Question

Light with an energy flux of 18w/m² or Wm^–2 falls on a non-reflecting surface at normal to surface. If the surface has an area of 20m². The average force exerted on the surface during 30 minutes is _______

(A) 6.48 × 10⁵N 

(B) 3.60 × 10²N

(C) 1.2 × 10^–6N

(D) 2.16 × 10^–3N

Answer 1

The correct option is (C) 1.2 × 10^–6N.

Explanation:

Average force by light on the surface is given by

f_avg = (Δp / Δt)  ........(1)

Δt = 30 min = 30 × 60 = 1800 sec

Δp = change in momentum

Given intensity = I = (18W / m²)

∴ Total power absorbed = I × area of surface

= 18 × 20

= 360 W

from (1)

F_avg = [(– E) / (c ∙ Δt)]

= [(– power) / c]

= [(–360) / (3 × 10⁸)] ------ power = [(Energy) / (time)]

 = – 120 × 10^–8

F_avg  = – 1.2 × 10^–6N

 |F_avg| = 1.2 × 10^–6N