Opaque and transparent depends on the wavelength (power, electric, box)

[MalaMan]

Visible light is just electromagnetic radiation of a specific wavelength. "Opaque" and "transparent" depends on the wavelength. But what factors determine "opacity" and "transparency" of materials to a certain wavelength?

[djmilf]

Fascinating question!

I once worked at an office on the second floor of a building, overlooking a busy street intersection that had a traffic light that could bee seen through the office window. A coworker had a red acrylic clip board that was transparent enough to allow enough light through, but only the red light. If you looked at the traffic signal when it was red, you could see the red light through the clip board. The red light appeared to be white, but that was an optical illusion due to the relative strength of the red light being emitted by the signal in comparison to the light waves in the red spectrum bouncing around everywhere. When the signal was green, you couldn't see any light from the signal - it appeared as a black dot when viewed through the red tinted transparent acrylic clipboard.

Think about visible light. It's a combination of various wavelengths emitted from a source and bouncing around our environment. Combined together the various wavelengths appear white. When said light hits a stop sign, only the red light bounces back. That's because the red pigment on the sign absorbs all other light wavelengths but the ones in the red spectrum, which get reflected from the stop sign or which get partially reflected and partially passed through in the case of the red acrylic clipboard.

When light encounters matter, it either passes through it, gets reflected from it, or gets absorbed by it. I guess you could also throw in that light can also get refracted from encountering matter. Light gets absorbed when the electrons in the matter resonate with the various frequencies of the light. Light gets either passed through or reflected when the electrons in the matter do not resonate with all of the frequencies of the light wave. A pane of glass or the atmosphere at ground level pass the light through. A stop sign absorbs all but the red light, reflecting the red light back, albeit in a scattered pattern. The clear red acrylic clipboard both reflects and ignores the light waves in the red spectrum (not enough red pigment, I'd guess) while it absorbs all other spectrums of the light waves. An object tinted black absorbs nearly all of the light spectrum. An object tinted white reflects all of the light waves in a scattered pattern. The silver backing of a mirror reflects all of the light, but does not scatter it due to the smoothness of the surface.

Remember that light that gets absorbed? It doesn't stay in the material, it gets re-emitted as another electromagnetic wave, but in the non-visible spectrum. If you had infra-red eyeballs, you'd still see the light.

As for opacity - I'm guessing that it has to do with the quantity and quality of pigmentation in the materials. Nearly all the light makes it through a clear pane of glass. Only the red light makes it through a stained glass panel that's red. Paint that clear glass pane white, and some visible light still comes through. Paint that pane black and almost none will come through. Make the pane out of metal instead of glass and no light would come through.

[stephenMM]

There really is no definition of "visible light", so that's a problem. What may be visible to me may not be visible to someone else, and light always has to be seen in relationship to something else. If it's a dark night, it takes very little light to be visible (to some people). If it's a bright day, then more light is required for it to be visible against the ambient light. But in both cases the amount of "visible light" is the same to the viewer. If I come into a dark room from a noon day sun, my eyes will continually adjust to the new environment, so the light that is essentially invisible to me is visible to the people that have been in the room long enough for their eyes to become accustomed to it.

Even if two different lights are of the same wavelength, the environment and the seer will largely dictate whether any extraneous light is visible to a person w/ good eyes, an older person, a cat......it is casting too wide a net to just say "visible light" w/o defining the exact parameters, which are not going to be fixed. We won't get into warm light and cold light here, it's too long a subject. However, the type of light will determine whether it may or may not be visible, and by how much, iregardless of its wavelength. IR light is essentially invisible to humans but not to other species, and a photographer who uses IR film can make striking photos by making what is invisible to us, visible. It will be tricky to meter the light though, and even the focus distance will be different from non IR film.

I'm an artist and photographer, and for my purposes, light is usually measured by candlepower, watts and lumens. Foot candle and lux are also useful forms of measurements, as is intensity. Wavelengths, not so much. Human psychology comes into this as well. All a painter has to do is put a touch of blue on the horizon line in a landscape painting to make the viewer think it's far in the distance. This is because in nature, light will have a bluish cast at the distant horizon. So even though the viewer knows they're looking at a landscape painting and not an actual landscape, their visual reality is just an illusion. What is triggered here is not their eyes, but is what's hard wired into their brain, which tricks their mind. The intensity and appearance of apparent light is different than actual light too. Just put a piece of white paper next to a gray piece, then put it next to a black piece. The same light wavelengths are in play, but will look very different depending on what is contrasted against it.

[MalaMan]

Thank your for the answers so far.

I'm really intrigued by the realization that "opacity" and "transparency" are properties of materials that are related not only to visible light, but also to all other wavelengths of electromagnetic radiation.

Take a wall made of bricks, for example. It may be "opaque" to electromagnetic radiation in the wavelengths of red light, and blue light, but it can be "transparent" to electromagnetic radiation in the wavelength of AM radio (it's perfectly possible for an AM radio receiver inside a closed room with brick walls to detect the electromagnetic "signal" from an AM radio station). In another example, muscle tissue of animals may be "opaque" to electromagnetic radiation in the wavelengths of red, green and blue light, but it's "transparent" to hard X-rays. In contrast, bones are mostly "opaque" to hard X-rays too....

It seems to me that the factors that determine the properties of "opacity" and "transparency" of each material to each wavelength of electromagnetic radiation are very complex factors.

[harry chickpea]

Originally Posted by MalaMan

Thank your for the answers so far.

I'm really intrigued by the realization that "opacity" and "transparency" are properties of materials that are related not only to visible light, but also to all other wavelengths of electromagnetic radiation.

Take a wall made of bricks, for example. It may be "opaque" to electromagnetic radiation in the wavelengths of red light, and blue light, but it can be "transparent" to electromagnetic radiation in the wavelength of AM radio (it's perfectly possible for an AM radio receiver inside a closed room with brick walls to detect the electromagnetic "signal" from an AM radio station). In another example, muscle tissue of animals may be "opaque" to electromagnetic radiation in the wavelengths of red, green and blue light, but it's "transparent" to hard X-rays. In contrast, bones are mostly "opaque" to hard X-rays too....

It seems to me that the factors that determine the properties of "opacity" and "transparency" of each material to each wavelength of electromagnetic radiation are very complex factors.

The spectrum of waves and wave frequency IS complex, too much so for easy glib answers. At best, I can only offer some starters to get you headed in the right direction:

In a wave, physical material is not propagated in the direction of wave travel. Crudely stated, the material is compressed and decompressed slightly in a unidirectional manner. It does so because a wave has inherent energy (there was a movement that initiated it), and it takes far less energy to pass that movement along than to stop it.

Take a fifty foot rope, tie one end to a tree, pull the rope taught and move it up and down quickly at the other end. You will see the waves propagate down the length of the rope, while the fibers of the rope are still at the same position on the rope. You may also see the reflection of the wave come back from the end tied to the tree if the system is taut enough that there isn't damping of the wave from slack.

What happens when you tie the end of the thin rope to a thicker rope? Play with it to see.

Next, make a cigar box violin and learn how materials have resonance, and certain frequencies of waves are accentuated or attenuated because of the resonance and material properties. Again, the wave propagates without any massive distorting of the shape of your violin.

Move on to building and playing with a simple wave table (big pan with a layer of water in it). See how point source waves behave, then use it to learn about refraction and reflection. Do a search on "wave table experiments" for ideas.

Move on to electricity. You are about to learn the difference between Edison and Tesla. With electricity there are two components involved, the wave (which as stated before has energy) and electrons. Electrical power is (again crudely stated) the energy that can be tapped when electrons move.

With direct current, there is no wave, and electrons are passed from molecule to molecule like a bucket brigade. In alternating current, a wave is induced into the conductor, which then acts much like your rope in the first experiment. The energy in the wave moves each electron in the path only a tiny amount (which is VERY energy efficient) The electrons go one way, then the other, repeatedly.

Direct current has losses and resistance all along the way. Alternating current has very low losses, and the energy put into the wave at one end can be retrieved at the other by anything that damps the wave.

Of special note is that with electricity you are dealing with two distinct things an (arguably physical) electron AND a wave.

Next, remembering the basics of electricity, move on to light. Light is both a wave and a photon, but a photon is more strange than an electron, which usually circles around a nucleus in various energy states.

In a way, a photon is and isn't at the same time. When we get light from a star billions of miles away, that light travels as the efficient wave energy. It doesn't travel as a particle. Particles cannot travel at the speed of light, but only approach it with the use of huge amounts of energy.

When the wave that is light encounters a material that damps it, (AGAIN, very crudely) the photon (physical representation of the light) manifests. So, much like electricity, light is a combination of the properties of different elements.

Light can pass through glass or many crystals, as the glass is mostly able to keep the wave intact because of the structure of the material.

When light of a certain frequency cannot maintain the wave, and the wave is damped, the energy of the wave gets released, just as with alternating current. The easy example is when you stand in bright sunlight and get warm, because all of the infrared wavelength waves are hitting your body and going "Oh s**t! I can't get through!" All those little swear words from the infrared waves add up and turn into heat (which is another definition of localized energy).

There is regular window glass, but there also is a special heat absorbing glass. The absorption is achieved either through adding colorants that damp certain wavelengths of light, or metal oxides that are commonly thought of as transparent, but are slightly darkening in the visual spectrum.

This all leads to the core answer to your question about transparency. If a wave passes through unimpeded, a substance is transparent. If a wave isn't blocked but there is a lot of refraction in different directions, a substance is translucent. If a wave is completely blocked, a photon appears, energy is released, and depending on the vibration speed of the child wavelengths, various colors manifest.

An X-ray usually passes through water but is stopped by bone, where the wave releases its energy or is reflected. The shadow of the bone projected onto a plate forms an image. The released energy of the X-ray, when focused on particular cells, can disrupt and kill or mutate them.

The above is all VERY crude and intended for basic understanding. Dive down into any of it and you will find it much more complex and even confusing.