By Talar Hagopian and Rima DebsÉcole la Dauversière, Montreal, June 2001

Content validation and linguistic revision : Karine LefebvreTranslated from French to English by Nigel Ward

Science animée, 2001

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

The primary colours are different in art (paint) and The primary colours are different in art (paint) and in science (light). That’s because in science we are in science (light). That’s because in science we are mainly interested in mainly interested in addingadding coloured lights together coloured lights together but paint works by but paint works by absorbingabsorbing (subtracting) colours. (subtracting) colours.

For paint the primary colours are: For paint the primary colours are: redred, , yellowyellow and and blueblue, , ……

……while for light addition they are: while for light addition they are: redred, , greengreen and and blueblue..

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

… … green...green...

……but by mixing yellow* light with blue light we get…but by mixing yellow* light with blue light we get…

… … white light!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.

By shining red, green and blue light beams onto a white screen and making them overlap, we obtain white 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…Let’s concentrate on the colours of light…

Here are the secondary colours and how they are formed…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 filterred 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.Longer wavelengths correspond to the colour red.

As the wavelength decreases the light As the wavelength decreases the light becomes becomes orangeorange then then yellowyellow then then greengreen then then indigoindigo then then violetviolet. .

Since we are mainly interested in the primary Since we are mainly interested in the primary colours colours redred, , greengreen and and blueblue we can say long we can say long wavelengths correspond to wavelengths correspond to redred, medium to , medium to greengreen and short to and short to blueblue..

Light consists of Light consists of electromagnetic waveselectromagnetic waves with with various various wavelengthswavelengths. Wavelengths of light are . Wavelengths of light are usually measured in usually measured in nanometresnanometres (nm). (nm).

Electromagnetic Electromagnetic SpectrumSpectrum

X raysX rays

Gamma (Gamma () rays) raysRadio wavesRadio waves

Microwaves

UltravioletInfrared

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.

(1642-1727)

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

prismprism

spectrumspectrum

whitewhitelightlight

DispersionDispersion

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

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

optic optic nerve

retinaretina

lightlight

cones and cones and rodsrods

The The rod cellsrod cells are sensitive only to are sensitive only to shades of grayshades of gray but function even in dim light. There are about 120 but function even in dim light. There are about 120 million of these detectors in the retina.million of these detectors in the retina.

The The cone cellscone cells detect detect colourcolour but don’t work well but don’t work well in dim light. We have only about 7 million cone cells in dim light. We have only about 7 million cone cells in the retina.in the retina.

Colour blindness is an anomaly of vision. People Colour blindness is an anomaly of vision. People suffering from this condition cannot distinguish certain colours suffering from this condition cannot distinguish certain colours from one another. For example, someone suffering from red-from one another. For example, someone suffering from red-green colour blindness cannot distinguish red and green. Why green colour blindness cannot distinguish red and green. Why would this be a great problem when that person drives a car?would this be a great problem when that person drives a car?

This visual dysfunction can be hereditary, or a This visual dysfunction can be hereditary, or a consequence of a disease that affects the optic nerve.consequence of a disease that affects the optic nerve.

Technically, colour blindness is due to a poor Technically, colour blindness is due to a poor functioning or an insensitivity to colour of the light-functioning or an insensitivity to colour of the light-sensitive cells, making the brain unable to recognise the sensitive cells, making the brain unable to recognise the colour correctly. colour correctly.

There are several types of colour blindness including There are several types of colour blindness including "red-green", which affects men more than women, and "red-green", which affects men more than women, and “yellow-blue", less common, which affects men and women “yellow-blue", less common, which affects men and women equally. Certain people can see only two colours, and equally. Certain people can see only two colours, and everything else looks gray.everything else looks gray.

Certain people suffer from ‘mono-chromatism’ which Certain people suffer from ‘mono-chromatism’ which means they see no colour at all, only shades of gray. 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. An optical filter only allows certain colours of light to pass. Other colours are absorbed by the filter.Other colours are absorbed by the filter.

For example, tinted glasses.For example, tinted glasses.

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

red filter

green filter

blue filter

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

cyan filter magenta filter

yellow filter

colourless filter black filter

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