Buckleymanor said:
White light is made up of the spectrum but we don't see it as a full of colour it is only when it is broken into it's constituent parts by passing the white light through a prism do we see colour.
Most light that we see is made up of a spectrum of colours. The only light we see which IS a single colour (more or less) is from a laser, from a narrow band filter, or from a monochromator (where light is split up by a prism or diffraction grating.) Even after passing light through a gelatin filter or reflecting it off coloured objects, it is generally still a mixture of many colours - just with one predominating.
White light doesn't look coloured because the different colours it contains are balanced.
All white lights are not the same. Several lights that we say are white (say from different lamps, the window, burning magnesium, ...) if we compare them side by side, we then think some a bit pinkish, some a bit bluish, others maybe still white but different. This is known as the colour temperature of white light.
Yet again you could have two white lights that do look identical (to your eye), but when analysed with a spectrometer (put through a prism) show very different spectrums (different mixtures of colours.) White light from an incandescent bulb has a spectrum that contains ALL the colours of the visible spectrum and the spectral graph is a smooth distribution. Light from a flourescent lamp or LED lamp usually contains 4 bands of colours and its spectrum looks completely different. (Try looking at different lamps with your prism, or reflected in a CD if you don't have a prism.) Manufacturers vary the amounts of each colour to produce different whites, like warm white (more red) and cold white (more blue).
From that it sounds like a spectrum of colours has colour .
So it depends on the mix of colours in the spectrum. If there is the right balance of some red light, some blue and some green you see it as white. And a mixture of just two colours around red and cyan would look white. If the mixture is deficient in blue (mainly red and green lights) it looks yellow. If you reduce the green a bit as well, it becomes orange. The yellow light could contain lots of colours in the range red - orange - yellow - green, or it could simply be a mixture of two pure colours red and green.
Any colour you see can be made up of an infinite number of different mixtures. What you see depends on the balance of all the different components. (And incidentally on other factors such as context and the recent history of your eye. *)
You might like to have a look at a CIE chromaticity diagram. It tries to show how different mixtures of light will appear to a standard eye.
only when it is broken into it's constituent parts by passing the white light through a prism do we see colour
Or when anything upsets the balance of colours which makes it look white. So reflecting white light off a coloured object upsets the balance and let's you see the colour. Or passing white light through a filter.
White light is quite invisible otherwise the night sky between the stars would not be dark.
This is puzzling. You may be talking about Olber's paradox, but probably not. I think you are saying something like, "you can't see light unless it enters your eye." I don't think anyone would disagree with that!
But why would you say that? Are you thinking that if you look at a light beam which is not going into your eye, rather across your line of view or even away from you, then you can see it? That is because as the light goes through the air, a small part of it gets scattered so that it changes direction and does come into your eye. When people are taking photographs of lasers, they sometimes blow smoke into the beam to increase this effect and make the beam more visible.
If so, then that applies to white light and to coloured light. What is different is the degree of scattering for different colours. White light (a mixture) going through the sky has more of its blue light scattered than red. So the sky looks blue because we see this blue light scattered towards us from the white(ish) sunlight going in all directions which would not normally enter our eye. At sunset when we look at the white light from the sun, it looks red (and dimmer) because so much blue has been lost in scattering.
(*) If you have a coloured filter such as a piece of red, green or blue cellophane, try holding it over one eye only for a minute, then when you remove it, look alternately through each eye. Obviously while you wear the filter, things will look different colours through each eye. But when you remove it, things will still look different through the two eyes for a few seconds. This is because you have changed the balance of sensitivity to different colours in your eye. They return to normal soon when thet are both exposed to similar light again.
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