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Questions  >>  CBSE XII  >>  Physics  >>  Electromagnetic waves

Light with an energy flux of $20w/cm^2$ falls on a non- reflectiy surface has an area of $30\;cm^2$ , the total momentum delivered (for complete absorption ) during 30 minutes is

$\begin{array}{1 1} 36 \times 10^{-5}kg m/s \\ 36 \times 10^{-4}kg m/s \\ 108 \times 10^{4}kg m/s \\ 1.08 \times 10^{7} kg m/s \end{array} $

1 Answer

Solution :
Given $\phi = 20 w/cm^2$
$A= 30\;cm$
$t= 30 \;mins= 30 \times 60\;s$
Total energy falling on the surface in time t is $U= \phi At$
$\qquad= 20 \times 30 \times (30 \times 60)\;J$
Momentum of incident light $= \large\frac{U}{c}$
$\qquad= \large\frac{20 \times 30 \times (30 \times 60)}{3 \times 10^8}$
$\qquad= 36 \times 10^{-4}-0$
$\qquad=36 \times 10^{-4} \; kg m/s$
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