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Author Topic: Why are some frequencies of light unable to excite electrons around atoms?  (Read 791 times)

Offline memoryerase1

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If the frequency of incident light is too weak, the electron does not get excited and does not move from the lowest energy band. But this low frequency of light is to weak to see through a opaque object around four inches thick, though right. The electron will not get exited, and move to a higher energy band, but light is passing through the solid opaque object.

So could you see through the solid opaque object up to a few mm or less, or not at all.

Thank you for your help.
« Last Edit: 02/01/2016 10:57:56 by chris »


 

Offline evan_au

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Quote from: memoryerase1
see through an opaque object
This is something of a contradiction, since opaque means that you can't see through it .
- 4 inches of wood is opaque to visible light (because light of different frequencies is absorbed, usually by exciting an electron in an atom).
- Metals are opaque, but for a different reason - light bounces off the metal (reflection), rather than being absorbed.
- You can't see through a translucent material (eg clouds, snow or sugar); these do not absorb the light (the electrons are not excited), but the light is scattered so much that it does not form an image.

If not otherwise stated, you would assume that someone is talking about seeing through an object with visible light. However, there are other kinds of light, such as X-Rays.  4 inches of wood is opaque to visible light but is transparent to X-Rays.

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light is too weak
This can mean several different things:
When we say the light is weak, we usually mean that the intensity is low (eg the light source is far away), without changing the frequency.
When we are considering interactions between light and electrons, what matters is the energy of the photon, and energy is proportional to frequency.

I am assuming that you are discussing this second meaning of "weak".

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If the frequency of light is too weak the electron does not get excited?
If you have hydrogen gas at room temperature, it is transparent to visible light. This is because it takes an ultraviolet photon (which is more energetic) to excite an electron from the ground state to a higher energy level.

If you take very high energy photons (eg X-Rays), the Hydrogen is again transparent.

However, if you have very hot hydrogen gas, where a considerable number of electrons are no longer in the ground state, visible photons do have enough energy to excite the electron to a higher energy level. And if you heat it to a plasma, it is opaque to ultraviolet, visible and infra-red frequencies.

Even for Hydrogen gas at room temperature, infra-red photons (with less energy than visible photons) are able to be absorbed and emitted, by triggering changes in the rotation and vibration of molecules. So for some infra-red frequencies, hydrogen is no longer transparent.

For pure materials (like Hydrogen gas), there are particular frequencies that are absorbed, and other frequencies which are transparent, so you can see through them by choosing the right frequency of light. For complex materials like wood, there are many different frequencies that are absorbed by many different chemical compounds, somewhere in the material.

The most ideal "black body" is probably carbon, as it is able to absorb every ultraviolet, visible and infra-red frequency (but X-Rays are still transmitted, to some extent).

If you want a material that can be selectively opaque and transparent, try a Liquid Crystal Display (LCD), as used on smartphones and thin-screen TVs.
 
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Offline evan_au

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Why are some frequencies of light unable to excite electrons around atoms?
Electrons are most likely to be excited if the energy of the incoming photon exactly matches the energy difference between an electron's current orbital, and another vacant orbital in the same atom.

If the photon has a higher frequency, or a lower frequency (ie a different frequency), the electron is very unlikely to absorb the photon and move to a higher orbital.
 
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