I understand that light of different wavelengths reflected is what gives off color but what determines which wavelengths, a table, for example, will reflect or absorb? Is it something encoded in the table itself or is it the light that communicates the color?
Why do some things absorb all wavelengths turning them black while others reflect all turning them white?
In: 7
Not a chemist, but what I understand is that it comes from a couple of things:
1. It could be the composition of the material itself, ie what molecules it’s made of. The simplest way to think about this might be food coloring. Each of the differently colored food colorings uses certain different chemicals (which each reflect or absorb certain colors of light) to get that color. The same can be said for flavorings as well. Instead of coloring, certain chemicals produce unique flavors due to their structures. For example the banana flavoring that you taste in a lot of candies comes from a chemical called isoamyl acetate. Certain chemicals added to different mixtures give off specific properties, one of those being color.
2. It may also be structural in nature. There are certain insect or birds for example that have structural coloring. The way the microstructure of their shells or feathers is designed creates a way for light to be reflected differently. Imagine if you had (in 2D) a row of triangles (on the hundreds of nanometers scale, similar to the wavelengths of light) lined up next to each other and light was shining in on them from the top left. White light is made up of all color lights which all have different wavelengths corresponding to their energy. As the different colors of light hit those triangles at an angle, only the colors of light with wavelengths corresponding to the spacing of the triangle (say ~500nm or so for green light) will be reflected. All the other colors will be redirected away and won’t be reflected back. This is not a very common way of coloring things, and is mostly found in the natural world on living things.
3. There’s also a few other ways that color can be described such as with different metals giving off different colors (eg silver, gold, copper, etc.) but I don’t know too much about that. I believe it has something to do with the electronic structure of the atom itself, or how many electrons the atom has in its outermost shell. It may also have to do with the crystal lattice structure (how the atoms are connected inside the material) of the material. Might need a materials scientist to better explain that one.
Electrical engineer here, I’ve studied the physics of light a decent amount in undergrad.
So a lot of what is happening is the molecular, chemical, and atomic structure of the material.
So you mentioned a table. Picture a table made of wood. The type of wood will have its own color that can range from a light, warm tan/yellow to browns to greys (and more). Wood is mostly a lot of carbon, hydrogen, and oxygen atoms (with perhaps some others sprinkled in such as nitrogen) and harder woods – oak, walnut – will have a slightly different chemical make-up and the packing of the molecules are denser than softer woods – e.g. pine.
Photons are little packets of an oscillating electromagnetic field, and sunlight contains the full spectrum of visible and invisible light. When photons from sunlight hit the naked wood on the table, some of them are absorbed by the electrons in the atoms of the wood. This tends to warm up the table. Other photons get reflected, and some in the non-visible spectrum may pass through entirely!
The photons that get reflected vs absorbed or passed through are filtered based on the frequency of the photon – i.e. the color of the light that will hit your eyes – and the chemical make-up and even the crystalline structure of the molecules. Specifically which color ranges respond to what kinds of chemical compounds or structures is beyond my knowledge, but that is how it is determined.
Then there are things like finishes, dyes, paints, etc, which can change what we see. These compounds have their own chemical makeup and they are specifically engineered and chosen to absorb and reflect certain frequencies of light, which is why you can paint or stain wood, or dye cloth, and change its color.
It’s because of the electrons in the atoms on the surface of an object. Photons of light all have a certain amount of energy. The color of the light is determined by energy. Red light has less energy than blue, for example. Electrons also have a certain energy, but because electrons are spooky, they can only exist at very specific energy values. Never a value in between. When a photon of the exact right energy hits an electron, it will be absorbed by the electron, and the electron will “jump” to the next energy level. Then, the electron will release the energy, producing a photon again with the same energy as the one that originally impacted it. This photon then hits your eyes, and you interpret this as color. When a photon and electron don’t have an energy that “matches” in this way, they don’t interact, and the light will just sort of pass through or be scattered. So it doesn’t reach your eye as much. The net effect is that when you wear a yellow shirt, the energy of light corresponding to that shade of yellow is continually absorbed and re-emitted by your shirt. I hope this helps!
> encoded in the table itself
What?
There are actually two parts to color perception. Wikipedia leads with “the visual perception based on the electromagnetic spectrum.” https://en.wikipedia.org/wiki/Color (which is a decent primer to read for background information.
What the brain perceives as a given color depends on the relative strengths of the stimuli of the cone cells in the eye’s retina. A bunch of red photons will stimulate the ‘red’ cell a lot and the ‘green’ and ‘blue’ cells little. A single wavelength of light is perceived as a [spectral color](https://en.wikipedia.org/wiki/Spectral_color). But “all the colors of the rainbow” is a very narrow set of colors. Different wavelengths correspond to different energies. As you go toward red (and past that into infrared) the energy is lower. Opposite for violet, ultraviolet, etc.
Electrons in different chemical compounds exist at energy levels, kind of like a ladder or staircase. You can’t stand halfway between two rungs. An electron needs energy input to go up, and can drop back directly down or take intermediate steps down. Like jumping three steps up, and walking down a step at a time.
The analogy I came up with before is if when you hear a certain note you can jump up a certain amount, and when you jump down, you sing the corresponding note.
So if a [dye](https://en.wikipedia.org/wiki/Dye) molecule has an electron structure that has the right gaps, it will absorb those light colors.
This only covers a relatively small part of the concept but hopefully is enough to get you started reading already-written stuff.
Latest Answers