Light

17.1 What is Light?

• Light is an important source of energy.

• It enables us to see.

• The Sun is the most important source of natural light. • Without sunlight, it is impossible for living things to survive on earth.

Speed of light

• Light travels at a very high speed.

• During a storm, lightning and thunder occur at the same location, but we see lightning first before we hear thunder. • This shows that light travels faster than sound.

• Light has the fastest speed in the Universe. It travels at 300,000,000 m/s in a vacuum.

Speed of light

• Comparison of the speed of light with the speeds of other things:

Cheetah Sound in air

300,000,000 m/s 7,780 m/s 29,780 m/s

How does light travel?

• Light travels in straight lines.

Experiment 1: How do we know light travels in straight lines?

bend, but travels in straight lines.

How does light travel?

• Since light travels in straight lines, shadows have the same shape as the object blocking the light.

• If light did not travel in straight lines, it would go round all objects and cast no shadow.

Rays and beams

• We can trace the path of light from a source by drawing straight lines to join one point to another.

• Such lines are called

light rays

.

• A bundle of light rays is known as a

beam

of light.

• Beams of light can be parallel, divergent or convergent. Parallel beam Divergent beam (fan outwards) Convergent beam (merge to a point)

Quiz 1

• 1. What is light?

Light is an energy that enables us to see 2. How fast does light travel in a vacuum?

• It travels at 300,000,000 m/s in a vacuum.

3. Why are shadows of objects formed when there is light?

• Because light travels in a straight line and is blocked by the objects.

Quiz 1

4. What are light rays?

The straight paths of a light source from one point to another.

5. State the three types of light beams.

• Parallel beam • Divergent beam • Convergent beam

17.2 Reflection

• When light rays hit the surface of another medium, it can be reflected. • On a smooth surface such as the surface of a mirror,

regular reflection

takes place. • A clear and undistorted image can be seen in the surface.

17.2 Reflection

• If the surface is rough such as the surface of a sheet of paper,

irregular reflection

occurs.

• The surface can be seen but no reflected images are seen in it.

• For example, when light shines on the paper, no reflected image is formed although light is reflected. Although this page appears smooth, under a microscope, you can see how rough it is. Thus, there is no reflected image.

A parallel beam of light hitting a rough surface gets scattered or reflected in an irregular manner.

Reflection in a mirror

• The flat mirror is a good reflector of light and is also called a

plane mirror

.

• Look at yourself in a mirror. What you see is an

image

of yourself.

• You can see your image in the mirror because light rays from your body are regularly reflected by the mirror into your eyes.

Reflection in a mirror

•

Characteristics of plane mirror images

o The

distance of the image from the mirror is equal to the distance of the object from the mirror

.

o The image and the object are the

same size

.

o The image is

upright

.

o The image is

laterally inverted

. This means the left and right side of the image are reversed.

o The image is

virtual

. This means that the image cannot be projected onto a screen behind the mirror.

Reflection in a mirror

• The position of an object’s image can be shown using light rays. • The light rays that bounce off the mirror into the eye of the observer will appear to have originated from the image from behind the mirror.

Tracing reflected light rays

A

The perpendicular line to the surface

NO

is also known as the

normal

.

N B

A ray of light

AO

strikes the mirror at point

O

, creating an angle

i

with the perpendicular line

NO

.

AO

is called the

incident ray

, and angle

i

is called the

angle of incidence

. Incident ray Reflected ray

O

The light ray bounces off the mirror and leaves along the path

OB

.

OB

is called the

reflected ray

. This ray makes an angle

r

with the normal. This angle is known as the

angle of reflection

.

Types of mirrors

•

Plane mirrors

o Plane mirrors are usually used to check one’s appearance.

o They are also used as a form of decoration to make a room appear more spacious.

o Plane mirrors are used in cars as rear-view or side-view mirrors.

Types of mirrors

•

Concave mirrors

o These are mirrors with a curved surface that bends inwards.

o When the object is near the concave surface, images formed are

virtual and magnified

.

Types of mirrors

• Convex mirrors o Mirrors that bend outwards are called convex mirrors.

o The back of a spoon is an example. Side-view mirrors of cars are sometimes convex mirrors.

o Convex mirrors cover a wider field of vision, and the images formed are

distorted and smaller

.

Quiz 2

• 1. What is reflection of light?

It is the bouncing of light when it hits the surface of another medium.

2. When does regular reflection occur?

• When light reflects on a smooth surface such as the surface of a mirror.

3. When does irregular reflection occur?

• When light reflects on a rough surface such as the surface of a sheet of paper.

Quiz 2

• 4. What is a plane mirror?

A flat mirror which is a good reflector of light.

5. State five characteristics of plane mirror images.

• The distance of the image from the mirror is equal to the distance of the object from the mirror.

• The image and the object are the same size.

• The image is upright. • The image is laterally inverted.

• The image is virtual.

Quiz 2

6. The diagram below shows the reflection of light on a smooth surface. (a) Name the light ray

AO

.

Incident light ray

N

(b) Name the light ray

OB

.

Reflected light ray

A B O

Quiz 2

(c) Name the perpendicular line

NO

.

Normal line (d) Name the angle

AON

.

Angle of incidence (e) Name the

A

angle

BON

.

Angle of reflection

N O B

Quiz 2

• • • 7. Name the three types of mirrors.

Plane mirrors Concave mirrors Convex mirrors 8. State two characteristics of concave mirror images.

• The images are virtual. • The images are magnified.

9. State two characteristics of convex mirror images.

• The images are distorted. • The images are smaller than the objects.

17.3 Refraction

• Light

bends

when a light ray travels from one medium to another medium of different density (such as from air to water).

• This bending of light is known as

refraction

.

17.3 Refraction

• When light travels from air to glass, it gets refracted.

incident ray Normal

A

At

A

, the point where the ray enters, a normal is drawn. When light enters the glass block, it is moving from a less dense medium to a denser medium. Hence, the ray of light will bend towards the normal.

emergent ray Normal

B

glass block When the ray of light exits the glass block at

B

, it is moving from a denser medium to a less dense medium. Hence, it will bend away from the normal.

Effect of refraction: Apparent depth

• Refraction of light causes the coin to appear higher than it really is.

• When viewed from the surface, the depth of the basin of water will always seem less than what it really is.

• We say the

apparent depth

the real depth. is less than • This explains why a pool appears shallower than it is.

Quiz 3

• 1. What is refraction of light?

The bending of light when it travels from one medium to another medium of different density.

2. When does regular reflection occur?

• When light reflects on a smooth surface such as the surface of a mirror.

3. When does irregular reflection occur?

• When light reflects on a rough surface such as the surface of a sheet of paper.

Quiz 3

4. The diagram below shows the refraction of light from air through a glass block. (a) Name the light ray

RA

.

R N

Incident light ray

A

(b) Name the light ray

AB

.

Refracted light ray

N B S

Quiz 3

(c) Name the light ray

BS

.

Emergent light ray (d) Name the perpendicular lines

NA

and

NB.

Normal lines

R N A N B S

Quiz 3

(e) Name the angle

RAN

.

Angle of incidence (f) Name the angle

N’AB

.

Angle of refraction

R N A N’ N B S

Quiz 3

• • 5. State two effects of refraction of light.

An object appears higher in water than it really is. When viewed from the surface, water in a swimming pool appears shallower than its actual depth.

17.4 Dispersion of White Light

• Sunlight appears white, but it is made up of different colours.

1.

When a beam of white light passes through a prism, it is split into different colours.

2.

This is because the different colours of white light bend towards the normal through diffferent angles when entering and leaving the prism.

3.

A

spectrum

of colours is produced, which is made up of seven colours: red, orange, yellow, green, blue, indigo and violet. This splitting of white light is called

dispersion

.

17.5 Colours

•

Primary colours

are basic colours that cannot be obtained by mixing other colours of light. • • Primary colours of light are

red

,

green blue

.

Secondary colours

and of light are colours that are formed by mixing primary colours. • Secondary colours are

cyan

,

magenta

.

yellow

and

17.5 Colours

Green Blue Cyan White Yellow Red Magenta

17.5 Colours

• We can see the colour of an object as it reflects that colour into our eyes.

• When white light falls on a green object, the object absorbs all other colours and reflects only green light.

• This is the case for the leaves of the strawberries below. • The strawberries appear red because they absorb all other colours except red.

White Green White Red

17.5 Colours

Experiment 3: Coloured light

Take three different coloured balls: red, blue and green. Let us see what happens to these coloured balls under different colours of light.

respectively.

17.5 Colours

Experiment 3: Coloured light

What happens when a yellow ball is used in this experiment? Yellow is a secondary colour, which consists of red and green. Thus, a yellow ball can reflect both red and green light.

Quiz 4

• 1. How many colours does the sunlight consist of?

Seven • 2. What is dispersion of white light?

This splitting of white light into seven rainbow colours. • 3. State the colours of sunlight.

Red, Orange, Yellow, Green, Blue, Indigo, Violet • 4. Which colour of light is the fastest?

Red light

Quiz 4

• 5. Which colour of light is the slowest?

Violet light • 6. Which colour of light will be refracted the most?

Violet light • 7. Which colour of light will be refracted the least?

Red light

Quiz 4

• 8. What are primary colours of light?

Primary colours

are basic colours that cannot be obtained by mixing other colours of light. • 9. State the primary colours of light.

Red, Green and Blue.

• 10. What are secondary colours of light?

Secondary colours

of light are colours that are formed by mixing primary colours.

Quiz 4

• 11. State the secondary colours of light.

Cyan, Yellow and Magenta • 12. How do you produce cyan light?

By adding green light to blue light • 13. How do you produce yellow light?

By adding red light to green light • 14. How do you produce magenta light?

By adding red light to blue light

Quiz 4

• 15. Why are we able to see the colour of an object?

We can see the colour of an object as it reflects that colour into our eyes.

• 16. When white light falls on a blue table, why are we able to see only a blue table and not other colours?

When white light falls on a blue table, the object absorbs all other colours and reflects only blue light.

Quiz 4

• 17. What happens to a red light when it is shone on a blue chair? The red light will be absorbed by the chair. The chair will appear black as it will reflect blue light only and absorb other colours of light.

• 18. What happens to a red light when it is shone on a yellow shirt? The red light will be reflected by the shirt.

The shirt will appear red as it will reflect red and green light only and absorb other colours of light.