Question

Threshold frequency for a metal is `10^(15) Hz`. Light of `lambda = 4000 Å` 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

Correct Answer - A
Frequency of light of wavelength `(lambda = 4000 Å)` is `v = ( c )/(lambda) = ( 3 xx 10^(8))/( 4000 xx 10^(-10)) = 0.75 xx 10^(15)` which is less than the given threshold frequency . Hence no photoelectric emission takes place.