Re: What happens to light as it passes through a translucent substance?

The question: "I am a teacher looking for a scientifically accurate explanation for translucency for a grade 4 level. Definitions often say that translucent substances don't let as much light through as a transparent substance. What does that mean? Is the light being reflected, absorbed or scattered by the substance? Why can't we see through a translucent substance? Additionally, some substances seem to be very translucent while others are barely translucent. When does a substance stop being translucent and become opaque? Are there varying levels of translucency?"

All excellent questions! As you say, many explanations are not explanations at all but rather restatements of the definitions!

Here are two answers from our archives that give some background to the subject:
previous answer about reflection and absorption
previous answer about dark clouds
I found these previous answers by using our search engine to search on
'scattering thickness' (minus the quotation marks).

From the background material perhaps you have discerned that there are two major ways in which light interacts with matter: absorption and scattering. Sometimes it is hard to decide which process is dominating in a material, but generally one can make the following statements:
- If the light beam does not change "color" as it passes through the material then the interaction is probably scattering.
- If the light beam does change "color" through the material then at least some frequencies of photon are being absorbed. (But, for instance, the Sun low in the sky looks redder due to preferential scattering of "violet and blue" photons, not absorption! There are always exceptions!)
- But there are two characteristics of a material that can combine to cause it to be "opaque": absorption and/or scattering. (This is one of the options you mentioned in your question! An "opaque" object can be simply a thick scattering object that would be translucent if thinner. But a thin object can actually be opaque if it is sufficiently absorbing.)

Absorbing materials (black plastic, soot) generally do actually absorb photons. Very thin sections of some dark plastics can be translucent, but only in very thin slices.

Scattering materials can be "opaque", or "translucent", or "nearly transparent" depending on the degree of scattering and the thickness of the material. Ocean water, for instance, looks nearly transparent if there isn't too much to look through, but several hundred meters of even very clear ocean water will be at best translucent, and if not very clear it can be very dark through that several hundred meters of ocean water.

But what is the main difference between a transparent and a translucent material? What makes it translucent? Answer: the degree of scattering. In general, transparent materials do not scatter much, while translucent materials scatter somewhat. And the degree of scattering usually depends on the presence of impurities in the material. Ocean water is a good case in point: it contains scatterers that consist of plankton, microbes, and other small pieces of stuff (silt, for instance). Very pure water is almost totally transparent, but not quite. Even very pure water, if you look through enough of it, is blue.

So, what happens to photons that do not get through a material? Absorbed photons give their energy to vibrations of the atoms and molecules, so the energy ends up as heat. Scattered photons go in different directions, and if the material is (optically) thick enough many of the photons can end up going in the opposite direction from which they started. A good example of this is clouds (see the 2nd previous answer above): if you are above the clouds they look very white.

John Link, MadSci Physicist