Suppose for a particular electric transition an electron absorbs a photon of energy $hf$. Now my doubt is can't the electron for the same transition absorb 2 photons each of energy $hf/2$ i.e, each of half frequency but in total of same energy. If it is so then why we focus only on one photon in school physics books?
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That's completely possible, and it's known as two-photon absorption.
You focus on one-photon absorption in school only because two-photon absorption is a rare process. It's rare because of the weakness of the electromagnetic force, described by the fine structure constant $\alpha = 1/137$, and because you need two photons to hit the atom at once, which is tough if there aren't too many photons around.
knzhou
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Most interesting thing I have read in a while (chemist here). Thanks – Mah Neh Sep 26 '22 at 13:43
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In my answer to your previous question I explained that two photons can be emitted, but this is less likely than one photon by a factor of the fine structure constant. By time-reversal invariance, the same applies to absorption. And, as knzhou notes, there is the additional improbability of two photons happening to hit the atom at the same time.
The calculations for multi-photon processes are considerably more complicated, so introductory courses deal with only the higher-probability one-photon case.
G. Smith
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I note that you did not accept my other answer, presumably because you didn’t believe it, but now that you have heard something similar from knzhou you should believe us. – G. Smith Jan 25 '20 at 21:48