A perfectly reflecting mirror of mass 2.0 g is illuminated normally by a 100 W laser beam for 100 s. Assume the mirror remains nearly at rest compared with the speed of light during this interval and neglect all other forces. What speed does the mirror acquire approximately?
Options

Question

A perfectly reflecting mirror of mass 2.0 g is illuminated normally by a 100 W laser beam for 100 s. Assume the mirror remains nearly at rest compared with the speed of light during this interval and neglect all other forces. What speed does the mirror acquire approximately?

Options

G1 Physics · Accepted Answer

For perfect normal reflection, the radiation force is F = 2P ÷ c. Substitute: P = 100 W, c = 3×10⁸ m·s⁻¹. Thus, F = 200 ÷ (3×10⁸) ≈ 6.67×10⁻⁷ N. Mirror mass: m = 2.0 g = 2.0×10⁻³ kg. Acceleration: a = F ÷ m = 6.67×10⁻⁷ ÷ 2.0×10⁻³ ≈ 3.33×10⁻⁴ m·s⁻². After 100 s: v = at ≈ 3.33×10⁻⁴×100 = 3.33×10⁻² m·s⁻¹. Correct option: B.