Irradiance of electromagnetic radiation from a point source of radiation decreases with distance from the source and obeys the inverse square law.

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When any kind of radiation (radioactive particles from a source, or electromagnetic waves) is incident on a surface, its irradiance, , is defined as the **power per unit area**. This relationship is represented by the following equation:

where:

- is irradiance
- is power
- is area

As the distance from a point source of radiation increases, the irradiance decreases. The relationship between irradiance, , and distance, , can be shown to follow an inverse square law.

The product of irradiance and the square of the distance from the source is a constant, .

As this product is constant, it follows that for two points at distances and from a point source of radiation:

Remember that a point source emits radiation in **all** directions.