I’ve learnt in physics that objects are different colours becsuse of the light they refract that then enters our eyes. For example, a red ball is red because it absorbs orange, yellow, green, blue, indigo and violet light, but refracts red light into our eyes.
But my question is: what exactly makes this ball only refract red light? What is it about red light that means it’s the only light that can be refracted from a red ball? What are differences between the chemical properties of a red ball and a blue ball, which are both made from the same material, but refract different light?
We’re also taught that chlorophyll turns leaves green, as it makes it so only green light is refracted. But why green light in particular?
In: 4
The ultimate cause of color has to do with the available energy levels in the atoms of the material.
In an atom in isolation, electrons can take on only very specific levels of energy. There’s no in between. So if, for example, two allowed energy levels happen at 100 units and 200 units, the electrons can’t absorb a photon with energy 75. (More properly, they’re much less likely to, because the energy of a photon isn’t definite. A photon with energy 75 *could*, in principle, “actually” have energy 100, it’s just very unlikely that it does.) That makes pure materials made of isolated atoms able to interact only with a very specific set of wavelengths, forming thin likes called an [emission or absorption spectrum](https://en.wikipedia.org/wiki/Emission_spectrum).
In a real material, the atoms pile up, enough to somewhat mix those energy levels around. That makes the bands wider and makes more of them, and if there’s enough bands, they can occupy a large portion of the available wavelengths to one degree or another. The ways those bands pile up – and some of the physical shape of the material’s surfacel that dictates how a photon bounces around near the surface – determine the material’s color.
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