(1) The illuminance of a surface illuminated by light falling on it perpendicularly from a point source is inversely proportional to the square of the distance between the surface and the source.
(2) If the rays make an angle θ with the normal to the surface, the illuminance is proportional to cosθ.
(3) (Also called Bouquer’s law) The luminous intensity (I) of light (or other electromagnetic radiation) decreases exponentially with the distance d when it enters an absorbing medium, i.e.
where I0 is the intensity of the radiation that enters the medium and α is its linear absorption coefficient. These laws were first stated (for light) by Johann Lambert.