Inverse square law - the light "fall off"
The way that light decreases as you move away from the light source is expressed scientifically by the Inverse Square Law, which states:
‘When a surface is illuminated by a point source of light, the intensity of the light at the surface is inversely proportional to the square of the distance from the light source’.
Now, just before you panic this is really quite simple. It means that if your subject moves away from your camera’s flash, there is a fall-off in the light that they receive. In fact, if they are now twice as far away, they do not receive half as much light but one quarter. Four times the distance, one sixteenth of the light! This does not happen in sunlight because we are all the same distance from that big point source in the sky, the sun. But it is essential that the Inverse Square Law is applied when dealing with flash, other artificial lights, reflected light or window light as the visual effect of subjects at differing distances to the light source is uneven illumination.