How is the intensity of radiation determined?

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(A) The intensity of radiation is defined as the amount of energy incident per unit area per unit time.In terms of the particle nature of light (photo

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By the number of photons incident per unit area per unit time.

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(A) The intensity of radiation is defined as the amount of energy incident per unit area per unit time.In terms of the particle nature of light (photons),the intensity is directly proportional to the number of photons incident on a surface per unit area per unit time.Mathematically,if $N$ photons of frequency $
u$ are incident on an area $A$ in time $t$,the intensity $I$ is given by $I = \frac{N \cdot h
u}{A \cdot t}$.Since $h$ and $
u$ are constant for a given radiation,the intensity depends on the number of photons $N$ per unit area per unit time.

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