First slide

Quantum Theory of Light

Question

At what rate should light energy be allowed to fall normally on a perfectly reflecting mirror with $1 c m^{2}$ of area for one hour such that the force that acts on the mirror is $6.7 \times 10^{- 8} N ?$

Moderate

A

$\frac{1 W}{c m^{2}}$
B

$\frac{8 W}{c m^{2}}$
C

$\frac{10 W}{c m^{2}}$
D

None of these

Solution

If I be the intensity of light, Then
$F = \frac{2 I A}{C} \Rightarrow I = \frac{F C}{2. A} = \frac{6.7 \times 10^{- 8} \times 3 \times 10^{8}}{2 \times 1} W / c m^{2}$
$\Rightarrow I = 10 W / c m^{2}$

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