Given below are two statements : one is labelled as Assertion A and the other is labelled as Reason R :
Assertion A : The photoelectric effect does not takes place, if the energy of the incident radiation is less than the work function of a metal.
Reason R : Kinetic energy of the photoelectrons is zero, if the energy of the incident radiation is equal to the work function of a metal.
In the light of the above statements, choose the most appropriate answer from the options given below.

Solution

Physics sir · 5 steps

Both lines come from one equation, but R talks about the threshold case, not about why A happens.

1. Einstein's equation. One photon gives all its energy hν to one electron. The electron spends φ, the work function, to leave the metal, and keeps the rest as kinetic energy.

   Kₘₐₓ = hν - φ

2. Check A. If hν < φ, a single photon cannot supply the energy needed to pull the electron out. No electron is emitted, however bright the light is. So A is correct.
3. Check R. If hν = φ, then Kₘₐₓ = φ - φ = 0. The electron just manages to leave and has no kinetic energy to spare. So R is correct.
4. Does R explain A. A is about hν < φ, where there is no emission at all. R is about hν = φ, the threshold. A zero kinetic energy at the threshold does not tell you why emission stops below it. The reason for A is that each photon carries too little energy to free an electron.
5. So. Both statements are true, but R is not the correct explanation of A. The answer is (b).

The idea to keep. In assertion-reason questions, first test each statement on its own. Then ask whether R gives the cause of A. Two true facts from the same equation do not make one the reason for the other.

Answer from the NTA final answer key. Solution by PrepWiser.