Question

A plane electromagnetic wave of wave intensity 10ωm^–2 strikes a small mirror of area 20 cm², held perpendicular to the approaching wave. The radiation force on the mirror will be

(A) 6.6 × 10^–11N

(B) 1.33 × 10^–11N

(C) 1.33 × 10^–10N

(D) 6.6 × 10^–10N

Answer 1

The correct option is (C) 1.33 × 10^–10N.

Explanation:

 Intensity = I = [(power) / (area)]   hence power = I × area

∴ power = 10 × 20 × 10^–4 = 20 × 10^–3W

F = (dp / dt) = [(power) / (c)] where p is momentum.

Here the wave is reflected back by the mirror.

Momentum transferred to mirror per unit area per second 

= 2 × momentum of light stroking the mirror per unit area per second. 

∴ (dp / dt) = 2 × [(power) / (c)] 

∴ F = (dp / dt) 

= [(2 × power) / (c)] 

= [(2 × 20 × 10^–3) / (3 × 10⁸)] 

= 1.33 × 10^–10N.