What is Colour?

When we were looking at sound, we saw that there were different pitches of it: deep sounds and high sounds, and some were too deep to hear (the infrasound of elephants), while others were too high (the ultrasound of bats). The different pitches are caused by different rates of vibration, with high frequency vibrations making high sounds, and low frequency vibrations making deep sounds. Light is very similar. Blue light is like a high sound: waves of blue light have a high frequency rate of vibration. Red light is like a low sound: waves of red light have a low vibration rate. Green light is somewhere in between the two. Infra-red light is the equivalent of infra-sound: its frequency is too low for it to be visible to our eyes, though we can feel its heat on our skin. Infra-red light is used in remote controls for such things as televisions: if you want to see that it really is a kind of light, just look at a remote control through a digital camera and press one of the buttons on the remote (choose which one with care so as not to affect what the television is doing at the time). The sensors in digital and video cameras usually misidentify some kinds of infra-red light as blue light. At the opposite extreme, Ultra-violet light is the equivalent of ultrasound: its frequency is too high for it to be visible to us, though many birds can see it, and some birds which look black to us actually look bright ultra-violet to their eyes. Ultra-violet light (or ultra-violent light, as I like to call it) is very energetic and can damage your skin and your eyes if they are exposed to too much of it: that is why it is a good idea to wear sunglasses and to use high-factor suncream on sunny days. You must be careful when you buy sunglasses to check that they filter out all the ultra-violet light, because if they don't they may do more damage to your eyes than wearing no sunglasses at all: the dark lenses will make your eyes open up more, but they may not be blocking any of the ultra-violet light at all.
If you play two different notes at the same time on a piano, you will still hear them as two distinct notes, but the way we see colour works quite differently, because two colours can combine to make you see a third one instead: if you shine green light at a white screen, the screen will look green; if you shine red light at a white screen, the screen will look red; but if you shine both a green and a red light at a white screen, the screen will look yellow. The red and green colours are the real colours present in the physical world, but our brains make us see them as yellow when they occur together instead of seeing the red and green. This leads to an interesting question: is there such a thing as pure yellow light, or is it always just a mixture of red and green? The answer is yes: there is a band of pure yellow in rainbows which doesn't contain any red or green light, but it looks exactly the same to our eyes as yellow made up entirely out of red and green light where there is no real yellow in it whatsoever! We simply can't see the difference with our eyes, and nor can our digital cameras or film, because they are designed to capture images which show the world the same way as our eyes see it. This leads to another interesting question: are there any yellow objects which only reflect pure yellow light rather than reflecting red and green light as well? If there are, they should normally look brown because they will reflect far less light than ordinary yellow objects when viewed under normal lighting, but if you light them up with pure yellow light (which you can isolate from sunlight using a prism), they could suddenly look bright yellow in that light. I haven't found such a pure-yellow object yet, but they may be out there. The same applies to orange and purple objects, because these too can either be pure colours or combinations of other colours entirely.
When red, green and blue lights are shone at a screen, the screen will look white. This happens because we have three kinds of colour sensors in our eyes: one type for red, another type for green, and a third for blue. If all three kinds of sensor detect the same amount of light hitting them in the same part of the retina (the bit inside the eye where the light is detected), then you will see that light as white (or gray if there is less light). White is most certainly not a pure colour, and it never appears in a rainbow or spectrum. If a red and green sensor are detecting light while the blue sensor is not, then the light will look yellow. If a green and blue sensor are detecting light while the red sensor is not, then the light will be seen as cyan. If the red and blue sensors detect light but the green one doesn't, then the light will look purple, although an equal mix of red and blue will technically give you magenta. If you want to play with colour mixing using light, click here. Try using the values red=255, green=128 and blue=0 to make a bright orange, or red=128, green=0 and blue=255 to make violet. There are animals (some sea creatures in particular) which have as many as eight different kinds of colour sensors in their eyes, so they can see radical differences between colours which look absolutely identical to our eyes. If you are keen to know more about this business of colours which look identical actually being quite different, the term you need to research is "metamer".
The three primary colours when working with light are red, green and blue: any colour we can see can be made out of just these three colours. When working with paints, however, it's a different story. Until recently, I thought the three primary colours for paints were red, yellow and blue, but it turns out that this is a myth. The primary colours for paints are actually magenta, yellow and cyan. Why should this be? Well, it's because paint absorbs light rather than emitting it, so when you mix two colours, more light will be absorbed and less reflected. Magenta, yellow and cyan reflect twice as much light as red, green and blue paint, and when you think about why this should be so, it's obvious: magenta paint absorbs green light while reflecting both blue and red; yellow paint absorbs blue light while reflecting both red and green; and cyan paint absorbs red light while reflecting both blue and green. If you mix magenta paint with yellow paint, the magenta paint will absorb any green light that falls on it while the yellow paint will absorb any blue, but neither will absorb any red, so all the red is reflected: mixing magenta with yellow gives you red paint. In the same way, mixing yellow paint with cyan paint will make green paint because all the blue and red light will be absorbed, and mixing cyan paint with magenta paint will make blue paint because all the red and green light will be absorbed.
Red and green paint become brown when mixed together; not yellow like when you mix beams of red and green light. This happens because red paint absorbs green and blue light, while green paint absorbs blue and red light: the result is that all the light should be absorbed and you should end up with black paint, but paints are never perfect at absorbing all the light you expect them to, so green paint doesn't quite absorb all the red, and red paint doesn't quite absorb all the green: the little red and green that is reflected combines to make yellow, but there is so little of it that it looks brown instead. Both the green and red paint in this mix will absorb blue, so the blue is much more heavily absorbed than either the red or green, and that explains why the result should be brown rather than gray.
If you mix blue and yellow light, you get white light (because all three kinds of sensors in the eye will see light from this), but when you mix blue and yellow paint in equal quantities you get a dark gray colour (again it fails to make black because paints don't quite absorb all the light you expect them to, so some blue light escapes being absorbed by the yellow paint and some red and green light excapes being absorbed by the blue paint). If, however, you mix a little blue into yellow, you can get green, but it will never be a bright green if the blue is a pure blue: you can only make dingy greens by mixing pure blue into yellow. Cyan paint, on the other hand, when mixed with yellow will make bright green paint because cyan paint doesn't absorb green (whereas blue paint does). It's easy to think that blue and yellow paint make green when mixed, but this only happens when the blue isn't pure blue and is actually closer to cyan.
You might like to try this experiment with your eyes. Stare at a coloured object for a minute, and then look at a white sheet of paper. You may continue to see the shape of the object you were looking at for a little while against the paper, but in a very different colour. If the object was red, you will now see cyan; if it was green, you will see magenta; and if it was blue, you will see yellow. THis happens because if the object is red, the red sensors in your eye are seeing light while the blue and green sensors aren't, so the red sensors temporarily become a little less sensitive to light, while the green and blue sensors become temporarily more sensative to light, and the result is that when you look at the white paper, the green and blue sensors exagerate the amount of light they are seeing while the red sensors underplay it, the result being that you see cyan. After a few seconds, the sensors get back to normal and you just see white again. If you try the same thing with objects which are magenta, yellow or cyan, when you look at the white paper you will see green. blue and red: in these cases you are causing two types of sensor to become less sensitive while remaining sensor becomes more sensitive.

Some people claim that white, gray and black are not real colours: white and gray being a mixture of red, green and blue, while black is just a complete lack of light. In a way they are right, but at the same time they are very wrong: colours in the world outside of our minds are not really out there at all, because all you have in the physical world is a whole lot of different vibration frequencies of light waves which are zipping about in a colourless world. The colours that we see in our heads are not really out there in the world in front of our eyes at all: they are actually states of our own consciousness, living within our minds. They are a part of ourselves, and black is no less real than blue, while white is probably no less pure than red. An alien species might see red as a low-pitched sound and see blue as a high-pitched sound rather than seeing the colours which we see, and he may also hear sounds as colours. Nobody knows. All the different sensations which we have are actually aspects of our own conscious minds and we merely map these feelings to different phenomena in the world outside, such as light frequencies and the rate at which pressure waves hit our ears: the colours and sounds that we see and hear in our minds are not out there in the external world at all, but are part of our own consciousness. It is not even clear whether we all see frequencies of light as the same colours as other people: it may be that all the things that look red to me look blue to you, though you will also call them red and they will not look at all strange to you because they have always looked that way to you.
Radio waves and microwaves are actually just a very low-energy kind of invisible light which can be produced by moving electrons about in wires, and they can be detected by the way they make electrons move about in other wires. Most people find radio waves rather puzzling, but they make sense as soon as you start to think of them as being light. To send signals using radio waves, a transmitter puts out a base frequency (which simply means that it switches on an invisible radio "light" of a particular colour). That base signal is then modified either by making slight changes to the frequency (changing the colour) or by changing the amplitude of the waves (the brightness), and these changes in colour or brightness can then be converted into different sounds by gadgets called radios. We could actually transmit radio programmes using visible light if we wanted to, and then we would see lights flashing from transmitters, changing in colour or brightness very rapidly from moment to moment, but radio waves are able to pass through walls better than light, so they are more convenient, added to which, the sun puts out a lot of light that would interrupt the transmission during the day, but it doesn't produce a lot of radio waves, so that is why we stick to using radio waves for television and radio transmissions, although we do use light to do the job if we send signals through fibre-optic cables. Now that you understand radio to be nothing more exotic than invisible light, why not explain it all to your parents and impress them!

Try to remember the following points:-

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