By Talar Hagopian and Rima Debs École la Dauversière, Montreal, June 2001 Content validation and linguistic revision : Karine Lefebvre Translated from French to English by Nigel Ward  Science animée, 2001 click here to begin

In which of these two worlds would you prefer to live?

The primary colours are different in art (paint) and in science (light). That’s because in science we are mainly interested in adding coloured lights together but paint works by absorbing (subtracting) colours. For paint the primary colours are: red, yellow and blue, … …while for light addition they are: red, green and blue.

By mixing yellow paint with blue paint we obtain … … green...

…but by mixing yellow* light with blue light we get… … white light! * yellow light = green light + red light

By shining red, green and blue light beams onto a white screen and making them overlap, we obtain white light.

Let’s concentrate on the colours of light… Here are the secondary colours and how they are formed…

blue + green = Cyan red + blue = Magenta red + green = yellow Cyan, magenta and yellow are the three secondary colours.

Optical filters work like paint, by absorbing certain colours. For example, a red filter allows only red light to pass. A cyan filter allows blue and green light to pass (remember cyan = blue + green). cyan filter red filter

By superimposing coloured filters (cyan, magenta and yellow) we get black (the absence of light) where the three filters overlap. The magenta filter transmits red and blue light and blocks green. The yellow filter blocks blue. The cyan filter blocks red. Where the three filters overlap every primary colour is blocked.

green blue red By superimposing coloured filters (cyan, magenta and yellow) we get the primary colours where pairs of filters overlap. For example, the magenta filter can transmit red and blue and the yellow filter can transmit red and green - only red can pass through both the magenta filter and the yellow filter.

Longer wavelengths correspond to the colour red. As the wavelength decreases the light becomes orange orange then yellow yellow then green green then indigo indigo then violet. Since we are mainly interested in the primary colours red, red, green green and blue blue we can say long wavelengths correspond to red, red, medium to green and short to blue. Light consists of electromagnetic waves waves with various wavelengths. wavelengths. Wavelengths of light are usually measured in nanometres nanometres (nm).

Electromagnetic Spectrum X rays Gamma (  ) rays Radio waves Microwaves Ultraviolet Infrared Visible light waves of various colours Visible light waves (colours) are part of a family called the ‘electromagnetic spectrum’. All members of this family share certain properties. For example, they all travel at the same speed through a vacuum.

( ) Isaac Newton Newton discovered that white light consists of all the colours of the rainbow combined together.

prism spectrum white light Dispersion Working with prisms, he noticed that white light could be broken up into its different components, the colours of the rainbow. He had discovered ‘dispersion’. In the diagram below, a prism disperses white light into the colours of the spectrum.

It is possible of recombine the colours to form white light again.

optic optic nerve retinalight cones and rods

The rod cells are sensitive only to shades of gray but function even in dim light. There are about 120 million of these detectors in the retina. The cone cells detect colour but don’t work well in dim light. We have only about 7 million cone cells in the retina.

Colour blindness is an anomaly of vision. People suffering from this condition cannot distinguish certain colours from one another. For example, someone suffering from red-green colour blindness cannot distinguish red and green. Why would this be a great problem when that person drives a car? This visual dysfunction can be hereditary, or a consequence of a disease that affects the optic nerve.

Technically, colour blindness is due to a poor functioning or an insensitivity to colour of the light- sensitive cells, making the brain unable to recognise the colour correctly. There are several types of colour blindness including "red-green", which affects men more than women, and “yellow-blue", less common, which affects men and women equally. Certain people can see only two colours, and everything else looks gray. Certain people suffer from ‘mono-chromatism’ which means they see no colour at all, only shades of gray.

We perceive objects to have certain colours according to which colours they absorb and which they reflect into our eyes. This chick appears yellow because the yellow component is reflected into the eyes of the observer. The other components of the light are absorbed.

This bush appears green because the green component is reflected into the eyes of the observer. The other components of the light are absorbed.

An optical filter only allows certain colours of light to pass. Other colours are absorbed by the filter. For example, tinted glasses.

A filter made of a primary colour only allows that colour to pass. red filter green filter blue filter

A filter made of a secondary colour transmits the primary colours that make up that secondary colour. cyan filter magenta filter yellow filter

colourless filter black filter What about a colourless filter or a black filter?

Which colour would our observer see if he looks at the bush through a red filter? Click on the bush to check your answer!

The bush would appear black because the green component reflected by the bush would be blocked (absorbed) by the red filter. The filter can only transmit red light but the bush does not reflect any red light so no light would reach the observer’s eyes (absence of light = black).

Which colour should the filter be so that the observer sees the bush as green ? Click on the lenses to check your answer !

Green since a green filter would allow the green light reflected by the bush to pass through. The green light would then arrive in the eyes of the observer!