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, - Physics

Sum

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.

(Use h = 6.63 × 10-34J-s = 4.14 × 10-15 eV-s, c = 3 × 108 m/s and me = 9.1 × 10-31kg)

Solution

Power of the incident beam, P = 10 watt

Relation between wavelength (λ) and momentum (p):

λ = h/p ,

where h is Planck's constant

⇒ p = h/λ

On dividing both sides by t , we get :

p/t = h/(λt)   ...(1)

Energy,

E = (hc)/λ

⇒ E/t = (hc)/(λt)

Let P be the power . Then,

P = E/t = (hc)/(λt)

P = (pc)/t ................["Using equation (1)"]

⇒ p/c = p/t

Force ,

F = p/t = p/c       ("Since F" = "Momentum"/"Time")

"Force, F" = 7/10 " (absorted)" + 2 xx 3/10 ("reflected")

F = 7/10 xx P/c + 2 xx 3/10 xx P/c

F = 7/10 xx 10/(3 xx 10^8) + 2 xx 3/10 xx 10/(3 xx 10^8)

F = 13/3 xx 10^-8 = 4.33 xx 10^-8  "N"

Is there an error in this question or solution?

APPEARS IN

HC Verma Class 11, Class 12 Concepts of Physics Vol. 2
Chapter 20 Photoelectric Effect and Wave-Particle Duality
Q 7 | Page 365
Share