Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: EvaH on 28/08/2020 11:32:46
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Calvin wants to know:
Since atoms are mostly empty space, why don't light photons pass through solid surfaces?
Can you help?
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I am reading this text through a solid glass window on my computer.
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I am reading this text through a solid glass window on my computer.
I wear glasses so that's another solid window, in addition to the one on the computer and the ones in my eye.
With a bit of messing about with a different photon source etc I could get an X Ray image of the monitor.
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Since atoms are mostly empty space, why don't light photons pass through solid surfaces?
If you just consider the protons and neutrons in the nucleus of the atom, indeed a solid or a liquid is mostly empty space.
- Since most of the mass of the atom is in the nucleus, you could also say that most of the mass takes up a small fraction of the space
- However, visible light does not interact with the nucleus, it interacts mostly with electrons - and in a solid or a liquid, electrons fill up most of the space between the nuclei. So from the viewpoint of visible light, it's not correct to say "atoms are mostly empty space".
Whether photons are absorbed or emitted by electrons depends on the energy levels that the electrons can take in the material.
- As the previous posts illustrate, glass windows have a composition which has no electron energy transitions that interact with visible-light photons
- As glasshouses illustrate, this same glass composition does interact strongly with infra-red photons, reflecting the infra-red photons and keeping the glasshouse warm.
- Other glass compositions are not transparent to visible light - brown beer bottles are formed by adding a small amount of titanium to the glass mixture to block UV photons (and many visible-light photons), and prevent the beer from spoiling.
Even if we look at metals, which reflect light, a sufficiently thin layer of metal will allow a fraction of photons to pass through.
- Take a metallized confectionery wrapper or balloon, and look towards a bright light - you can see the light faintly through the metal. That is because a fraction of photons do penetrate some way into the metal. If the metal is thin enough, some photons will pass through
Warning: Do not look at the Sun through metallized plastic - these are not intended to block UV light, and you can damage your eyes.
To see some of the many ways electrons fill the space around a nucleus, see: https://en.wikipedia.org/wiki/Electron_shell#Subshells
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I've made an experiment using microwave to study its blocking mechanism. Opaque objects, regardless of its phase (solid, liquid, gas, or plasma) blocks electromagnetic wave by producing reactive wave which interfere destructively with original wave.