CAPF AC 2020 Q23: Science & Technology – Physics
- (a) Albert Einstein received the Nobel Prize in Physics for explaining photoelectric effect
- (b) For each metal, there is a threshold frequency ν₀ below which this effect is not observed
- (c) At a frequency ν > ν₀, the kinetic energy of ejected electrons does not change on increasing the frequency of incident light
- (d) The number of electrons ejected is proportional to the intensity of incident light
Answer: (c) At a frequency ν > ν₀, the kinetic energy of ejected electrons does not change on increasing the frequency of incident light
Explanation
Photoelectric effect
How to think: remember two rules.
Frequency decides the energy of each electron.
Intensity decides the number of electrons.
Einstein's equation: maximum kinetic energy = hν − hν₀
So when the frequency ν goes up, the kinetic energy goes up.
| Statement | Verdict | Fact |
|---|---|---|
| (a) Einstein received the Nobel Prize for explaining it | True | Nobel Prize in Physics, 1921 |
| (b) Each metal has a threshold frequency ν₀ below which there is no effect | True | Below ν₀ no electron comes out, however bright the light |
| (c) Above ν₀, the kinetic energy does not change on increasing the frequency | NOT correct | Kinetic energy rises with frequency |
| (d) The number of electrons ejected is proportional to the intensity | True | More intensity means more photons, so more electrons |
Answer: (c) At a frequency ν > ν₀, the kinetic energy of ejected electrons does not change on increasing the frequency of incident light.
Why each option is right or wrong
- (a) Incorrect. This statement is true: Einstein won the 1921 Nobel Prize for this explanation.
- (b) Incorrect. This statement is true: every metal has its own threshold frequency.
- (c) CORRECT. This is the wrong statement: the kinetic energy increases as the frequency increases.
- (d) Incorrect. This statement is true: the number of electrons depends on the intensity.
Key point
Photoelectric effect: kinetic energy = hν − hν₀. Frequency controls the energy of electrons; intensity controls their number; no emission below the threshold frequency.
Remember
Higher frequency = faster electrons; higher intensity = more electrons.
Common mistakes
- Swapping the two rules: thinking intensity raises the energy of each electron.
- Believing Einstein got the Nobel Prize for relativity; it was for the photoelectric effect.
- Thinking very bright light below the threshold frequency can still eject electrons.
Exam tip
Frequency for energy, intensity for number. Check each statement against these two rules.
Sources
- NCERT Class XII Physics — Dual Nature of Radiation and Matter
- NCERT Class XI Chemistry — Structure of Atom (photoelectric effect)