Hey! Great question. Colour is all really a ‘trick of the eye’. In or eyes there are millions of receptors, which ‘receive; all of the light coming into the eye. There two types of receptors, called ‘rods’ and ‘cones’. The rods detect blacks and white, and let us see in dark rooms. The cones are respnsible for differentiating different colours.
We need to start off by thinking a bit about light. All different kinds of light travel at one speed (299,792,458 metres per second). All the kinds of light form something called the ‘electromagnetic’ (electricity + magnetism) spectrum. We like to make things as easy as possible in physics, so we split the light into different groups according to their energy. This includes things like gamma rays (a type of radiation), radio waves (how we get music in the car!) and microwaves! There’s only one little part of the spectrum called ‘visible light’, and that’s where all the colours of the rainbow sit. There is low energy ‘visible light’ which is red. He is right next to ‘infrared’- the same kind of light that sends signals to your TV to tell it to change channel. At the high energy side there is blue light, then ‘ultraviolet light’, which is the bright bright light from the sun. Just above that are X-Rays, which are so energetic they can break into your body!
When light hits something like a red apple, the little molecules inside the apple start to wiggle around and get excited. Some of the light is absorbed by the apple, and used to heat the apple up. Some of the light is reflected by the apple, and comes into our eyes. Because it isn’t all of the light that went into the apple in the first place, it has a bit of a lower energy. This is why the light appears to be red! But the meaning of the word ‘red’ varies from person to person, because of the different cone receptors in our eyes. The cone receptors all respond differently to different energies of light- some are great at seeing the low energies, some great at the high energy blue light. Some people have more of the red receptors than the blue. Also, we learn about colours just from words- my mum told me the sky was blue, and so the colour I see the sky is what I have learnt is blue. But if she’d actually told me the sky was pink, I would have believed it! We have come up with the names for colours because that is what we all agree on.