynoecium producesfemale gamete also called eggs.

Pollination: The process of transfer of pollen grains from androecium to gynoeciumis called pollination. This can happen in same flower, or between different flowers ofthe same plant. When only one plant is involved the process is called self-pollination.When flowers of two different plants are involved, then it is called cross pollination.Cross pollination can be facilitated by insects, birds, animals, air or water.

Zygote Formation: Once pollen grains enter the androecium, one of them entersthe egg to fertilize it to form a zygote. Seeds are the zygote or embryo of the plant.To survive and to germinate seeds need source of food. In all seeds there isabundance of food. That is why for our daily need we depend on so many seeds likerice, wheat, groundnut for food. During germination the food in the cotyledon is usedto grow a new plant. Once green leaves come out, they take care of further foodproduction.

Reproductive System in Humans:

Male Reproductive System:

1. Testis: Testis is a galndular organ made up of fine tubules. Testis producessperm or male gamete.

2. Seminal Vesicle: Once sperm is produced it is stored in seminal vesicle.

3. Vas Deferens: Vas deference is the tube through which semen containing spermis transferred out.

Apart from producing sperm, testis also produces certain hormones, like tetosterone

which are responsible for secondary sexual characters in humans. These are deepmale voice, hair growth in pubic area and under armpits, and facial hair.

Female Reproductive Organs:

1. Ovary: Ovaries are situated on left and right side of the uterus. Ovaries have aninner epithelial lining called endometrium, which is responsible for the production ofeggs.

2. Fallopian Tubes: Fallopian tubes extends on both sides of the uterus intransverse direction. Fallopian tubes have finger like structures which catch the eggs

to transfer them to the uterus.


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3. Uterus: Uterus is a bag like structure, with an opening in the vagina. Once eggsreach uterus, a layer of soft tissues develops to support the embryo. This layer iscalled corpus luteum. If fertilization takes place, then the embryo develops into afoetus and ultimately to a fully developed child over a period of about 9 months.

Menstrual Cycle in Females: If no fertilization takes place then after about twoweeks the dead eggs and corpus luteum gets expelled out of the uterus throughvagina. This process takes place over a period of about three to four days. Thisclears the way for new batch of eggs to come in the uterus. The whole cycle fromegg production to the expulsion of eggs takes about four weeks. This cycle is knownas Menstrual Cycle. Apart from humans, some primates like Chimpanzee and Gorillaalso show same phenomenon.

Ovary secretes one of the important hormones estrogen, which is responsible forsecondary sexual characters in female, like thin voice and breast enlargement.


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Heridity and evolution

Heredity:

Characters of parents get copied in children. Skin colour, hair colour, height,appearance, etc. in children resemble either of parents or grandparents. Thisphenomenon is known as heredity. Chromosomes contain genes, which work like arecording device recording all the genetic codes of an individual and transferringthem to the next generation.

Variation:

As half of the chromosomes come from paternal side and rest half from maternal

side, so the offspring will have a mix of characters from both parents. This mixing upof characters creates slight variation in the genetic makeup of the offspring. Thesevariations accumulate over hundreds of years giving rise to a altogether newspecies.

Rules for the Inheritance of TraitsMendels Contributions:

John Gregor Mendel (1856-63) conducted hybridization experiments on pea plantsfor many generations and studying how certain characters get transferred from onegeneration to the next generation. He also studied how certain characters becomedormant or prominent in a particular generation. One of the experiments involved

making hybrids of wrinkled seeds and smooth seeds. The results can be depicted byfollowing diagram:

Interpretation of Results: In the first generation all the seeds were wrinkled. The'wrinkled' character was dominant, while the 'smooth character was dormant. Bothcharacters were present in the genotype or genetic makeup. In the next generation25% of seeds were pure wrinkled with 'wrinkled' genotype, 25% seeds were puresmooth with 'smooth' genotype and 50% of seeds were wrinkled with'wrinkled+smooth' genotype. This shows how sometimes children of tall father canbe of average height as the 'tall' character becomes dormant in that generation.

Sex Determination


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This issue answers the question how it is possible that the newborn is a male orfemale. In some animals, like crocodile, the temperature at which an egg hatchesdetermines the sex of a newborn. In human it depends on the last pair ofchromosomes. As you know human have 23 pairs of chromosomes. 22 pairs arealike and the 23rd pair can be of similar or dissimilar chromosomes. In females both

chromosomes of the 23rd pair consists of X type, while in male the 23rd pair is madeup of XY chrmosomes. Following diagram shows what happens when a zygote isformed:

It is clear by above diagram that when two gametes with X chromosome makezygote the offspring will be female. When one gamete with X chromosome fertilizesanother gamete with Y chromosome to make zygote the offspring will be male.

EVOLUTION:

Life started on earth as simple unicellular organisms. These organisms throughevolution over millions of years created hugely diverse life forms which we seetoday. In fact variations accumulating over a period of time created a new speciesand the process continued and will be continued in years to come.

Charles Robert Darwin (18091882) Charles Darwin set out on a voyage when hewas 22 years old. The five-year voyage took him to South America and the islandsoff its coast. During his voyage Darwin collected huge number of specimens for hisstudy. After analyzing all information he came up with his theory of evolution in hisbook 'Origin of Species.

Origin of Life on Earth: All living beings are made up of basic elements like

Carbon, Hydrogen, Oxygen and Nitrogen. In the initial period of earth's life theseelements combined together to form the earliest living beings, which had the powerof replicating itself.

Darwin's Theory of Evolution:

Struggle for Existence

1. Species have great fertility. They have more offspring than can grow to adulthood.

2. Populations remain roughly the same size, with small changes.

3. Food resources are limited, but are relatively stable over time.


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4. An implicit struggle for survival ensues.

5. In sexually reproducing species, generally no two individuals are identical.

6. Some of these variations directly affect the ability of an individual to survive in a

given environment.

7. Much of this variation is inheritable.

Natural Selection and Survival of the Fittest

8. Individuals less suited to the environment are less likely to survive and less likelyto reproduce, while individuals more suited to the environment are more likely tosurvive and more likely to reproduce.

9. The individuals that survive are most likely to leave their inheritable traits to future

generations.

10. This slowly effected process results in populations that adapt to the environmentover time, and ultimately, after interminable generations, these variations accumulateto form new varieties, and ultimately, new species.

Proofs of Evolution:

1. Homologous organs: Organs, like forelimbs, digestive system, of humans, birds,crocodiles and bat show same basic design. This similarity in design suggests that

they have originated from same root.

2. Analogous organs: Wings of bats and those of birds are different in design butserve the same purpose. The similarity in purpose indicates towards single source oftheir origin.

3. Rudimentary Organs: Certain organs in human are having no functional value.Appendix and nictitating membrane are such examples. Appendix is reduced form ofan additional chamber in the digestive system of ruminating animals like cow, wherethey help in cellulose digestion. Humans no longer need them so they arefunctionless. Nictitating membranes in frog's eyes help them to see under water. We

need special goggles for that. These rudimentary organs suggest that we haveevolved from frogs.

4. Developmental Stages in Foetus: This theory suggests that right after theformation of zygote up to delivery an animals passes every stage of evolutionthrough which it has evolved. In case of humans at certain stage the human embryolooks like that of a fish, later it looks like that of a frog and ultimately it develops intoa human being. This is like reliving your past lives.

5. Fossils: Fossils are remains of living beings which were buried millions of yearsago under the earth. They provide us with linking proofs between various groups of

animals.


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Evolution and Classification:

Evolutionary principles has been used for classification of animals and plants. Forexample all animals with four legs have been included in the class tetrapoda(tetra:four; podium:Legs). All flowering plants come under angiosperms.

Main Animal Groups


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Reflection and refraction of light

Laws of Reflection:

(i) The angle of incidence is equal to the angle of reflection, and

(ii) The incident ray, the normal to the mirror at the point of incidence and thereflected ray, all lie in the same plane.

These laws of reflection are applicable to all types of reflecting surfaces includingspherical surfaces.

Plane Mirror:

Image formed by a plane mirror is always virtual and erect. The size of the image isequal to that of the object. The image formed is as far behind the mirror as the objectis in front of it. Further, the image is laterally inverted.

Spherical Mirror:

1. Concave Mirror: A spherical mirror, whose reflecting surface is curved inwards,that is, faces towards the centre of the sphere, is called a concave mirror.

2. Convex Mirror: A spherical mirror whose reflecting surface is curved outwards, is

called a convex mirror.

Key Terminologies:

1. Pole: The centre of the reflecting surface of a spherical mirror is called the pole. Itis represented by 'P'.

2. Centre of Curvature: The centre of the sphere is called the centre of curvature.The spherical mirror is part of a big sphere. The centre of curvature lies outside themirror. In case of concave mirror it lies in front of the reflective surface. In case ofconvex mirror it lies behind the reflective surface.

3. Radius of Curvature: The radius of the sphere is called the radius of curvature. Itis represented by 'R'.

4. Principal Axis: The line joining the pole and the center of curvature is called theprincipal axis.

5. Principal Focus: In mirrors with small aperture (diameter) roughly half of theradius of curvature is equal to the focus point. At focus point all the light coming frominfinity converge, in case of concave mirrors. The light seem to diverge from f, incase of convex mirrors.


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Image Formed by Concave Mirror: (S here stands for distance between object andmirror.)

1. When S < F, the image is: Virtual, Upright , Magnified (larger)

2. When S = F, the image is formed at infinity. In this case the reflected light rays areparallel and do not meet the others. In this way, no image is formed or more properlythe image is formed at infinity.

3. When F < S < 2F, the image is: Real, Inverted (vertically), Magnified (larger)

4. When S = 2F, the image is: Real, Inverted (vertically), Same size

5. When S > 2F, the im5. When S > 2F, the image is: Real, Inverted (vertically),Diminished (smaller) j

Use of Concave Mirrors: They are used in torches, searchlights, to reflect a beamof light to great distance. Doctors use concave mirrors to throw beam of light insideears and mouth to examine patients. Headlights of automobiles use concave mirrorsfor better visibility.

Image Formed By Convex Mirror: The image is always virtual (rays haven'tactually passed though the image), diminished (smaller), and upright . Thesefeatures make convex mirrors very useful: everything appears smaller in the mirror,so they cover a wider field of view than a normal plane mirror does as the image is"compressed".

Use of Convex Mirrors: Rear-view mirrors of automobiles are convex mirrors. Theyenable the driver to see through a wider vision field without craning his neck. Athairpin bends on hilly roads convex mirrors are installed for motorists to see thetraffic on the other side of the bend.

Sign Convention for Reflection by Spherical Mirrors

While dealing with the reflection of light by spherical mirrors, we shall follow a set ofsign conventions called the New Cartesian Sign Convention. In this convention, thepole (P) of the mirror is taken as the origin. The principal axis of the mirror is taken

as the x-axis (XX) of the coordinate system. The conventions are as follows:

(i) The object is always placed to the left of the mirror. This implies that the light fromthe object falls on the mirror from the left-hand side.

(ii) All distances parallel to the principal axis are measured from the pole of themirror.

(iii) All the distances measured to the right of the origin (along + x-axis) are taken aspositive while those measured to the left of the origin (along x-axis) are taken asnegative.


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The Refractive Index

A ray of light that travels obliquely from one transparent medium into another willchange its direction in the second medium. The extent of the change in direction thattakes place in a given pair of media is expressed in terms of the refractive index.

The refractive index can be linked to an important physical quantity, the relativespeed of propagation of light in different media. It turns out that light propagates withdifferent speeds in different media. Light travels the fastest in vacuum with thehighest speed of 3108 ms1. In air, the speed of light is only marginally less,compared to that in vacuum. It reduces considerably in glass or water. The value ofthe refractive index for a given pair of media depends upon the speed of light in thetwo media, as given below:

Speed of Light in Air = c

Speed of light in a medium = v

Then refractive index Then refractive index of medium

nm = c/v

The speed of light is higher in a rarer medium than a denser medium. Thus, a ray oflight travelling from a rarer medium to a denser medium slows down and bendstowards the normal. When it travels from a denser medium to a rarer medium, itspeeds up and bends away from the normal.

Refractive Index of Some Media

Refraction by Spherical Lenses

A transparent material bound by two surfaces, of which one or both surfaces arespherical, forms a lens. This means that a lens is bound by at least one sphericalsurface. In such lenses, the other surface would be plane. A lens may have twospherical surfaces, bulging outwards. Such a lens is called a double convex lens. It

is simply called a convex lens. It is thicker at the middle as compared to the edges.Convex lens converges light rays, hence convex lenses are called converging


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lenses. Similarly, a double concave lens is bounded by two spherical surfaces,curved inwards. It is thicker at the edges than at the middle. Such lenses divergelight rays as shown and are called diverging lenses. A double concave lens is simplycalled a concave lens.

A lens, either a convex lens or a concave lens, has two spherical surfaces. Each ofthese surfaces forms a part of a sphere. The centres of these spheres are calledcentres of curvature of the lens.

Image formed by convex lens

Image Formed by Concave Lens

Sign Convention for Spherical Lenses

According to the convention, the focal length of a convex lens is positive and that ofa concave lens is negative. Appropriate signs for the values of u, v, f, object height hand image height h.

Lens Formula and Magnification

This formula gives the relationship between objectdistance (u), image-distance (v)and the focal length (f ). The lens formula is expressed as:

1/v 1/u = 1/f


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The magnification produced by a lens, similar to that for spherical mirrors, is definedas the ratio of the height of the image and the height of the object. It is representedby the letter m. If h is the height of the object and h is the height of the image givenby a lens, then the magnification produced by the lens is given by:

m =Height of the Image/Height of the object = h'/h = v/u

Where object-distance Where object-distance is u and the image-distance is v.

Power of a Lens

The degree of convergence or divergence of light rays achieved by a lens isexpressed in terms of its power. The power of a lens is defined as the reciprocal ofits focal length. It is represented by the letter P. The power P of a lens of focal lengthf is given by:

P =1/f

The SI unit of power of a lens is dioptre. It is denoted by the le tter D. If f isexpressed in metres, then, power is expressed in dioptres. Thus, 1 dioptre is thepower of a lens whose focal length is 1 metre. 1D = 1m1. Power of a convex lens ispositive and that of a concave lens is negative. Opticians prescribe corrective lensesindicating their powers.

Let us say the lens prescribed has power equal to + 2.0 D. This means the lensprescribed is convex. The focal length of the lens is + 0.50 m. Similarly, a lens ofpower 2.5 D has a focal length of 0.40 m. The lens is concave. Many opticalinstruments consist of a number of lenses. They are combined to increase themagnification and sharpness of the image. The net power (P) of the lenses placed incontact is given by the algebraic sum of the individual powers P1, P2, P3, as

P = P1 + P2 + P3 +


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Human Eye

THE HUMAN EYE: Structure:

1. Pupil: Pupil is the round black spot in front of eye. It regulates the amount of lightentering the eyes. Pupil works like aperture of a camera. In case of dim light pupildilate to allow more light to enter the eyes. In case of strong light pupil constrictallowing less light to enter.

2. Iris: Iris is made of muscles. They control the size of opening if pupil.

3. Lens: Lens lies just behind the pupil. Lens becomes thin to increase its focallength. This enables us to see distant objects clearly. To focus on nearer objectslens becomes thick to decrease its focal length. But there is a limit. The minimumdistance of clear vision is 25 cm. Below this distance we cannot see things clearly.

4. Retina: Retina works like a screen or camera film. Retina is full of light and coloursensitive cells. These cells, upon receiving image send electrical signals to the brain,which processes these information to make a mental image of what we see.

Benefits of two eyes: One eye is having a field of vision of about 150 degrees. Boththe eyes enable us to see upto a field of 180 degrees. Moreover, as two differentimages get juxtaposed in the brain, so we are able to see a three dimensional viewof the world.

Malfunctions of Eyes:

1. Cataract: In old age the cornea becomes cloudy. This reduces the vision in oldage. In early stages of the disease cataract can be cured by eye surgery. Sometimesartificial lens is also transplanted during cataract surgery. This is called Intra Ocular

Lens Transplantation.


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2. Myopia: Myopia is also known as near-sightedness. A person with myopia cansee nearby objects clearly but cannot see distant objects distinctly. In a myopiceye,the image of a distant object is formed in front of the retina and not at the retina itself.This defect may arise due to (i) excessive curvature of the eye lens, or (ii) elongationof the eyeball. This defect can be corrected by using a concave lens of suitable

power. A concave lens of suitable power will bring the image back on to the retinaand thus the defect is corrected.

3. Hypermetropia: Hypermetropia is also known as far-sightedness. A person withhypermetropia can see distant objects clearly but cannot see nearby objectsdistinctly. The near point, for the person, is farther away from the normal near point(25 cm). Such a person has to keep a reading material much beyond 25 cm from theeye for comfortable reading. This is because the light rays from a closeby object arefocussed at a point behind the retina. This defect arises either because

(i) the focal length of the eye lens is too long, or

(ii) the eyeball has become too small.

This defect can be corrected by using a convex lens of appropriate power. Eye-glasses with converging lenses provide the additional focussing power required forforming the image on the retina.

4. Presbyopia: The power of accommodation of the eye usually decreases withageing. For most people, the near point gradually recedes away. They find it difficultto see nearby objects comfortably and distinctly without corrective eye-glasses. Thisdefect is called Presbyopia. It arises due to the gradual weakening of the ciliarymuscles and diminishing flexibility of the eye lens. Sometimes, a person may sufferfrom both myopia and hypermetropia. Such people often require bifocal lenses. Acommon type of bi-focal lenses consists of both concave and convex lenses. Theupper portion consists of a concave lens. It facilitates distant vision. The lower part isa convex lens. It facilitates near vision.

ATMOSPHERIC REFRACTION

Twinkling of stars

The twinkling of a star is due to atmospheric refraction of starlight. The starlight, onentering the earths atmosphere, undergoes refraction continuously before it reachesthe earth. The atmospheric refraction occurs in a medium of gradually changingrefractive index. Since the atmosphere bends starlight towards the normal, theapparent position of the star is slightly different from its actual position. The starappears slightly higher (above) than its actual position when viewed near the horizon. Further, this apparent position of the star is not stationary, but keeps on changingslightly, since the physical conditions of the earths atmosphere are not stationary, aswas the case in the previous paragraph. Since the stars are very distant, theyapproximate point-sized sources of light. As the path of rays of light coming from thestar goes on varying slightly, the apparent position of the star fluctuates and the

amount of starlight entering the eye flickers the star sometimes appears brighter,and at some other time, fainter, which is the twinkling effect.


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Management of natural resources

Major Natural Resources:

(i) Water

(ii) Forest

(iii) Soil

(iv) Fossil Fuel

(v) Sunlight

(vi) Air

Management: Management of natural resources can involve judicious use,reduction of wastage and proper recycling. As most of the natural resources arelimited in quantity so we should take every possible step to prevent their exhaustion.

Even those resources which are available in plenty, like sunlight and air, need to bepreserved from pollution.

Proper management will ensure that we would leave a conducive environment for

our future generations.

Water:

Source of Potable Water: Rivers, Lakes and Underground Reservoir.

Source of Water Pollution: Industrial Effluents, Sewage

Sewage in most of the cities in India flows into rivers or nearby ponds polluting thewater. Some of the sewage percolates down to pollute even the underwaterreservoir. Depletion of Underground Water: Excess exploitation of underground

water by bore wells is a major cause of depletion of underground water reservoir.

Remedies:

(I) Sewage treatment plants should be utilized to prevent water pollution. Sewagetreatment plant filters out all the muck and cleanses water before releasing it into theriver.

(II) Ban on bore wells will help reduce depletion of underground water reservoir. LikeUK water supply can be privatized to ensure adequate supply of drinking water to

masses. This will reduce the need for private bore wells.


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(III) Rainwater Harvesting is a process of catching rainwater and allow it to seepdown the ground which helps recharge the underground water reservoir.

Forest:

Benefits:

1. Forest is necessary to maintain the biodiversity and ecological balance of theearth.

2. Trees help prevent soil erosion.

3. Trees balances the excess Carbon Dioxide during photosynthesis, reducinggreenhouse effect.

4. Forest is a source of livelihood for tribes.

Causes of Deforestation:

1. Increasing human need for land for residential, commercial and agriculturalpurpose.

2. Need of timber for construction, furniture and industrial use.

Remedies:

Find a proper balance to address the need of all who are dependent on forest.Conserve forests to conserve biodiversity.

Three Way Approach of Natural Resource Management:

Reduce: Reduce usage of natural resources. Avoid going by car to short distances,this will help preserve the fossil fuel. Reduce use of plastic bags can help inminimizing garbage.

Recycle & Reuse: Recycle as much as possible. Biodegradable packing materials,like jute bags, should be used instead of polythene bags. Indians are the best at

recycling and reusing newspaper. Paper bags made of old newspaper should beused as extensively as possible. Aluminium cans, paper, glass, etc. should berecycled to make new items.


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Mankind's Bad Effects on Environment:

Man is consuming all natural resources in a mindless way. Burning of fossil fuel iscreating heavy pollution.

1. High concentration of Carbon Dioxide is creating greenhouse effect, resulting inhihger average temperature across the globe.

2. Chlorofluorocarbon being used in refrigerators, ACs and pressurised cans has

caused hole in the ozone layer of atmosphere. Ozone layer protects the earth fromultraviolet rays from the sun.

3. Use of no-biodegradable materials like polythene is choking the ground. It willreduce the soil fertility. It may hamper in recharging of underground water reservoir.

Preventive Steps Needed to Protect the Environment:

1. Use of renewable energy sources.

2. Use of Bio-degradable packing materials.

3. Recycle as many things as possible.

4. Reuse things as many times as possible.

5. Reduce energy consumption.

Date post:	04-Apr-2018
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