Question

A beam of white light is incident normally on a plane surface absorbing 70% of the light and reflecting the rest. If the incident beam carries 10 W of power, find the force exerted by it on the surface.

Answer 1

Explanation:

• Incident beam’s power, P = 10 watt
• Relation between momentum (p) and wavelength λ: λ = hp,
• Here, h is constant of Planck
• ⇒ p = hλ
• When both the sides are divided by t, we obtain: pt = hλt   …1
• Energy, E = hcλ
• Let us consider that power is shown as P. Then,
• P = pct [Using equation1]
• ⇒ Pc =pt  
• Force,F = Pc = pt      
• Since F = Momentum  
• Time Force, F = 710(absorbed) + 2×310(reflected)
• F = 710 × Pc + 2 × PcF × 310 = 133 × 10-8 = 4.33 × 10-8 N.