The world around us is filled with objects that have different colors. Usually, the same type of objects has common color too; for example, when I say tree the image in our minds is directly a tree with a brown trunk and green leaves. If I say apple, the object in your mind is obviously red, I hope. This is such highly consistent that we even name objects after their color: orange!
But when we ask the question “What is a color?” are we able to get a direct answer? We know that we see objects when the photons (light particles) from that object is received by our eyes. So can we say that each material creates their own type of photon?
Let’s try to answer these questions by understanding the light first. If we use an analogy to understand it, we can think of light as electricity. So, if light was electricity, then photons would be electrons. In basic terms, electrons are the charge carriers in a circuit just like photons are the light-carriers (in a way). This tells us that to understand the light, we should investigate photons.
Photons are massless particles that carry energy to wherever they go. You may think “Do they carry energy OR light?” but the answer is both. The thing about light is that, it is not actually what we think it is. What comes up to your mind when we say light? If the answer has anything to do with colors or seeing, then you think about only the visible light. Light actually is made up of any photon, but the visible light only constitutes to photons with a wavelength between 400 and 700 nm. The electromagnetic spectrum shows us all different types of photons that we can and can’t see. Here you can see the EM spectrum:
Alright, now that we understand the light better let’s start considering colour again. In the EM spectrum image right above, we can see that different colours have different wavelengths. This means that if a photon has 500 nm of wavelength, then it is visualised as blue in our mind. This brings the question “what makes the light change its wavelength?” The answer to this question is actually a bit of chemistry and a bit of physics mixed: atoms and molecules have different energy states. When a photon collides with these atoms or molecules, the photon causes them to get excited. But not excited as if they were on a date or something, but instead the photon makes the atom jump to a higher energy state than its original state. We call the original state as the ground state, and the higher one is called the excited state. Now remember, the photon was carrying energy and that is how it “excites” the particle. But this excited state of the particle is unfortunately not stable, so after maybe 1 microsecond or less the particle will drop back to the ground state, meanwhile releasing a photon. Now this released photon is the photon that will reach our eye and make us “see” this object. The question now is “How are different photons formed, if the same interaction occurs in all particles?”
It is true that the same interaction between particle and light is taking place each and every time. But, different elements and compounds have different energy levels, resulting in different energy differences between the ground state and the excited state. As a result of this even when the same photon collides with different matter, the released photons will not be the same as they will have different excited states, giving birth to different wavelengths which means different colours for our eyes.
Let me conclude by asking a question, is colour there when we switch the lights off but we can't see it, or is it not present at all..?
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