Chapter 41 Light

Objectives:In this section, you will learn:

1. That light is a form of energy that travels in straight lines.

2. That objects are either luminous or non-luminous.

3. That light is made up of different colours called the spectrum of light.

4. How light is reflected and how reflected light is used in instruments.

5. How light is refracted and how refraction of light is used in lenses.

Light is a form of energy.- Light can do work or cause movement.

- Light energy can be changed into other forms of energy e.g. used to power solar calculators

- A green plant changes light energy into chemical energy.

- Other forms of energy can be changed to light

- E.g. Nuclear energy can be changed to light on the sun.

- Chemical energy changes to light e.g. burning candles

- Electrical energy is converted to light in a light bulb.

•Albert Einstein was a Scientist from

Germany he studied light.It was for his work on light that he was awarded the Nobel Prize in 1921 when he wasjust 42 years old.

"For services to theoretical physics and especially for the discovery of the law of the photoelectric effect.“

Light Sources

All light comes from the sunLight travel at constant speed of 300 million metres per second.

The sun is 150 million km from the earth.Light from the sun reaches the earth in 8.5 minutes.

Light from the sun travels through the vacuum of space and then through the atmosphere.

Light travels through the atmosphere which is transparent (allows light through).

Luminous and non-luminous bodies

• A luminous object creates its own light - the sun is luminous, as is a burning candle or a light bulb.

• A non-luminous body does not create its own light; it only reflects light. The moon is a non-luminous body, as are the walls of your classroom.

How are Shadows formed?

Shadows occur when light is blocked by an object that doesn't allow light through.

Fact: What direction does light travel?

a) Straight lines

b) In a circle

c) Around corners

What do you think?

Can light travel around

Corners?

Experiment Light travels in straight lines T/B Pg. 319

What is a Solar Eclipse?

A solar eclipse occurs when the moon passes between the sun and the earth. One part of the earth goes completely dark and no light from the sun reaches this part of the earth. A lunar eclipse is when the is when the earth passes between the sun and the moon.

The spectrum of lightThe spectrum of light

Ordinary light is a mix of many different colours of light.

How many colours are

In a rainbow?

Can you name the

Colours in the rainbow?

Light passing through raindrops breaks up into its different colours. The colours of light are known as the spectrum of light.

• The spectrum of light is made up of seven major colours:

RED, ORANGE, YELLOW, GREEN, BLUE, INDIGO, VIOLET

• You should know the colours in order. Memory tricks such as “Richard of York gave battle in vain” can help.

Reflection of light• Reflection is the bouncing of light from a

surface. • Mirrors reflect light in a regular way so there is

a clear image.• Other surfaces e.g. can also reflect

light

Applications of Reflection

1. Microscopes often use mirrors to direct light onto the slide.

2. Security mirrors in shops give a wide field of vision

MirrorsA mirror reflects light because its surface is smooth flat and polished.

When light strikes an object it is either reflected, absorbed or transmitted.

Opaque: a material that reflects or absorbs the light – can’t see through it. -woodTransparent: The material transmits light – allows light to pass through it– glassTranslucent: allows some light to pass through – can’t see image clearly – wax paper, frosted glass.

Reflection of lightReflection of light If light falls on a smooth surface, it bounces

back in an orderly way [it does not scatter in all directions]. Reflection in smooth surfaces such as mirrors can be used to make images in the eye.

Uses of mirrorsUses of mirrors

• Mirrors are used to reflect light in many situations:

– In cars to get a “rear view” of what is going on behind.– By dentists to look behind teeth.– In lamps to reflect the light of a bulb outwards.– In a periscope to look above water or another

obstacle.

Periscope

• A periscope uses two mirrors placed at 45 degree angles. The light is reflected from one mirror to the next .

Periscopes

Uses of Periscopes:

Periscopes are used in submarines to see above the water.

They are sometimes used by people at the back of crowds to watch sporting events.

The refraction of light

Title: Refraction of lightAim: To demonstrate the refraction of light from glass to air

Apparatus: ray box, power supply, glass block darkened room

Safety:

Method:1. Direct a single ray from a ray box onto a rectangular block of glass

Diagram Fig 41.12 pg 323

The path of light ray ___________as it goes from air into glass. It also _______ as it passes from glass back into the air.

Conclusion: Light is refracted as it passes from air into glass and from glass into air

Title: Refraction of light

Aim: To demonstrate refraction of light as it passes from air to water using a laser

Apparatus: Laser, basis of water, small amount of milk

Safety:

Method:

1. Shine light from a laser from the air into the water containing milk in the basin.

Diagram: Fig 41.13 pg 323

Result:

The beam _______ going from the air into the water.

Conclusion: Light from a laser is refracted as it passes from the air into water

Title: Refraction of light

Aim: To demonstrate refraction of light from water into air

Apparatus: Straight rod, large beaker of water

Safety:

Method:

1. Set up the apparatus as shown in the diagram holding the rod at an angle in the water

Result: The straight rods seems ________ as it passes from the air into the water

Conclusion: Refraction occurs where the rod enters the water

Refraction of light in lenses

• A lense is a piece of glass or other transparent material that has at least one curved surface.

• Lenses bend or refract light as it passes through them.

• There are two main kinds of lens: convex and concave.

• A convex lens is thicker in the middle than at the edges. Light is refracted in a convex lens to a focus and they are used in cameras and spectacles to magnify close objects.

Lenses: The concave lensLenses: The concave lens

• A concave lens is thinner in the middle than at the edges.

• When light falls on the lens it is refracted and the light spread out on the other side.

• Concave lenses are also used in cameras and in certain eye-glasses to correct certain sight problems.

Summary: Key points

• Light is a form of energy and travels in straight lines.

• A luminous body is a source of light. A non-luminous body reflects light.

• Ordinary light can be broken up into different colours: red, orange, yellow, green, blue, indigo, violet. This is the spectrum of light.

• Light is reflected in an orderly way from smooth surfaces, e.g. mirrors. Mirrors have many everyday uses.

• Light is refracted when it moves from one transparent material to another. This fact is used in lenses. There are two main types of lenses: convex and concave. They are used in cameras and eye-glasses.

Summary (continued)