Question

63. Use Max Planck’s quantum theory to explain the following behaviour of photoelectrons.

a) Low-intensity light does not release any photoelectrons. What will happen if the light

is made brighter? Explain your reasoning.

b) Low-intensity light releases photoelectrons. What will happen if the light is made

brighter? Explain your reasoning.

c) Low-intensity light does not release any photoelectrons. What will happen if the

frequency of the light is gradually increased? Explain your reasoning.

Answer 1

a) The intensity of light is a quantity of the no of photons in the light.

For photo electrons to be produced, there must be a minimum energy(frequency) for the light (individual photon). Even though the no of photons is increased, if they do not have enough energy to create photo electrons, there will be no photo electrons.

b) Here, even though the light is of small intensity, they are producing photo electrons.

This means that the light has enough energy(frequency) to create photo electrons. So, when the intensity of light increases, the no of photons increases and consequently, the no of photo electrons increases.

c) As said earlier, the light must have a minimum energy(frequency) to create photo electrons.

When the frequency is slowly increased, the frequency will reach a point, where the light has enough energy to create photo electrons. After this frequency, photo electrons will be started to produce. But since the intensity of light is constant, the no of photo electrons will be constant after this cut off.