Question

The threshold wavelength for photoelectric emission from a material is \(5500Å\). 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. A and D only
• B. C only
• C. C and D only
• D. B and C only

Hint:

 For photoelectric effect:
• Compare the radiation wavelength with the threshold wavelength.
• Only radiation with \(λ < λ_{threshold}\) causes emission.

Answer 1

The Correct Option is C

1. Threshold Wavelength:
   • Photoelectric emission occurs if \( \lambda < \lambda_{\text{threshold}} \).
   • Given \( \lambda_{\text{threshold}} = 5500 \, \text{Å} \).
2. Analysis of Radiation:
   • Infra-red radiation has \( \lambda > 5500 \, \text{Å} \) (no emission).
   • Ultra-violet radiation has \( \lambda < 5500 \, \text{Å} \) (emission occurs).

Options that are correct: C and D

Answer 2

Wavelength of infra-red = 700 nm (minimum)
Wavelength of UV = 100 – 400 nm 
Since we need λ < 5000 Å 
⇒ Only UV would be able to emit photoelectrons.