The colour of an object depends on the wavelength of the light that it scatters

e.g., a black object scatters little light, and absorbs light at all visual wavelengths; a green object scatters more green light than other colours, which it absorbs more.

It has already been mentioned that the colour of light is dependant on its wavelength with red light having a longer wavelength than blue light. The light from the Sun and from most torches and light bulbs doesn't appear to have much colour and is called white light. Long before Newton's time it was known that if you passed white light through a prism you get a mixture of different coloured lights (the visible spectrum):

red/orange/yellow/green/blue/violet

It was believed then that the prism added something to the light. Newton (1666) questioned this idea and was the first person to manage to recombine the coloured light using another prism and make white light again.

The splitting of light occurs because of the shape of the prism and the different interactions of the components of the white light with the glass. The violet component interacts with glass more than the red component.

The rainbow is an example of white light being split into its components. The raindrops are somewhat like prisms, although in a raindrop you get a special phenomenon called internal reflection if the angle at which the light hits the drop is just right. Total internal reflection means that the light travels quite a long way in the water medium and there is the opportunity for the different components of the white light to interact with the water. This splits the white light into its component colours before it reemerges from the rain drop so a rainbow is produced. When you look at a rainbow the Sun is always behind you.

We know that when light hits an object one of four things might occur.

· It could be absorbed by the surface.

· It could be scattered by the object.

· It could be partly scattered and partly absorbed.

· It could be reflected.

What do we mean by absorbed by the surface? There are some instances where all the light hitting an object is captured or absorbed by the object. The light interacts with the surface and is not allowed to escape. So we see no light bouncing off the object and the object is said to be black.

In most cases not all the light is absorbed by the surface and the light that is not absorbed is scattered. We also find that the light that is absorbed has a particular wavelength dependant on the nature of the surface, ie what material it is and what dyes might have been added to it. When white light falls upon a green surface all of the components of the white light that are not green are absorbed by the surface while the green light is scattered. A red object is red when white light falls upon it because it absorbs all the components of white light with the exception of the red component which is scattered. A yellow object is slightly different because yellow light can be made from mixing red and green light so the object is yellow because it is scattering red and green light and absorbing all of the other colours.

What do you think if we illuminated a green jersey with red light? The green dye in the jersey will absorb light of all wavelengths with the exception of green light. As there is no green in the red light that is illuminating the jersey, no light will be scattered and the jersey will look black.

Blue skies and lovely sunsets are the result of light being scattered. When the light from the Sun reaches the Earth's atmosphere it doesn't all pass through. Some of the ultraviolet light is absorbed by the outer atmosphere and some of the ultraviolet and visible light is scattered by it. The scattered visible light is from the blue part of the spectrum and it is this scattering which causes us to have blue skies.

On a nice summer evening with a lot of dust in the atmosphere the blue part of the Suns light is scattered even more leaving only the yellow and red components in the light which reaches our eyes therefore giving us a red sunset.

Contents

Light does appear to travel in straight lines
Light can differ in intensity and wavelength
The distinction between reflection and scattering, How images are formed in a mirror
Objects are seen when light enters the eye through the pupil
Self assessment