Modern Physics 7 Question 4

4. Light is incident normally on a completely absorbing surface with an energy flux of 25Wcm2. If the surface has an area of 25cm2, the momentum transferred to the surface in 40min time duration will be

(a) 3.5×106Ns

(b) 6.3×104Ns

(c) 1.4×106Ns

(d) 5.0×103Ns

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Solution:

  1. Radiation pressure over an absorbing surface is, p=Ic where, I =intensity or energy flux

and c= speed of light.

If A= area of surface, then force due to radiation on the surface is

F=p×A=IAc

If force F acts for a duration of Δt seconds, then momentum transferred to the surface is

Δp=F×Δt=IAc×Δt

Here, I=25Wcm2,A=25cm2,

c=3×108ms1,Δt=40min=2400s

So, momentum transferred to the surface,

Δp=25×25×24003×108=5×103Ns



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