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Recently,I was reading about the photoelectric effect and have a doubt regarding it:

Suppose I irradiate a metal surface with a monochromatic beam of light of frequency 'μ' such that μ > μ0 where μ0 is the threshhold frequency.When a single photon strikes the electron present in the metal surface,it(electron)is emmited in the form of a photoelectron,generating photelctric current.

Image courtesy:Wikipedia

(Image courtesy:https://en.m.wikipedia.org/wiki/Photoelectric_effect)

However, in my coursebook (and in some other books as well), it is written that there is no time lag between the striking of the light beam and the ejection of electrons from the metal surface, whereas some other sources specify that the time lag is very small (in the order of nanoseconds).

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Since nothing can travel faster than the speed of light,some amount (extremely minute) of time lag is bound to happen.Can anyone please clarify if I am correct or misunderstanding what is written in my textbook? Additionally, if a time lag does not occur, why is that the case?

Help is appreciated.

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  • $\begingroup$ Where did you read this? There are even spelling errors in this text (as well as in your title). It is better to read physics books, not arbitrary posts. It takes a minimum of care and precision to be a physicist. $\endgroup$
    – my2cts
    Commented May 31 at 12:11
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    $\begingroup$ Thanks you pointing out my mistake(I was a bit careless while writing the heading and would definitely try to avoid it from now) $\endgroup$
    – Ishaan
    Commented May 31 at 12:16
  • $\begingroup$ @my2cts it is not an arbitrary post.Rather it is an extract from a book ment for class 11th students(Ncert chemistry-Part 1).Its a book which millions of Indian students follow and even I was taken aback after seeing this level of text which is creating confusion among students. $\endgroup$
    – Ishaan
    Commented May 31 at 12:21

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In principle you are correct: Nothing is infinitely fast, and everything will take some time. Nevertheless, it is a matter of two different perspectives:

  1. If we consider time intervals, which a human is able to experience (say $1ms$ or larger), a time lag does not exists.
  2. In contrast, if we consider infinitesimal small time intervals, we have to describe the light-matter interaction quantum mechanically. In fact, we have to use QED, and the concept of a interaction, happening within a well-defined location in space and time, breaks down. If we are willing to go down this road, we will probably end up with a "time delay" around 100 attoseconds, but this value is open for discussion and interpretation.

Thus, no rigorous answer exists. I do not say that the answer "around 100 attoseconds or less" is worthless -- it is certainly much better than the answer "no time lag exists". However, from a practical point of view both answers are almost identical.

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  • $\begingroup$ Thanks for your answer!I agree with your viewpoint. $\endgroup$
    – Ishaan
    Commented May 31 at 11:40

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