The reason why light falls off as the inverse square.  If we were out to 3 units there would be 3x3 or 9 squares.  If we were out to 4 units there would be 4x4 or 16 squares.  Think about what these means to the intensity of the light that went though one square at 1 unit away and 4 at 2 units away and 9 at 3 units away and 16 at 4 units away.