Question

Light of wavelength `lambda` 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 `lambda'` where ____________.

Options

• `lambda'  = lambda/2`

• `lambda'  = 2lambda`

• `lambda/2 < lambda'  < lambda`

• `lambda'  > lambda`

Answer 1

Light of wavelength `lambda` 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 `lambda'` where `lambda/2 < lambda'  < lambda`.

Explanation:

`"E" = "Hc"/lambda - phi and 2"E" = "Hc"/lambda'  - phi`

`therefore (lambda')/lambda = ("E" + phi)/(2"E" + phi)`

`therefore lambda'  = lambda ((1+ phi/"E")/(2 + phi/"E"))`

`"since"  (1+ phi/"E")/(2 + phi/"E") > 1/2, lambda'  > lambda/2`