Colour of light

[ll_billym]

How does light hold colour information, i.e. how is there red light, green light, etc.  I understand that coloured light is just an absence of all other colours from white light but how is this done.  I suppose this question is about the nature of light.

[Space]

light can be seen as waves.
Each frequency is a color. (red for low frequencies, violet for high) and your eye roughly detects which frequencies the light is made of.
White light is the sum of several waves.
If the C musical note was red, E was green and G was blue, then the chord made with these 3 notes together would be white.

[OBonio]

Colour information is stored in our brains. A colour is simply an interpretation of particular wavelengths or combination thereof. Some animals only see in black and white. Some people have trouble with certain colours too. So, light has a wavelength or combination of wavelengths then then brain interprets this into a colour.

[fo3nix]

And to expand on Space's answer, the frequency of light is really just its speed or energy. Red light is made up of stuff that has less energy than orange light, or blue light.

Why, when looking at an orange, is it the colour orange? Simple! The light comes from outside, as natural light, hits the orange, and this hit causes it to change its frequency or energy to that or orange light. When the light gets to your eye, your brain knows how to interpret this information, and hence you see it the colour orange!

Same goes with the sky. Why is it blue, when the light from the sun is a brilliant white? Because it gets to the atmosphere of the earth, which is full of all sorts of stuff, and these particles just happen to make the light turn into blue light.

When the Sun is lower, just before it sets, it has more stuff to get through to reach you. So like you running through water, this extra stuff will slow it down. And when blue light slows down, it changes to some other colour... in our case it happens to change to orange or red!

[DocRob]

It's correct that the colours we perceive are dependent on the frequency of light. That frequency is the rate of oscillation, around 100 million million oscillations per second. That's a different thing than the speed at which light travels, about 300 million metres per second.

It's a bit subtle, though: humans (most of them) have three colour receptors, pigments in certain cells in the retina, at the back of the eye. All we can 'see' is the recipe for how much each receptor is stimulated, the ratio of signals R:G:B from our red, green and blue receptors. These are sometimes called 'additive' primary colours.

So when we see 'yellow', it can be a single frequency of pure light that excites some red and some green reception in our eyes. Or it can be light not at the single yellow frequency, but red light and also green light from two sources -- either will cause the same stimulation in our eyes, and the brain will 'read' the same signals.

So, for colour television, we don't need all the colours of the rainbow, teased out by a prism from white light: red, green and blue phosphors (or LEDs or lasers) are enough to simulate what we can see.

Inkjet printers use 'subtractive primary' colours: cyan, magenta and yellow. Yellow can be made up of red and green light,, no blue, so yellow ink absorbs -- or subtracts -- blue light. Magenta is red and blue, not green (white subtract green), and cyan is blue and green, not red (white subtract red). So inks, like paint, compose their colours by figuring out what does NOT belong. Just as the answer about the colour of a orange fruit, the 'inks' (pigments) in the fruit soak up all the colours in white light that are NOT orange.

Try this: shine a bright light on an orange, and a piece of white paper near to it, and shadowed from the light. You'll see the colour of the light that comes off the orange...