[thedoc (OP)]

Oliver Nelson Dummett  asked the Naked Scientists:
   
Thanks for the great show - I love listening to your podcasts when walking to and from school.

I was listening to a (very old) podcast on optics and was wondering:

If matter is mostly empty space, then why are most things opaque. Wouldn't the photons just pass through without interacting in any way?

Many thanks, and keep up the great show

Oliver Dummett

What do you think?

[syhprum]

I am sure there are more complete explanations available but I will give you my thoughts on the matter, what the transparency of a materiel chiefly  depends on is the number of free electrons a photon interacts with as it tries to pass thru some atoms have loosely bound electrons in their outer shells and these are free to drift in the interatomic spaces making the materiel opaque whereas materiel's such as diamond have all their electron tightly bound and photons at the correct angle to the atomic plane can pass thru unimpeded, this of course a very simplistic version of a much more complex subject.   

[evan_au]

In the classical view of an atom, there is a lot of empty space inside an atom; quantum theory has a somewhat different view of the atom, in that the electron shells fill a volume around the nucleus, with various probabilities. However, because the wavelength of visible light is much larger than an atom, the internal structure of the atom (and how much empty space it contains) does not really matter. 

Light does pass through "empty space" between stars, without much absorption. This is because atoms are well spaced out between stars, compared to the wavelength of light. The common atoms which are present (like hydrogen and helium) don't absorb light in the visible range. Uncommon atoms which are present (like carbon, oxygen, silicon, nitrogen & iron, and their chemical compounds) are condensed into compact dust grains & comets which don't block much light.

However, familiar matter like a table has atoms which are packed much more closely than a wavelength of light. Light can't pass very far into the table without striking an atom, and the huge variety of molecules present are able to absorb energy from photons at almost every frequency in the visible range.

A more detailed discussion may be found here: "Why can sound waves go through some solids that light waves can't?"