Question

Threshold frequency for a metal is 1015 Hz. If light of wavelength $ 4000\overset{\text{o}}{\mathop{\text{A}}}\, $ falls on its surface, then which of the following statement is correct?

• 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)

Wavelength of light, $ \lambda =4000\,\overset{\text{o}}{\mathop{\text{A}}}\, $ Frequency of light $ v=\frac{c}{\lambda }=\frac{3\times {{10}^{8}}}{4000\times {{10}^{-10}}}=0.75\times {{10}^{15}}Hz $ Which is less than the given threshold frequency. Hence, no photoelectric emission takes place.