Question

If an average force is exerted by a light wave on a non-reflecting surface of area during a 40-minute time span, the energy flux of light just before it falls on the surface is (round off to the nearest integer). (Assume complete absorption and normal incidence conditions are there)

Options:
a. The energy flux is independent of time.
b. The energy flux increases with time.
c. The energy flux decreases with time.
d. The energy flux remains constant during the time span.

Answer 1

The energy flux or intensity of light on a non-reflecting surface is the average power per unit area and remains constant over time in this scenario because the light imparts momentum to the surface without change. Therefore, the correct option is D.

Step-by-step explanation:

The question pertains to the energy flux of light when it is absorbed by a non-reflecting surface. The energy flux of light, also known as the intensity (I), is defined as the average power per unit area. It is given by the time-averaged Poynting vector and is equal to the energy transported per unit area per unit time. In this scenario, the light wave imparts momentum to the absorbing surface, but since the light is completely absorbed and not reflected, the force experienced due to the light (and hence the momentum transfer) remains constant for the duration of the light's interaction with the surface. Consequently, since the force, area, and the wave's properties do not change with time, the energy flux remains constant as well.

Therefore, the correct answer to the question, which asks for the behavior of the energy flux of light just before it falls on the surface during a 40-minute time span, would be:

• (d) The energy flux remains constant during the time span.