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Wave Optics

Question
CBSEENPH12039875
Wired Faculty App

Light with an energy flux of 25 x 104 W/m2 falls on a perfectly reflecting surfaces at normal incidence. If the surface area is 15 cm2, the average force exerted on the surface is,

  • 1.25 x 10-6 N

  • 2.50 x 10-6 N

  • 1.20 x 10-6 N

  • 3.0 x 10-6 N

Solution

A.

1.25 x 10-6 N

Energy flux = fraction numerator 25 space straight x space 10 to the power of 4 space straight J over denominator sm squared end fraction
Force on unit area = momentum transferred in unit time on unit area = fraction numerator 25 space straight x space 10 to the power of 4 over denominator straight c end fraction
space equals space fraction numerator 250 space straight x space 10 to the power of 4 over denominator 3 space straight x space 10 to the power of 8 end fraction equals space 8.3 space x space 10 to the power of negative 4 end exponent space N divided by m squared
Force on the 15 x 10-4 m2 area,
        = 8.3 x 10-4 N/m2 x 15 x 10-4 m2

        = 124.5 x 10-8 N
        = 1.25 x 10-6 N

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