Inverse square law

Last revised by Dr Daniel J Bell on 01 Apr 2020

The inverse square law describes the principle of dose reduction as the distance from the source increases.

This assumes a point source. If radiation spreads over a spherical area, as the radius increases, the area over which the dose is distributed increases according to


where A is the area and r is the radius of the sphere.

Therefore, the dose is proportional to the inverse of the square of the radius. Thus if you double the distance you reduce the dose by a factor of four.

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Cases and figures

  • Figure 1: inverse square law
    Drag here to reorder.
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