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Light with an energy flux of 25×10⁴ W m⁻² falls on a perfectly reflecting surface at normal incidence. If the surface area is 15 cm², the average force exerted on the surface is

Asked in NEET 2014 · Force and radiation pressure

Answer: (2) 2.50×10⁻⁶ N

Step-by-step solution

For a perfect reflector the radiation pressure is (2I)/c, twice the absorbing value.

The area is A=15 cm²=15×10⁻⁴ m².

F=(2IA)/c=(2×25×10⁴×15×10⁻⁴)/(3×10⁸).

The numerator is 2×25×15×10⁰=750, so

F=(750)/(3×10⁸)=2.5×10⁻⁶ N.

Why the other options are wrong

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