In the case of the photo-electric effect, which of the following sentences is NOT TRUE?
a. The photo-electrons are emitted only when the frequency of incident radiation is greater than the threshold frequency.
b. There is no time lag between incidence of light and emission of electrons.
c. The rate of photo-electric emission is independent of the intensity of incident radiation.
d. The maximum kinetic energy of emitted photo-electrons depends on the frequency of incident radiation.

According to the laws of photoelectric effect:

1. Photoelectric emission occurs only when the frequency of incident light is greater than the threshold frequency.

2. The process is instantaneous, meaning there is no time lag between the incidence of light and the emission of electrons.

3. The rate of photoelectric emission (number of electrons emitted per second) is directly proportional to the intensity of the incident radiation.

4. The maximum kinetic energy of the emitted photoelectrons depends linearly on the frequency of the incident radiation and is independent of its intensity.

Statement c claims that the rate of photoelectric emission is independent of the intensity of incident radiation, which contradicts the laws of the photoelectric effect. Thus, statement c is not true.

Answer: c