First slide

Electromagnetic radiation - Dual nature of radiation and matter

Question

A totally reflecting, small plane mirror placed horizontally faces a parallel beam of light as shown in the figure. The mass of the mirror is 20 g. Assume that there is no absorption in the lens and that 30% of the light emitted by the source goes through the lens. The power of the source (in MW) needed to support the weight of the mirror is _____

Moderate

Solution

As 30% light is incident and reflected by mirror, force due to reflection of light on mirror is
$F = 2 \times \frac{0.3 P}{c} = \frac{0.6 P}{c}$
$\begin{array}{l} To support the weight of mirror, we use \frac{0.6 P}{c} = m g \\ \Rightarrow P = \frac{m g c}{0.6} = \frac{20 \times 10^{- 3} \times 10 \times 3 \times 10^{8}}{0.6} = 100 \times 10^{6} watt =100MW \end{array}$

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