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?
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Think of any object like a filter. The gels on stage lights for example. They filter out all but the desired color. The dyes and pigments that make up everything we see are just filters.
>what exactly makes this ball only refract red light?
It’s reflecting the light, not refracting, which is a separate process. Refraction is when light passes from one medium to another (say, air into glass, or air into water).
At an atomic level, reflection happens by an electron interacting with a photon. It will absorb the photon, and then re-emit it. If the electrons in a material are organized the right way*, it will absorb all of the incoming photons, but will only re-emit some of a specific wavelength (and therefore color). How these electrons are organized is what determines the color the object will end up being.
*Electrons are arranged in levels according to their energy. There are “gaps” in these levels where electrons cannot exist. It will either absorb enough energy that it can jump to the next one, lose enough that it falls to the one below, or not quite absorb/lose enough to change at all. These jumps between levels correspond directly to the wavelength of light emitted. If the energy an electron gains from absorbing a photon matches the energy required to jump to a specific level (say, equal to the energy of a green photon), the object will look red. It has absorbed all the “green” energy, and the complimentary color is emitted. Different materials will have different sized gaps, which will determine which colors get absorbed, and if the energies work out perfectly then you get specific colors.
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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