What is the colour of things? And, if we are in a completely dark room, what is their colour? Let’s change the situation. We are in a completely dark room and there are a number of objects we have not ever seen, what is their colour? Do they have any? The answer to the first question is difficult because if we have not ever seen an object, the only thing we can do is imagine a colour. The second answer can be a bit philosophical, however in my opinion it has a colour which in fact is the same of the rest of the objects in the room: black. If they have another colour I’m not able to say it. But I don’t want to talk about Philosophy but Physics, concretely about the interaction between radiation and matter.
Anything we see, touch, breath… is made of atoms, and atoms are made of a nucleus, protons and neutrons, and an outer shell of electrons (here I talk a bit more about atoms). Electrons are the ones in charge of giving things their characteristics colours. But they can’t do it on their own and need the energy support provided by photons, that is, the light.
In 1911, Rutherford published his atomic model where he proposed that electrons were orbiting the nucleus in a similar way as planets do around the sun. However, the problem was that when electrons orbit in this way they emit radiation and thus they lose energy until they fall to the nucleus. The atom would not be stable in this case. In 1913, Niels Bohr took this idea together with the quantum hypothesis made by Max Planck and proposed that electrons would orbit the nucleus in circular orbits, which is the content of his first postulate of his model, but not all the orbits are allowed, electrons only can orbit in specific quantized orbits. This is known as the second Bohr’s atomic model postulate that says that not every orbit for the electron is allowed, only those whose radius is such that the angular momentum of the electron is n times h/2π, being n an integer and h the Planck constant. In these orbits the electron would not emit radiation and the atom would be stable.
But then, is the electron always in the same orbit, in the same manner planets are always in their orbit? For a planet to be in its orbit, it needs that there is not any perturbation that gives the planet energy and pushes it out of its orbit, as it could be the case of a meteorite. And even so, there could be meteorites without enough energy to take the planet out of its orbit. In the case of electrons, it happens the same, when there is not a perturbation with enough energy to make the electron jump to another orbit. What is the kind of perturbation that can make the electron jump to another orbit? Here it is the relation with the colour of things. This perturbation is the light, more concretely the photons of light. Photons have a specific energy that depends on its wavelength, or in other words, the colour of light. If light has little energy, its wavelength would be close to the red colour and if it has much more energy, its wavelength would be close to the blue colour.
When a photon collides with an electron in its stable orbit, it gives it energy to jump to another orbit. But it cannot be any orbit, it has to be one that meets the second Bohr’s postulate, that is, it has to be quantized.
The electron in the new orbit cannot remain there forever if there is not a continuous source of energy, therefore it will jump back to its initial orbit. The problem is that the initial orbit has less energy than the final one; therefore to go back it has to lose the excess of energy by emitting a new photon whose energy is the difference between the energies of the initial and final orbits. This new photon has a wavelength that depends on the energy and thus has a specific colour. It is this new photon the one that arrives to our eyes and makes us see things of a certain colour.
Where does the initial photon, that makes the electrons jump, from? For example from the light of the sun, from the light bulbs, from a fire… Because of it, we can’t see the colour of things in the darkness, because in the absence of light, ‘there is not’ photons making the electron jump of its orbit (although there are continuously photons arriving and colliding with electrons, but they don’t have enough energy to make the electron jump to an orbit that makes it emit another photon of a colour, e.g. green, that arrives to our eyes.
I have to say that I used the concept of orbit as Bohr defined it, that is, making the assumption that it is like the orbit of a planet, however the reality is always more complex and I should have talked about energy levels or even could have used a more quantum specific and rigorous terminology, but probably nobody would have read beyond the first paragraph.