Waves Tutorial 9 - Colour Addition

This tutorial is for students of the Irish Syllabus

Visible light is a very small part of the electromagnetic spectrum.

Insects can see wavelengths in the UV spectrum.  A plain looking flower has elaborate markings when studied with a UV sensitive camera.

Many animals can see the whole spectrum from red light to blue light (700 nm to 400 nm), although some mammals can see just green and blue light.  Tigers, being orange with black strips, may seem very obvious to us, but their prey can only see blue and green, so the tiger is very well camouflaged.

Most animals cannot see infra-red directly.  Snakes have infra-red detectors, but these are not part of the eye.  Biologists think that some fish, like goldfish, can see infra-red.  Goldfish are the only animals that can see both UV and IR.  Frogs are believed to be able to see IR.

Dispersion

If we pass monochromatic red light through a prism, we see that it is refracted like this:

If we use a ray of white light, we see that the light ray gets split into the colours of the rainbow (a spectrum).  This is because different wavelengths refract by different amounts.

This is called dispersion

Dispersion explains rainbows.  Droplets of water pick up white light from the sun.  When sunlight strikes a raindrop, the ray will pass into the drop and, the the angle is greater than the critical angle, the ray will be reflected by total internal reflection on the water-air boundary (about 49o).  The light will be reflected and will pass to the boundary, where it refracts as it emerges from the droplet.    Remember that different colours refract by different amounts.  So we get a spectrum.

From that particular droplet, we may only see one colour, but there are millions of other droplets around the droplet we have considered.  So we see all the colours of the rainbow.

By using a second prism, we can combine all the rays to get a ray of white light:

It is said that the young Isaac Newton showed dispersion by slitting one of the curtains in his mother's house and projecting the spectrum from the sunlight onto the wall.  What his mother had to say about it is not recorded.

White light is all the colours of the rainbow added together.

We can make white light from just three colours, red, green, and blue.   These are called the primary colours.

Note that the primary colours are NOT the same as the artist's primary colours.  The mixing of pigments involves colour subtraction, not colour addition.  We will not consider colour subtraction here.

Secondary colours are:

• cyan - from green and blue;

• magenta - from red and blue;

• yellow - from red and green.

You can get a double light emitting diode that emits red when the first diode only is on, or green light when the second diode only is on.  When both are on, the emitted light is yellow.

A complementary colour is one that you add to a secondary colour to give white light.  The combinations are:

• yellow and blue;

• magenta and green;

• cyan and red.

The uses of colour addition include:

• stage lighting;

• colour television.

In television, the photo-detectors in the camera are sensitive to red, green, and blue.  A blue detector gives out its maximum response to blue light, and none to red or green.  White light makes all three detectors give out their maximum response.  Other colours, like yellow, will stimulate the red and green detectors, but not the blue.  At the receiver, the individual pixels give out varying intensities of red, green, and blue, to make up the individual colours.