Q: The plane electromagnetic wave of wave intensity 6 W/m2 strikes a small mirror of area 30 cm2 , held perpendicular to the approaching wave . The momentum transferred in kg m/s by the wave to the mirror each second will be
(a) 1.2 × 10-10
(b) 2.4 × 10-9
(c) 3.6 × 10-8
(d) 4.8 × 10-7
Sol: Momentum transferred in one second by EM wave to mirror is
$\large p = \frac{2 I A }{c} $
$\large p = \frac{2 \times 6 \times (30 \times 10^{-4})}{3 \times 10^8} $
= 1.2 × 10-10 kg m/s
Radiation force on mirror = Momentum transferred per second
= 1.2 × 10-10 N