Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: cowlinator on 09/12/2016 03:13:30
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If blue light has enough energy to trigger the photoelectric effect on alkali metals, which frees electrons from the material, that is the very definition of ionizing radiation.
Why isn't visible light considered ionizing radiation?
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Visible light can release unbound electrons from or into the conduction band of a metal or semiconductor. This is not the same as ionisation, where the photon energy is sufficient to release a bound electron from a single atom or molecule.
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Thank you! :)
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If blue light has enough energy to trigger the photoelectric effect on alkali metals, which frees electrons from the material, that is the very definition of ionizing radiation.
Why isn't visible light considered ionizing radiation?
Besides alancalverd, who said it wasn't? Gamma rays, X-rays, and higher ultraviolet rays are all considered to be ionizing radiation. See: https://en.wikipedia.org/wiki/Ionizing_radiation
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A factor that might make a difference is that a high energy photon can ionise more than 1 atom.
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Besides alancalverd, who said it wasn't?
It's not about who said it wasn't, it's about who said it was.
Which, according to anything I've been able to find, is nobody.
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Visible light can release unbound electrons from or into the conduction band of a metal or semiconductor. This is not the same as ionisation, where the photon energy is sufficient to release a bound electron from a single atom or molecule.
Light can knock electrons out of the bulk metal, and that is ionisation. This is the effect that makes vacuum photodiodes work, rather than semiconductor photodiodes.
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Visible light is capable of ionizing highly reactive metals and molecules with very high-lying HOMO energies. But my understanding of the term "ionizing radiation" is that it typically refers to EM radiation with enough energy to ionize typical and unreactive materials, like water (which requires hard UV radiation <100 nm).
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Why isn't visible light considered ionizing radiation?
The reaction that we as humans are most concerned about is anything that breaks bonds like C=C, C-C, C-N, C-H, and so can cause DNA mutations that might turn into cancer.
DNA has two backbone chains, and a break in one chain can be repaired fairly reliably by enzymes in the cell nucleus, by using the undamaged strand as a template.
Of more concern is double-stranded breaks, for which the cell must "guess" what has been lost, which is much less reliable. High-energy ionizing radiation like alpha, beta and gamma rays can produce these double-stranded breaks, which is much more likely to produce mutations.
When DNA is struck by visible light, it transforms the light energy into heat (in picoseconds), and avoids damage.
This process also works fairly well for UV light - but sometimes it strikes adjacent Thymine bases in the DNA chain, and changes them into an "invalid" cyclobutane group. The effects of this are felt as sunburn. This produces a distinctive T-T mutation that is often detected in skin cancers. But visible blue light doesn't have enough energy to trigger this reaction.
See: https://en.wikipedia.org/wiki/Direct_DNA_damage
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Visible light has insufficient energy (under normal circumstances). At 400nm (Deep violet high energy light) the light has, if my calculations are correct around 5E-7J/photon or 300KJ/mole. Since the ionization energy of sodium is just under 500KJ/mole, the light is therefore of insufficient energy.
However, by applying a voltage (negative) to the sodium of just a few volts, we can make up the energy difference. The 20V of a DC power source can deliver, If my logic/math is correct, just over 1900KJ/mole, which means at that value, only a few of those 20Volts would be needed to cover the difference.
Thinking of it in non-quantized perspective, by forcing a bunch of electrons into the same place on the sodium, the repulsion helps assist in launching them off the metal, so a low energy visible light can be used.