NEET 2020 Physics Question 150 Solution

The radiant flux (or intensity $$I$$) of an electromagnetic wave is defined as the radiant energy falling per unit area per unit time perpendicular to the beam: $$I = \frac{U}{A \cdot t} = \frac{P}{A}$$ where $$U$$ is total energy, $$A$$ is surface area, and $$t$$ is time duration.

Given: - Flux $$I = 20\text{ W cm}^{-2}$$ - Area $$A = 20\text{ cm}^2$$ $$P = I \cdot A = (20\text{ W cm}^{-2}) \times (20\text{ cm}^2) = 400\text{ W} = 400\text{ J s}^{-1}$$ (Notice that the units of $$\text{cm}^2$$ cancel immediately, avoiding tedious conversion to $$\text{m}^2$$).

Given time span $$t = 1\text{ minute} = 60\text{ seconds}$$. $$U = P \cdot t = (400\text{ J s}^{-1}) \times (60\text{ s}) = 24000\text{ J} = 24 \times 10^3\text{ J}$$ This confirms Option (3).