if white stuff reflects all light and black stuff absorbs all light, how does clear stuff work?

58 views
0

if white stuff reflects all light and black stuff absorbs all light, how does clear stuff work?

In: 7

Clear stuff means most light passes straight through almost unimpeded, but enough gets reflected/refracted to make the object not totally invisible.

It depends on the clear stuff, really. Have you ever noticed the greenish edge to a window pane or a car window? That’s its true color.

Typically, clear glass allows visible light to pass through it at a fourth of an inch thickness. The edges aren’t see through since the angle at which they sit is relative to the rest of the glass make. This makes the edge very thick and hence, not see through.

In simplest terms:

White=Most light is reflected

Black=Most light is absorbed

Clear=Most light passes through

I had a similar question, if white tshirt reflects, why when i put it over my eyes to sleep, it lets in more light

both reflectance and absorption require a kind of match between the electrons in the material and the frequency of the light hitting them. this is because each phenomenon involves the uptake of energy by electrons from the light striking them. in the case of absorption that energy is never released except as heat, while in the case of reflection the energy is immediately released via a photon of nearly the same frequency.

the least hand-waving account of what’s going on at the subatomic level here is offered by something called quantum electrodynamics (QED). both absorption and reflection interactions have complicated quantum electrodynamic explanations. those explanations involve a compatibility between the energy contained in the photon and the current level of excitation of the electrons in the surface’s material. if there’s a “match” then the electron can absorb the photon’s energy and move to a greater level of excitation. the consequence is that the electron rises into a higher-energy state as it buzzes around its atom and the light that struck it ceases to exist as a forward-traveling photon. in un-watered down QED this concept of a “match” is expressed in terms of the probability of an interaction between a photon and electron, which is how you can get partially transparent things like smoked glass, but that’s neither here nor there.

the QED explanation for transparency is comparatively simple. something is clear when the frequencies of light that matter to you do not “match” with the electrons in the material. hence no electron absorbs the energy of photons striking the surface of, for example, optical glass. and if no surface electrons absorb the light, from the perspective of the photon’s energy, it is as though the photon has not encountered anything at all. (there are other things going on in a lens, like dispersion, refraction and surface reflections, also explainable with QED, but those are all outside the scope of the question.) the same for electrons inside the clear material. those electrons can’t access the energy of the photons passing near them and so can’t stop the light from passing straight through the clear material as though it were not there at all.

this immunity to absorption by an electron is of course frequency-dependent, because frequency indicates the energy contained by the photons. a good example of this is polycarbonate safety glasses. for all the colors of the rainbow, which means all frequencies of visible light, polycarbonate is very transparent. even transparent enough to make eyeglasses from. but to light just a little beyond the color violet polycarbonate glasses become almost completely opaque. if you shine a UV flashlight on them they will look totally black. this is because, at those frequencies, the electrons in polycarbonate DO have an energy match with light, and CAN drain those photons of their energy.

this stops the light at the surface of the polycarbonate, slightly energizes the electrons there, and causes the plastic to gradually warm up as those electrons bleed off their higher energy by shaking the atoms to which they are bound. that vibration, distributed though the whole material, is what we experience as heat. (it’s also the way microwaves use microwaves to heat up susceptible molecules, like water and long-chain fatty acids: the “match” between the energy of the microwave radiation and that of the electrons in water creates the vibration we later taste as hot food.)

among other reasons this is why there are no clear metals: the sea of unbound electrons that makes metals so electrically conductive happens to “match” with a very broad range of light frequencies. this is what makes metals shiney, and reflective if their surfaces are flat enough that its deviations from true flatness are the same or smaller than the wavelengths of light striking it. (if a metal is thin enough it can transmit light, gold leaf for example absorbs all visible light except green and so looks greenish if you hold a square of it up to the sun, but that is a kind of filtering, and very different from anything we’d call “clear”.)

at much higher frequencies (x-rays, gamma rays) photons are still absorbed but the very high energies involved in those absorptions create electron-positron pairs, which can annihilate and release another gamma ray. in this way very high frequency light can leapfrog its way though materials that “ought” to be opaque to that light from the perspective of energy matching. this is one of the reasons it takes a meter of concrete to fully arrest a very hard gamma ray.