1. Buxton & District U3A Science Discussion Group “What is Yellow?” John Estruch 16 January 2015

2. Buxton & District Science Discussion What is yellow? A colour Cowardice A canary A banana Light of wavelength 560-600nm A primary colour A mixture of red and green light A secondary (or complimentary colour) Grey Blue

3. Buxton & District Science Discussion What are we going to talk about? What is colour? How do we represent it? How do we perceive it? Answer a more difficult question: What is magenta?

4. Buxton & District Science Discussion Light & colour

5. Buxton & District Science Discussion Hot objects give off light of various colours The colours and amount of each make up the colour spectrum of visible sunlight.

6. Buxton & District Science Discussion The spectrum depends on temperature Black body radiation Colour temperature

7. Buxton & District Science Discussion Chemical(ish) reactions give off lots of light of same wavelength (colour) Neon Lights Sodium lamps

8. Buxton & District Science Discussion That’s all wrong John… All colours are made by mixing Red, Green and Blue light (TV, colour projector) Primary colours Secondary (or complimentary) colours

9. Buxton & District Science Discussion Make more colours by mixing different amounts of RGB

10. Buxton & District Science Discussion Wrong again John…. All colours are made by mixing Yellow, Cyan and Magenta pigments (mixing paint, ink-jet printers) Primary colours Secondary (or complimentary) colours

11. Buxton & District Science Discussion What’s happening with pigments? V V V V Cyan pigment absorbs Red light Yellow pigment absorbs Blue light Magenta pigment absorbs Green light We see the mixture of Blue + Green We see the mixture of Red + Green We see the mixture of Red + Blue White light (mixture of colours)

12. Buxton & District Science Discussion Pigments (subtractive system) Green absorbed see blue+red Blue absorbed see green+red Red absorbed see blue+green Red & blue absorbed see green Green & blue absorbed see red Red & green absorbed see blue Red, green & blue absorbed looks black

13. Buxton & District Science Discussion How do we see colour The retina has colour sensitive cells called “Cones” “S” cones are sensitive to 400- 500nm wavelengths (blues). “M” cones are sensitive to 450- 600nm wavelengths (green- yellow). “L” cones are sensitive to 500- 650nm wavelengths (yellow- red).

14. Buxton & District Science Discussion How do we see yellow. Yellow lightGreen light + red light Triggers M & L cones. We see yellow Triggers M & L cones. We see yellow

15. Buxton & District Science Discussion How do we see cyan. Cyan lightGreen light + blue light Triggers S & M cones (and a bit of L). We see cyan Triggers S & M cones ( and a bit of L). We see cyan

16. Buxton & District Science Discussion How do we see magenta? ???? lightGreen light + red light There is no colour in the visible spectrum that triggers S & L cones but not M cones (and hence looks to us like magenta) Triggers S & M cones. We see Magenta

17. Buxton & District Science Discussion How do we perceive colour? Banana on a white table (honestly!). In white light. Use a slightly redder light (sunset, incandescent lamp) Cones detect relative amounts of S, M & L. Brain figures it is yellow banana on white table Cones detect more M & L. Eye is sending image of slightly orange banana on slightly pink table. Brain interprets information & decides it is shaded red Brain figures it is yellow banana on white table

18. Buxton & District Science Discussion But the brain can be tricked in to overcompensating.

19. Buxton & District Science Discussion Questions?