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

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Presentation transcript:

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

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

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?

Buxton & District Science Discussion Light & colour

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.

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

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

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

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

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

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)

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

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

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

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

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

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

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

Buxton & District Science Discussion Questions?