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Light, sorted

Where Do Colours Come From?

White light is every colour mixed together. Objects keep some and throw the rest at you.

1 White light —all colours2 Most coloursabsorbed3 Red bounces toyou
A red apple is red because it rejects red.

  1. Step 1 of 5

    White light is a mixture.

    1 White light in2 Bent bydifferentamounts3 All the coloursout

    Sunlight looks colourless but contains every colour at once. A prism spreads it out and proves it — red, orange, yellow, green, blue, violet.

  2. Step 2 of 5

    Colour is a wavelength.

    Long waves Red — about 700nanometresShort waves Violet — about 400nanometres

    Light travels as waves. Long waves look red to us, short waves look violet, and everything between fills in the rest of the rainbow.

  3. Step 3 of 5

    Materials absorb some wavelengths and bounce others.

    A green leaf Absorbs red and blue,rejects green

    The atoms in a leaf soak up red and blue light, turning it into heat. Green is not absorbed, so it bounces off — and that is what reaches you.

  4. Step 4 of 5

    So an object's colour is what it rejects.

    What we assume The apple is full ofredWhat happens Red is the only colourit throws away

    This is the strange bit: a red apple absorbs every colour except red. The one colour it will not keep is the one you name it after.

  5. Step 5 of 5

    Black and white are the extremes.

    White Bounces almosteverything backBlack Absorbs almosteverything as heat

    Something white bounces back nearly all wavelengths. Something black absorbs nearly all of them — which is why black clothes get hot in the sun.

    Colour also depends on the light. Under a red lamp, a blue jumper has no blue to bounce, so it looks black.

The short version

An object's colour is the slice of white light it refuses to absorb and bounces back at you.

Try it yourself

Look at coloured clothes under an orange street lamp. Most colours vanish, because the lamp is not offering them.

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