Light

What is light? Light is produced by the Sun, and by objects such as light bulbs and matches. Light carries energy and travels as a wave. Light travels at a speed of 300,000,000 metres per second, which is much faster than the speed of sound. Photo credit: © 2008 Jupiterimages Corporation Light waves travel in straight lines. 2

Which materials let light through?

Light travels in a straight line directly into the eye. How do we see things? An object that gives out light is described as luminous. How does light from a luminous object such as a light bulb reach the eye? Light travels in a straight line directly into the eye. An object that does not give out light is non-luminous. How does your eye see a non-luminous object such as a comb? Light hits the comb and some of it is reflected into the eye. 4

Reflection at a mirror 5

Reflection ray diagram

Reflection can be very useful. Using reflection Reflection can be very useful. High-visibility strips are very reflective and make sure that this cyclist gets noticed when there is little light. How does a periscope use reflection? The two plane mirrors must be positioned at 45° from the vertical. Light is reflected at right angles from the top mirror onto the bottom mirror and into the eye of the viewer. 7

Refraction at an air-glass boundary 8

This causes the light to bend or refract. Why is light refracted? The speed of light depends on the material through which the light is travelling. When light enters a different material (e.g. when moving from air into glass), the speed of light changes. This causes the light to bend or refract. air glass The speed of light is affected by the density of the material through which it is travelling. When light enters a more dense medium, its speed decreases and this is why refraction occurs.

Effects of refraction Light from the part of the pencil in the water is refracted as it travels from the water into the air, making it appear bent. How does refraction make this stone look closer to the surface of the water than it really is? Light rays from the stone are refracted as they leave the water. The brain assumes the rays have travelled in straight lines, and is fooled into forming an image where it thinks the light rays came from.

Richard Of York Gave Battle In Vain Splitting white light A prism splits a ray of white light into the colours of the rainbow. This process is known as dispersion. The colours that make up white light are called the spectrum. Dispersion occurs because different colours of light refract differently. Red light refracts the least; violet light the most. The order of the colours in the spectrum is always the same. Use this phrase to remember the order of colours: Richard Of York Gave Battle In Vain Y 12