Question

Threshold frequency for a metal is ${10}^{15}Hz$. Light of $\lambda =4000\stackrel{\circ }{A}$ falls on its surface. Which of the following statements is correct

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

Answer 1

The correct option is A

No photoelectric emission takes place

Frequency of light of wavelength $(\lambda =4000\stackrel{\circ }{A})$ 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 emisssion takes place.