Question

Threshold frequency for a metal is ${10}^5$ Hz. If light of wave length $4000\stackrel{o}{A}$ falls on its surface then among the following correct statement is:

• A. Photo electrons come out with zero speed
• B. Photo electrons come out with ${10}^{3}m/s$ speed
• C. Photo electrons come out with ${10}^{5}m/s$speed
• D. No photo electric emission takes place.

Answer 1

Answer: (D)

No photo electric emission takes place.

Frequency of light of wavelength $(\lambda =4000A^o)$ is

$v=\frac{c}{\lambda }=\frac{3\times {10}^8}{4000\times {10}^{-10}}=0.75\times {10}^{15}$ which is less than the given threshold frequency. Hence no photoelectric emission takes place.