LightLight

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.

Space shuttle 7,780 m/s

Cheetah 31.0 m/s

Speed of light

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

Light in vacuum

300,000,000 m/s

Earth in orbit around the Sun

29,780 m/s

Jetplane 350 m/s

Sound in air 330 m/s

Garden snail 0.04 m/s

How does light travel?• Light travels in straight lines.Experiment 1: How do we know light travels in straight lines?Look at the flame of a candle through a straight hose. You should be able to see the light through the hose.

Now, try looking through a bent hose. The light from the candle cannot reach your eyes. This shows that light does not 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)

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.

Reflected images are very clear on a smooth surface.

A parallel beam of light stays parallel after regular reflection.

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 imageso 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

O

N

A B

Incident ray

Reflected ray

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.

The perpendicular line to the surface NO is also known as the normal.

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 mirrorso 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 mirrorso 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.

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.

Normal

incident ray

Normal

glass block

B

A

emergent ray

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.

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 is less than the real depth.

• This explains why a pool appears shallower than it is.

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 and blue.

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

• Secondary colours are cyan, yellow and magenta.

17.5 Colours

Green

Blue RedWhite

CyanYellow

Magenta

17.5 Colours

GreenWhite

• 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.

RedWhite

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. When white light is shone on these

balls, each ball will reflect their respective colours. This is because white light contains all the colours of the spectrum.

When red light is shone on the balls, only the red ball will reflect the red light. The green and blue balls appear black since they can only reflect green and blue light respectively.

17.5 Colours

Experiment 3: Coloured light

What happens when a yellow ball is used in this experiment?

Under white light, a yellow ball appears yellow

But if red light is shone on the yellow ball, it appears red. Yellow is a secondary colour, which consists of red and green. Thus, a yellow ball can reflect both red and green light.

Miscellaneous

• http://science.howstuffworks.com/light.htm• http://www.colorado.edu/physics/2000/

waves_particles/lightspeed_evidence.html• http://en.wikipedia.org/wiki/

Reflection_(physics)• http://en.wikipedia.org/wiki/Refraction• http://demonstrations.wolfram.com/

DispersionOfLightThroughAPrism/• http://www.fi.edu/color/