Matter, Empty Space, Opaque versus Transparent
Country: Czech Republic
Date: Winter 2012-2013
How is it possible that I cannot see through something, when spaces between atoms (between electrons and protons) are immensely bigger than atoms (particles) themselves? I should see right through everything when the matter covers less than 0.1% space. Why are there opaque materials?
There are a few things here that can help:
First, while the size in classical terms of a proton or electron is small, the average orbital radius of an electron around an atom is not small. In better terms, the electron's wave function extends for a relatively large size, with significant likelihood for the electron distance to be comparable or inter-atomic distances in a solid.
Next, the size of an individual photon for optical light is much, much larger than inter-atomic distances. That applies both to the wavelength as well as the coherence volume ("size") of the photon.
Lastly, the interaction between light and matter is relatively strong. The photon will find plenty of stuff to interact with. Sometimes all those interactions add up in a way the lets the photons often continue as if there was no object and we see something transparent. More often those interactions add up in a way that either reflects or just absorbs the light. Regardless of the result, it comes from interaction of the photon with lots of electrons over a very large area (or volume).
The interaction itself, who light and matter interact, is to me one of the most fascinating parts of nature. It is a bit long to go into here, but if this at all sounds interesting, and you would like to learn a bit more about it, I would encourage you to read "QED: the strange theory of light and matter" by Richard Feynman. It is a wonderful little book and, while written in very accessible language, it is exactly how we think of photons and matter on an elementary scale.
1) You /can/ see through very thin layers of many materials. A carbon foil only a few nanometers think, for example, looks sort of like a soap bubble.
2) The space between atoms of a solid is very small compared to the wavelength of visible light. Light behaves as a wave in this case, and responds to the electron density averaged over a volume on the scale of the wavelength, which might contain thousands of atoms.
If you could get something thin enough, say one atom or molecule thick you could probably see through it. But usually, there are so many layers of atoms between you and the other side that your view is blocked. Think of the pile of balls at the McDonald's kiddy slide. The holes between the balls are immense, but the many layers make it impossible to see thru them. (Even though atoms are not solid balls, the idea is the same - too may things in the way )
Hope that helps
You have invoked a model for atoms and molecules that is incomplete. In particular, electrons are not point masses. They occupy space. In addition, molecules (composed of various atoms) occupy space, so the model you are invoking is way over-simplified.
You have also assumed that matter and light do not interact. That is not the case. Molecules absorb light and that light can change wavelengths by various processes in the object being illuminated.
You need to consider that the wavelength of visible light and the size of atoms and molecules are approximately the same, so atoms and molecules can scatter light.
Your model of point atoms and molecules just does not match the actual sizes of the absorbers.
Thanks for the question. The ability to see through something depends on the probability that the light wave (or photon, if you prefer) will interact with the matter. You are definitely correct in that much of matter is empty space. However, a material appears opaque if the light is absorbed instead of being transmitted through. A material appears translucent when light scatters from an atom or electron. A material will be opaque if the probability of absorption is large, as compared to another material.
I hope this helps. Please let me know if you have more questions.
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Update: November 2011