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Question: One of the online review questions asks us to justify whether or whether not the photoelectric effect must be carried out under a vacuum. However, the video modules does not go into very deep detail about the operating conditions of the photoelectric effect. Is the reason for a vacuum so that no particles would take up the electron ejected prior to its detection by some measurement system?
Answer: Consider what would happen if the experiment was not carried out under vacuum. We think of air as quite transparent because visible light easily passes through it. But for major parts of the electromagnetic spectrum, our atmosphere is really quite opaque. Water vapor in the air absorbs or scatters many frequencies in the infrared and microwave region, as well as the frequencies at or above the ultraviolet region. This is why Ozone is such an important molecule in our atmosphere. If we were trying to vary the energies of the photons, we would want all of the frequencies to be able to reach our sample, and the only way to guarantee this is by conducting the experiment under vacuum.
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