$A$ light beam of intensity $20 \,W/cm^2$ is incident normally on a perfectly reflecting surface of sides $25 \,cm \times 15 \,cm$. The momentum imparted to the surface by the light per second is

Given below are two statements: one is labelled as Assertion $(A)$ and the other is labelled as Reason $(R)$.
Assertion $(A)$: The electromagnetic wave exerts pressure on the surface on which they are allowed to fall.
Reason $(R)$: There is no mass associated with the electromagnetic waves.
In the light of the above statements, choose the correct answer from the options given below:

How many photons of wavelength $\lambda = 6600 \ nm$ must strike a totally reflecting screen per second at normal incidence so as to exert a force of $1 \ \mu N$?

$A$ plate of mass $10 \text{ g}$ is in equilibrium in air due to the force exerted by a light beam on the plate. Calculate the power of the beam,if the plate is perfectly absorbing.

If the total energy transferred to a surface in time $t$ is $6.48 \times 10^5 \,J$, then the magnitude of the total momentum delivered to this surface for complete absorption will be:

$A$ beam of white light is incident normally on a plane surface absorbing $70 \%$ of the light and reflecting the rest. If the incident beam carries $10 \ W$ of power,the force exerted by it on the surface is