A 50Wm^ - 2 energy density of sunlight is incident normally on the...

The pressure exerted by the photons

$P = 0.75 \times \dfrac{I}{c} + 0.25 \times \dfrac{2I}{c}$ ( $P$ = pressure, $c$ = speed of light, $I$ = intensity )

$P = 1.25 \times \dfrac{I}{c}$

We know the force can be computed using

$F = P \times A$ ( $F$ = force, $P$ = pressure, $A$ = area )

$F = 1.25 \times$\left(\dfrac{50 \times 1}{3 \times 1{0}^{8}}\right)

$F = 20.83$\times$1{0}^{- 8}N$