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Question:

Light is incident normally on a completely absorbing surface with an energy flux of 25 Wcm⁻². If the surface has an area of 25 cm⁻², the momentum transferred to the surface in 40 min time duration will be:

3.5 × 10⁻⁶ Ns

1.4 × 10⁻⁶ Ns

6.3 × 10⁻⁴ Ns

5.0 × 10⁻³ Ns

Solution:

The correct option is D
5.0 × 10⁻³ Ns
Pressure = I/c
Force = Pressure × Area = I/c . Area
Momentum transferred = Force . Δt = I/c . Area . Δt
= 25 × 10⁴ / 3 × 10⁸ × 25 × 10⁻⁴ × 40 × 60 → 5 × 10⁻³ Ns