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.
Correct Answer: D
Solution :
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.
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