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

Answer 1

Answer: (C)

C and D only

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.