Colour

1. click here to begin Colour 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

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

6. Put some colour in your life!

7. Table of contents Light and colours Wavelengths and colours How the eye sees colours Rainbows Games Conclusion and bibliography

8. Light and colour 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.

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

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

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

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

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

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

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

16. red filter cyan filter Optical filters 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).

17. 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.

18. 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. blue red green

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

20. Electromagnetic Spectrum 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. Gamma () rays Radio waves X rays Microwaves Ultraviolet Infrared Visible light waves of various colours

21. A bit of history... In 1873, James Maxwell proved that electromagnetic waves consist of a combination of an electric wave (an oscillating electric field) and a magnetic wave (an oscillating magnetic field).

22. At about the same time the German physicist Heinrich Hertz, with the help of Maxwell, managed to produce radio waves and showed that they have all the properties of light: reflection, refraction, interference (superposition of waves), diffraction, polarisation and speed ( 300 000 km/s).

23. But well before them, Isaac Newton had attributed wave properties to the particles of light. What’s more, he had discovered that white light consists of all the colours of the rainbow combined together. (1642-1727)

24. 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. prism whitelight spectrum Dispersion

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

26. 3600- 4300 4300- 4550 4550- 4920 4920- 5500 5500- 5880 5880- 6470 6470- 7600 Wavelengths of the colours of the visible spectrum violet indigo blue green yellow Measurements are in angströms. One angström is one billionth of a metre. orange red

27. How the eye perceives colours

28. light optic nerve retina cones and rods

29. 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.

30. Colour blindness 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.

31. 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.

32. 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.

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

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

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

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

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

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

39. 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).

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

41. 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!

42. Rainbows A rainbow is sometimes produced when sunshine interacts with falling rain. • In order to see a rainbow, the sun must be behind you. • Sunlight hitting rain does not always produce a rainbow. In order for the raindrops to be able to form a rainbow they must be between 1 and 2 mm in diameter.

43. Ray of sunlight raindrop reflection refraction Dispersion This diagram shows how a ray of sunlight is dispersed into a spectrum of colours as it passes through a raindrop.

44. The flag game Stare hard at the red dot for 15 seconds then look at the white space. 0 15 10 11 14 13 2 12 3 4 5 8 9 1 6 7 You will notice that the colours of the ‘phantom’ image of the flag are the same that those of the real USA flag. We see that because red, blue and white are respectively the complementary colours of cyan, yellow and black.

45. The word game bluewhite red gray green Mauve redOrangeyellow TurquoisePinkblack Say out loud the colours of these words – do NOT read the words themselves.

46. Conclusion Colours are part of our daily life … Life would be pretty dull without them! Luckily we find them everywhere,even in science!

47. Bibliography Internet sites • Beverly T. Lynds. (Page consulted 05 March 2001). About rainbows, [online]. :http://www.unidata.ucar.edu/staff/blynds/rnbw.html • H. Jaegle and L. T. Sharpe. (Page consulted 15 November 2000). Colour and night vision, [online]. : http://www.eye.medizin.uni-tuebingen.de/ • the University of Texas, Houston. (Page consulted 15 November 2000). Colour vision, [online]. : http://eye.med.uth.tmc.edu/MasseyLab/color%20vision/colorvision.htm

48. Books • M. PARAMON, José. Le grand livre de la couleur, Italy, Angela Berenuer Gran, 1993, 160 p. • CHABOUD, René. La météo question de temps, France, Nathan, 1993, 286 p. CD-ROM «Colour Blindness».Microsoft Encarta Encyclopaedia 2000 [CD-ROM]. Microsoft Corporation, 1999.