The threshold wavelength for photoelectric emission from a material is 5500 Å. Photoelectrons will be emitted, when this material is illuminated with monochromatic radiation from a
A. 75 W infra-red lamp
B. 10 W infra-red lamp
C. 75 W ultra-violet lamp
D. 10 W ultra-violet lamp
Choose the correct answer from the options given below:
A. 75 W infra-red lamp
B. 10 W infra-red lamp
C. 75 W ultra-violet lamp
D. 10 W ultra-violet lamp
Choose the correct answer from the options given below:
- A.B and C only
- B.C only
- C.A and D only
- D.C and D only
Show answer and solution
Correct option: D
Solution
The threshold wavelength is 5500 Å, meaning only photons with wavelength shorter than 5500 Å (i.e., higher frequency/energy) can cause photoelectric emission. Infrared light has wavelength longer than 5500 Å, so its photons have insufficient energy regardless of the lamp's power (intensity). Ultraviolet light has wavelength shorter than 5500 Å, so UV photons exceed the work function and will cause emission. Since photoelectric effect depends on photon energy (frequency), not intensity, both the 75 W and 10 W UV lamps will eject photoelectrons — the power only affects the rate of emission, not whether emission occurs. Hence C and D both work.
[JEE Main 2023 · 29 Jan · Shift 1]