Properties of EM Waves: Practice Problem & Solution
Light with an energy flux of $25 \times 10^4 W/m^2$ falls on a perfectly reflecting surface at normal incidence. If the surface area is $15 cm^2$, the average force exerted on the surface is: (2014)
Solution Explained:
To solve this problem, we apply the core principles of Properties of EM Waves. Understanding the underlying formula is key to arriving at the correct answer below:
Total power incident is $P = Flux \times Area = (25 \times 10^4) \times (15 \times 10^{-4}) = 375 W$.
For a perfectly reflecting surface, the radiation pressure is $P_{rad} = \frac{2I}{c}$, so the force is $F = \frac{2 P}{c}$.
$F = \frac{2 \times 375}{3 \times 10^8} = \frac{750}{3 \times 10^8} = 2.50 \times 10^{-6} N$.
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