Question Video: Understanding the Photoelectric Effect | Nagwa Question Video: Understanding the Photoelectric Effect | Nagwa

Question Video: Understanding the Photoelectric Effect Physics • Third Year of Secondary School

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A monochromatic light ray with frequency 𝑓 falls on a surface that has a critical frequency 𝑓_c. If 𝑓 is less than 𝑓_c, which of the following statements is true about the electrons on the surface? [A] By increasing the number of photons, the number of the released electrons increases. [B] By increasing the number of photons, the number of the released electrons decreases. [C] No electrons leave the surface. [D] None of the answers are correct.

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Video Transcript

A monochromatic light ray with frequency 𝑓 falls on a surface that has a critical frequency 𝑓 sub c. If 𝑓 is less than 𝑓 sub c, which of the following statements is true about the electrons on the surface? (A) By increasing the number of photons, the number of the released electrons increases. (B) By increasing the number of photons, the number of the released electrons decreases. (C) No electrons leave the surface. (D) None of the answers are correct.

We can answer this question by recalling that the critical frequency, also known as the threshold or cut-off frequency, is a property of a metal. It gives the minimum frequency of an incident photon that can overcome that metal’s work function and induce the photoelectric effect.

So, if incident photons have a frequency that is greater than or equal to the threshold frequency, the photoelectric effect will be induced and electrons will be emitted from the metal surface. However, if incident photons have a frequency that’s less than the threshold frequency, the photoelectric effect will not be induced and no electrons will leave the surface.

We are told in the question that the frequency, 𝑓, of the monochromatic light ray is less than the critical frequency of the surface, 𝑓 sub c. So, it looks like the correct answer is option (C): no electrons leave the surface. This means we can eliminate option (D). But let’s quickly consider options (A) and (B).

Since 𝑓 is less than 𝑓 sub c, increasing the number of photons will have no effect on the number of electrons released. Photons will still have the same frequency 𝑓. And since this is still less than the threshold frequency, 𝑓 sub c, the photoelectric effect will not be induced and no electrons will leave the surface. So, this means we can eliminate options (A) and (B). Therefore, the correct answer must be option (C). If photons with a frequency 𝑓 that is less than 𝑓 sub c are incident on a metal surface, no electrons will leave the surface.

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