Department of Pre-University Education, KarnatakaPUC Karnataka Science Class 12

A Parallel Beam of Monochromatic Light of Wavelength 663 Nm is Incident on a Totally Reflecting Plane Mirror. the Angle of Incidence is 60° and the Number of Photons Striking the Mirror per Second is - Physics

Sum

A parallel beam of monochromatic light of wavelength 663 nm is incident on a totally reflecting plane mirror. The angle of incidence is 60° and the number of photons striking the mirror per second is 1.0 × 1019. Calculate the force exerted by the light beam on the mirror.

(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

Here ,

Wavelength of monochromatic light, λ = 663 xx 10^-9  "m"

Angle of incidence, θ = 60^circ

Number of photons per second, n = 1 xx 10^19

Momentum of photon,

p = h/λ,

where h is Planck's constant.

p = (6.63 xx 10^-34)/(663 xx 10^-9) = 10^-27

Force exerted on the wall,

F = n xx [pcosθ - (-pcosθ)] = 2npcosθ

= 2 xx 1 xx 10^19 xx 10^-27 xx 1/2

= 1.0 xx 10^-8  "N"

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HC Verma Class 11, Class 12 Concepts of Physics Vol. 2
Chapter 20 Photoelectric Effect and Wave-Particle Duality
Q 6 | Page 365