Given below are two statements: one is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A): Light of frequency lower than the threshold frequency of a metal cannot eject photoelectrons from it, however large its intensity.
Reason (R): In photoelectric emission, an electron absorbs the energy of a single photon, and it can escape only if this energy \(h\nu\) is more than the work function \(\phi_0\).
In the light of the above statements, choose the most appropriate answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Both (A) and (R) are correct and (R) is the correct explanation of (A).
Assertion (A) Light of frequency lower than the threshold frequency of a metal cannot eject photoelectrons from it, however large its intensity. — True. Below the threshold frequency, no photoelectric emission takes place even with very bright light.
Reason (R) In photoelectric emission, an electron absorbs the energy of a single photon, and it can escape only if this energy \(h\nu\) is more than the work function \(\phi_0\). — True. This is Einstein’s photon picture of the effect, \( K_{\max}=h\nu-\phi_0 \) with \( K_{\max} \) never negative.
Why Reason (R) explains Assertion (A)?
Each electron takes energy from one photon only. So the energy of one photon, \( h\nu \), decides whether an electron can escape.
Below \( \nu_0=\phi_0/h \), every photon has less energy than \( \phi_0 \). A brighter light only sends more such photons each second, so still no electron escapes.
NCERT (2026-27):
Einstein’s Photoelectric Equation: Energy Quantum of Radiation – “… Thus, there exists a threshold frequency ν0 (= φ0/h) for the metal surface, below which no photoelectric emission is possible, no matter how intense the incident radiation may be or how long it falls on the surface. …”
Einstein’s Photoelectric Equation: Energy Quantum of Radiation – “… This has happened because in Einstein’s picture, photoelectric effect arises from the absorption of a single quantum of radiation by a single electron. … Since Kmax must be non-negative, … photoelectric emission is possible only if h ν > φ0 …”