Q. Light of wavelength λ strikes a photo sensitive surface and electrons are ejected with kinetic energy E . If the kinetic energy is to be increased to 2E, the wavelength must be changed to λ’ where

(a) λ’ = λ/2

(b) λ’ = 2λ

(c) λ/2 < λ’ < λ

(d) λ’ > λ

**Ans: c**

Sol: A/c to Einstein’s Photoelectric equation ,

$\large \frac{h c}{\lambda} = \phi + E $ ….(i)

$\large \frac{h c}{\lambda’} = \phi + 2 E $ ….(ii)

On dividing ,

$\large \frac{\lambda’}{\lambda} = \frac{\phi + E}{\phi + 2 E} $

Hence , $\large \frac{\lambda}{2} < \lambda' < \lambda $