First slide

Momentum and Radiation pressure

Question

Light with energy flux of 18 W/cm² falls on a non reflecting surface of area 20 cm² at normal incidence. The momentum delivered in 30 minutes is

Moderate

A

1.2 × 10^–6 kgms^–1
B

2.16 × 10^–3 kgms^–1
C

1.8 × 10^–3 kgms^–1
D

3.2 × 10^–3 kgms^–1

Solution

Energy flux = 18 Wcm^–2 = 18 x10⁴ Wm^-2
Total energy incident
U = flux x Area x time
= 18 x10⁴ x 20 x 10^-4 x 30 x 60 = 18 x 36 x 10³
Momentum delivered
$P = \frac{U}{C} = \frac{18 x 36 x 10^{3}}{3 x 10^{8}} = 2.16 x 10^{- 3} kg {ms}^{- 1}$

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