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 $1 \ \mu N$?

Light with an average flux of $20 \, W/cm^2$ falls on a non-reflecting surface at normal incidence having a surface area of $20 \, cm^2$. The energy received by the surface during a time span of $1 \, minute$ is $............ \, J$.

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:

Radiations of intensity $0.5 \ Wm^{-2}$ are striking a metal plate. The pressure exerted on the plate is:

$A$ bulb of power $660 \,W$ radiates uniformly in all directions. The pressure exerted by the radiation on the surface at a distance of $5 \,m$ is

$A$ point source of light is placed at the centre of curvature of a hemispherical surface. The source emits a power of $24 \, W$. The radius of curvature of the hemisphere is $10 \, cm$ and the inner surface is completely reflecting. The force on the hemisphere due to the light falling on it is $.......... \times 10^{-8} \, N$.