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Name: Jay
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Question:
If Planck's theory says that energy can have values of 1hv, 2hv, 3hv,etc, then are there an infinite (all values of n times v) amounts of energy for say red light? In the photoelectric effect the metal may not absorb enough energy from hv, but 3hv would give it enough energy to release an electron.



Replies:
I might be misunderstanding your question, but there are two ways to increase the amount of energy in "light". The first way is to increase the frequency of the light (in other words, to use photons with higher energy, E=hv). However, if you go this route, you change the color of the light (e.g. blue light photons have higher energy that red light photons). The second way is to simply increase the number of photons, this corresponds with greater brightness or intensity of the light, regardless of the given color/frequency. If you fix the number of photons *and* you also fix the color (frequency/energy per photon), then the total energy is also fixed.

Hope this helps,

Burr Zimmerman


Jay - I thought a photon had energy 1hv, no multiples. Meanwhile, it sounds like you might be thinking of multi-photon absorption. If the first ionization energy of a species is 10eV (electron-Volts), then 2 photons of 5eV or 3 photons of 3.3eV might well also do it, provided that number of photons arrives close enough together in time. This close-together requirement gives the multi-photon absorption rate a dependence on intensity of the light. In other words, 10eV material is almost completely transparent to 5eV photons at low intensities, but with each factor of two the concentration increases, the amount of absorption doubles. The dependence is steeper for higher numbers of photons required. For 3-photon, it goes as the square of intensity.

For each extra photon, the practical intensity threshold, the order of magnitude of intensity required to get usable absorption, increases greatly too. So there is a hardware-price for getting higher multiples of energy quanta up-conversion. And the total amount of energy in the flow is conserved. It always takes three 3.3eV photons to do one 10eV ionization.

For practical use, intensities are deliberately maximized (given a fixed power), usually by lens-focus or sharp pulses,

Jim Swenson



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