For a photosenstive surface, threshold wavelength is `lambda_0`. Does photoemission occur if the wavelength `(lambda)` of the incident radiation is (i) more than `lambda_0` (ii) less than `lambda_0`? Justify your answer.

Explanation

The maximum kinetic energy of the emitted photoelectron from a metal surface is given by
`K_(max)=1/2mv_(max)^2=(hc)/lambda-(hc)/(lambda_0)=hc((lambda_0-lambda)/(lambda lambda_0))`
(i) When `lambdagtlambda_0, K_(max)` is positive, `v_(max)` is imaginary. Thus photoelectric emission will not occur.
(ii) When `lambdaltlambda_0,K_(max)` is positive, `v_(max)` is positive. Thus photoelectric emission will take place and K.E. of photoelectron increases as `lambda` decrease.