Question

Assertion: The number of electrons ejected from a metal surface depend upon the frequency of light.
Reason: There is a time lag between the striking of light beam and the ejection of electrons from the metal surface.

• A. If both assertion and reason are true and reason is the correct explanation of assertion.
• B. If both assertion and reason are true but reason is not the correct explanation of assertion.
• C. If assertion is true but reason is false.
• D. If both assertion and reason are false.

Answer 1

Answer: (D)

Photoelectric effect is defined as the ejection of electrons from the metal surface as soon as the beam of light of appropriate energy (above a threshold) strikes the surface.
In photoelectric effect, a given metal has characteristic threshold frequency $\left(v_0\right)$ that is required for ejection of electron.
Threshold energy is called the work function $(h{v}_0)$ of the metal and the excess of energy gets converted to the kinetic energy of the photoelectron.
Since each photon can eject one electron from the surface, the number of electrons ejected will therefore depend upon the number of photons strike per unit area or the intensity of light and not on the energy/frequency of light.
As excess of energy will change the kinetic energy of ejected photon and not their number.