$A$ light with an energy flux of $18 \,W \,cm^{-2}$ falls on a non-reflecting surface at normal incidence. If the surface has an area of $20 \,cm^2$, the average force exerted on the surface during a $30$ minute time span is . . . . . . $\left(c=3 \times 10^8 \,ms^{-1}\right)$.

$A$ radiation of energy $E$ falls normally on a perfectly reflecting surface. The momentum transferred to the surface is ($c$ is velocity of light).

$A$ totally reflecting small plane mirror placed horizontally faces a parallel beam of light as shown in the figure. The mass of the mirror is $20 \, g$. Assume that there is no absorption in the lens and that $30 \%$ of the light emitted by the source goes through the lens. Find the power of the source needed to support the weight of the mirror in $MW$ (take $g = 10 \, m/s^2$ and $c = 3 \times 10^8 \, m/s$):-

Light with an intensity of $18 \ W \ m^{-2}$ is incident normally on an opaque surface. If the area of the surface is $20 \ m^2$,calculate the average force exerted on the surface in $30 \ min$.

$A$ parallel beam of monochromatic light of frequency $\nu$ is incident on a surface. The intensity of the beam is $I$ and the area of the surface is $A$. Find the force exerted by the light beam on the surface if it is perfectly reflecting and the light beam is incident at an angle of incidence $\theta$. (The speed of light is denoted as $c$.)

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 $1 \ \mu N$?