Question

In experiments concerned with the photoelectric effect, which of the following will increase the kinetic energy of an electron ejected from a metal surface? 1. increasing the wavelength of the light striking the surface 2. increasing the frequency of the light striking the surface 3. increasing the number of photons of light striking the surface A) 1 only B) 2 only C) 3 only D) 1 and 3 E) 1, 2, and 3

Answer 1

Ans: 2 Only.

The maximum kinetic energy E_max of an ejected electron is given by

E_max = hν - W

where h is the Planck constant and ν is the frequency of the incident photon. The term W is the work function which gives the minimum energy required to remove a delocalized electron from the surface of the metal.

If the wavelength (λ) of the incident radiation is increased frequency (ν) will be decreased, as ν = c/λ. So E_max will decrease.

Increasing the frequency of the incident beam, keeping the number of incident photons fixed (i.e. keeping intensity of the incident light constant) increases the maximum kinetic energy of the photoelectrons emitted. Here hν in the equation increases, so E_max also increases.

If the intensity of the incident radiation of a given frequency is increased, there is no effect on the kinetic energy of each electron. For a given metal and frequency of incident radiation, the rate at which photoelectrons(not the kinetic energy) are ejected is directly proportional to the intensity of the incident light. An increase in the intensity of the incident beam (keeping the frequency fixed) increases the magnitude of the photoelectric current.