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LIGHT WAVES Waves carry energy from one place to another © 2000 Microsoft Clip Gallery
LIGHT WAVES
Waves carry energy from one place to another
© 2000 Microsoft Clip Gallery
Light & the Electromagnetic Spectrum
THE NATURE OF LIGHT
The Dual Nature
Albert Einstein
LIGHT: Particles or Waves?
Wave Model of Light
Explains most properties of light
Particle Theory of Light
Photoelectric Effect – Photons of light produce
free electrons
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LIGHT: What Is It? Light Energy
Atoms
As atoms absorb energy, electrons jump out to a
higher energy level.
Electrons release light when falling down to the
lower energy level.
Photons - bundles/packets of energy released when the
electrons fall.
Light: Stream of Photons
Electromagnetic Waves
Speed in Vacuum ( C )
300,000 km/sec
186,000 mi/sec
Speed in Other Materials
Slower in Air, Water, Glass
Light travels much faster than sound.
1) Thunder and lightning start at the same time, but we will see the lightning first.
2) When a starting pistol is fired we see the smoke first and then hear the bang.
For example:
Light travels VERY FAST – around 300,000 kilometres per second.
At this speed it can go around the world 8 times in one second.
Transverse Waves
Energy is perpendicular to direction of motion
Moving photon creates electric & magnetic
field
Light has BOTH Electric & Magnetic fields at
right angles!
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Electromagnetic Spectrum
Visible Spectrum – Light we can see
ROY G. BIV – Acronym for Red, Orange, Yellow,
Green, Blue, Indigo, & Violet.
Largest to Smallest Wavelength.
EM W
ave
s in
Act
ion Radio: yes, this is the same kind of energy that radio stations emit
into the air for your boom box to capture and turn into your favorite Mozart, Madonna, or Coolio tunes. But radio waves are also emitted by other things ... such as stars and gases in space. You may not be able to dance to what these objects emit, but you can use it to learn what they are made of.
Microwaves: they will cook your popcorn in just a few minutes! In space, microwaves are used by astronomers to learn about the structure of nearby galaxies, including our own Milky Way!
Infrared: we often think of this as being the same thing as 'heat', because it makes our skin feel warm. In space, IR light maps the dust between stars.
Visible: yes, this is the part that our eyes see. Visible radiation is emitted by everything from fireflies to light bulbs to stars ... also by fast-moving particles hitting other particles.
Ultraviolet: we know that the Sun is a source of ultraviolet (or UV) radiation, because it is the UV rays that cause our skin to burn! Stars and other "hot" objects in space emit UV radiation.
X-rays: your doctor uses them to look at your bones and your dentist to look at your teeth. Hot gases in the Universe also emit X-rays .
Gamma-rays: radioactive materials (some natural and others made by man in things like nuclear power plants) can emit gamma-rays. Big particle accelerators that scientists use to help them understand what matter is made of can sometimes generate gamma-rays. But the biggest gamma-ray generator of all is the Universe! It makes gamma radiation in all kinds of ways.
Color of Light Transparent Objects:
Light transmitted because of no scattering
Color transmitted is color you see. All other colors are
absorbed.
Translucent:
Light is scattered and transmitted some.
Opaque:
Light is either reflected or absorbed.
Color of opaque objects is color it reflects.
Explain the terms transparent, translucent, and
opaque, and give an example of each.
Plastic wrap Wax paper Aluminum Foil Transparent Translucent Opaque
Color Transmission Filters work in a similar way.
Red filters only let RED light thru.
Blue let only BLUE light thru.
What do you think that UV sticker means on
your sunglasses?
Why do they sell those orange glasses that are
supposed to reduce glare?
© 2003 Mike Maloney 16
SHADOWS
Shadows are places where light is “blocked”:
Rays of light
Polarization
Polarization is a phenomenon of light that is
used in sun-glasses and 3-D movies.
Play with the two polarizing filters for a
few minutes and note what is happening and
see if you can think of any reasons for it.
© 2003 Mike Maloney 18
Polarization Hint Light vibrates in all directions.
A polarizing filter acts like a picket fence. It only
lets certain direction vibrations pass through it.
Therefore, if you pass light through two of them
you can completely block the light from passing
through.
HOW?
© 2003 Mike Maloney 19
Some Sweet Color Tricks
Combining colors in art class
How does color printing work?
Combining lights
Why is the sky blue?
Why are sunsets red?
Why is water greenish-blue?
How does 3-D work?
Why does a CD reflect a rainbow, and a mirror does not?
How can you help people who are color blind?
OTHERS link to site
© 2003 Mike Maloney 21
How You See
Retina –
Lens refracts light to converge on the retina. Nerves transmit the image
Rods –
Nerve cells in the retina. Very sensitive to light & dark
Cones –
Nerve cells help to see light/color
© 2000 Microsoft Clip Gallery
Sensing and Interpreting Light Human vision is possible due to the ability of the eye to sense light. When a light source such as
the sun strikes an object, the object reflects certain wavelengths of light and absorbs others.
Things that are white in color reflect all wavelengths of light, while black objects absorb all
wavelengths. Colored objects absorb certain wavelengths but reflect those of the color that the
objects appear. When the light reflected by objects enters the eye, it passes through a transparent
layer called the cornea and then through the pupil, which is essentially a hole in the middle of the
eye. Then a lens behind the pupil focuses the light on the back of the eye, which is known as the
retina.
Light-Sensing Cells in the Retina The retina is a sensitive area in the back part of the eye containing millions of cells known as rods
and cones. Rods are good at sensing dim light or light that is devoid of color, while cones are
specialized at sensing color. If too much light enters the eye, the rods and cones can be
overwhelmed, which can result in eye damage. A ring of muscles in the eye called the iris (the
colored part of the eye) flexes and contracts to alter the size of the pupil and therefore the amount
of light that enters the eye. In conditions with ample light, the pupil gets smaller to prevent the
retina from being overwhelmed, while the pupil dilates in dark conditions to take in as much light
as possible in order to sense objects.
Electrical Impulses Transmit Images to the Brain After light strikes the rods and cones, they initiate a chemical reaction that turns the image
focused on the retina into electrical impulses that are then sent to the brain through the optic
nerve. When the impulses reach the brain, it translates them into the images you see. As light
enters the eye, the lens bends it, creating an upside down image on the retina. The brain must flip
the image back so that we see things properly.
COLORS OF LIGHT AND PIGMENTS
Color Reflection, Mixing and Combination
We see things because they reflect
light into our eyes:
Homework
Color Reflection
So if we see something as WHITE, that means …
It reflected back all the wavelengths of light to our
eyes
If we see something as RED or BLUE
It reflected only the RED or only the BLUE
wavelengths
The others were absorbed.
And if we see something as black?
It did not reflect back any of the light.
© 2003 Mike Maloney 27
Color of Light (Cont.)
Color of Objects
White light is the presence of ALL
the colors of the visible spectrum.
Black objects absorb ALL the colors
and no light is reflected back.
© 2000 Microsoft Clip Gallery
Color of Light (Cont.)
Primary Colors of Light
Three colors that can be mixed to
produce any other colored light
Red + blue + green = white light
Complementary Colors of Light
Two complementary colors combine
to make white light-Magenta,Cyan,Yellow
© 2000 Microsoft Clip Gallery
Paint Pigments
Pigments absorb the frequency of
light that you see
Primary pigments
Yellow + cyan + magenta = black
Primary pigments are complements
of the primary colors of light.
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Complementary Pigments
Green, blue, red
Complementary
pigments are
primary colors
for light!
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© 2000 Microsoft Clip Gallery
Colour White light is not a single colour; it is made up of a
mixture of the seven colours of the rainbow.
We can demonstrate this by
splitting white light with a
prism:
This is how rainbows are
formed: sunlight is “split up” by
raindrops.
Adding colours White light can be split up to make separate
colours. These colours can be added together
again.
The primary colours of light are red, blue and green:
Adding blue and red makes magenta (purple)
Adding blue and green makes cyan
(light blue)
Adding all three makes white again
Adding red and green makes yellow
Seeing colour
The colour an object appears depends on the
colours of light it reflects.
For example, a red book only reflects red light:
White
light
Only red light is reflected
A white hat would reflect all seven colours:
A pair of purple trousers would reflect purple light (and red and blue, as purple is made up of red and blue):
Purple light
White
light
Using coloured light
If we look at a coloured object in coloured light we
see something different. For example, consider a
football kit:
White
light
Shorts look blue
Shirt looks red
In different colours of light this kit would look different:
Red
light Shirt looks red
Shorts look black
Blue
light
Shirt looks black
Shorts look blue
Some further examples:
Object Colour of light Colour object seems to be
Red socks
Red Red
Blue Black
Green Black
Blue teddy
Red Black
Blue
Green
Green camel
Red
Blue
Green
Magenta book
Red
Blue
Green
Using filters
Filters can be used to “block” out different colours of light:
Red Filter
Magenta Filter
Investigating filters Colour of filter Colours that could be “seen”
Red
Green
Blue
Cyan
Magenta
Yellow
Red
Magenta
White
Yellow
Blue Green
Cyan
SOURCES OF LIGHT
Luminous and Non-Luminous
LIGHT & ITS USES
Sources of Light
Incandescent light –
light produced by
heating an object
until it glows.
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© 2000 Microsoft Clip Gallery
LIGHT & ITS USES
Fluorescent Light –
Light produced by electron bombardment
of gas molecules
Phosphors absorb photons that are
created when mercury gas gets zapped
with electrons. The phosphors glow &
produce light.
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LIGHT & ITS USES - Neon
Neon light – neon
inside glass tubes
makes red light.
Other gases make
other colors.
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Luminous and non-luminous objects
A luminous object is one that produces light.
A non-luminous object is one that reflects light.
Luminous objects Reflectors
LIGHT REFLECTION
Mirrors and Law of Reflection
Reflection Reflection from a mirror:
Incident ray
Normal
Reflected ray
Angle of incidence
Angle of reflection
Mirror
The Law of Reflection
Angle of incidence = Angle of reflection
In other words, light gets reflected from a surface at ____ _____ angle it hits it.
The same !!!
LIGHT & ITS USES - Reflection
Reflection – Bouncing back of light waves
Regular reflection – mirrors smooth
surfaces scatter light very little.
Images are clear & exact.
Diffuse reflection – reflected light is
scattered due to an irregular surface.
Clear vs. Diffuse Reflection
Smooth, shiny surfaces
have a clear reflection:
Rough, dull surfaces have a
diffuse reflection.
Diffuse reflection is when light
is scattered in different
directions
LIGHT & ITS USES:
Reflection Vocabulary
Enlarged –
Image is larger than actual object.
Reduced –
Image is smaller than object.
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© 2000 Microsoft Clip Gallery
LIGHT & ITS USES:
Reflection Vocabulary
Erect –
Image is right side up.
Inverted –
Image is upside down.
© 2000 Microsoft Clip Gallery
© 2000 Microsoft Clip Gallery
LIGHT & ITS USES:
Reflection Vocabulary Real Image –
Image is made from “real” light rays that converge
at a real focal point so the image is REAL
Can be projected onto a screen because light
actually passes through the point where the image
appears
Always inverted
LIGHT & ITS USES:
Reflection Vocabulary
Virtual Image–
“Not Real”
because it cannot
be projected
Image only seems
to be there!
Using mirrors
Two examples:
1) A periscope
2) A car headlight
Properties of Light summary
1) Light travels in straight lines
2) Light travels much faster than sound
3) We see things because they reflect light
into our eyes
4) Shadows are formed when light is
blocked by an object
Reflection and Refraction
Mirrors reflect light and allow us to see ourselves.
A prism is another optical device that can cause light to change
directions.
A prism is a solid piece of glass with flat polished surfaces.
17.1 Reflection
Images appear in mirrors
because of how light is reflected
by mirrors.
The incident ray follows the
light falling onto the mirror.
The reflected ray follows the
light bouncing off the mirror.
17.1 Reflection
In specular reflection each incident ray bounces off in a single
direction.
A surface that is not shiny creates diffuse reflection.
In diffuse reflection, a single ray of light scatters into many
directions.
Law of Reflection
The angle of
incidence equals
the angle of
reflection.
The incident ray
strikes the mirror.
The reflected ray
bounces off.
Law of reflection
A light ray is incident on a plane mirror with a 30 degree angle
of incidence.
Sketch the incident and reflected rays and determine the angle
of reflection.
30o 30o
Refraction Light rays may bend as they
cross a boundary from one
material to another, like from
air to water.
This bending of light rays is
known as refraction.
The light rays from the straw
are refracted (or bent) when
they cross from water back
into air before reaching your
eyes.
Refraction
When a ray of light crosses from one material to another,
the amount it bends depends on the difference in index of
refraction between the two materials.
Index of refraction
The ability of a material to bend rays of light is
described by the index of refraction (n).
Snell's Law of Refraction
Snell’s law is the relationship between the angles
of incidence and refraction and the index of
refraction of both materials.
ni sin Qi = nr sin Qr
Index of
refraction of
refractive
material
Angle of incidence
(degrees)
Angle of refraction
(degrees)
Index of
refraction of
incident
material
Calculate the angle of refraction
A ray of light traveling
through air is incident
on a smooth surface of
water at an angle of
30° to the normal.
Calculate the angle of
refraction for the ray
as it enters the water.
Dispersion and Prisms
When white light passes through a glass prism, blue
is bent more than red.
Colors between blue and red are bent proportional to
their position in the spectrum.
Dispersion and Prisms
The variation in
refractive index with
color is called
dispersion.
A rainbow is an example
of dispersion in nature.
Tiny rain droplets act as
prisms separating the
colors in the white light
rays from the sun.
EXTRAS
Sounds
Refraction
Refraction is when waves ____ __ or slow down due to travelling in a different _________. A medium is something that waves will travel through. When a pen is placed in water it looks like this:
In this case the light rays are slowed down by the water and are _____, causing the pen to look odd. The two mediums in this example are ______ and _______.
Words – speed up, water, air, bent
Sound – The basics
We hear things when they vibrate.
If something vibrates with a high frequency (vibrates very ______) we say it has a _____ pitch.
If something vibrates with a low frequency (vibrates ______) we say it has a ____ pitch.
The lowest frequency I could hear was…
Words – slowly, low, high, quickly
Drawing sounds…
This sound wave has a _____ frequency:
This sound wave has a ___ _frequency:
Drawing sounds…
This sound wave has a _____ amplitude (loud):
This sound wave has a _____ amplitude (quiet):
