Buxton & District U3A Science Discussion Group

“What is Yellow?”

John Estruch16 January 2015

Buxton & District

Science DiscussionWhat is yellow?

A colourCowardice

A canaryA banana

Light of wavelength 560-600nm

A primary colour A mixture

of red and green light

A secondary (or complimentary colour)

GreyBlue

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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?

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Science DiscussionLight & colour

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Science Discussion

Hot objects give off light of various colours

The colours and amount of each make up the colour spectrum of visible sunlight.

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Science Discussion

The spectrum depends on temperature

Black body radiation

Colour temperature

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Science Discussion

Chemical(ish) reactions give off lots of light of same wavelength (colour)

Neon Lights

Sodium lamps

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Science DiscussionThat’s all wrong John…

All colours are made by mixing Red, Green and Blue light (TV, colour projector)

Primary colours

Secondary (or complimentary) colours

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Science Discussion

Make more colours by mixing different amounts of RGB

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Science DiscussionWrong again John….

All colours are made by mixing Yellow, Cyan and Magenta pigments (mixing paint, ink-jet printers)

Primary colours

Secondary (or complimentary) colours

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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)

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Science DiscussionPigments (subtractive system)

Green absorbedsee blue+red

Blue absorbedsee green+red

Red absorbedsee blue+green

Red & blueabsorbedsee green

Green & blueabsorbedsee red

Red & green absorbed see blue

Red, green & blueabsorbedlooks black

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Science DiscussionHow 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).

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Science DiscussionHow do we see yellow.

Yellow light Green light + red light

Triggers M & L cones.

We see yellow

Triggers M & L cones.

We see yellow

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Science DiscussionHow do we see cyan.

Cyan light Green 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

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Science DiscussionHow do we see magenta?

???? light Green 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

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Science DiscussionHow 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

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Science Discussion

But the brain can be tricked in to overcompensating.

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Science DiscussionQuestions?