1. The illuminance of a surface illuminated by light falling on it perpendicularly from a
point is inversely proportional to the square of the distance between the surface and
2. If the rays make an angle x with the normal to the surface, the illuminance is
proportional to cos(x).
3. (Also called Bouquerï¿½s law) The luminous intensity (I) of light decreases exponentially
with the distance d that it enters an absorbing medium i.e.
I = Io exp(-z d)
Where Io is the intensity of the radiation that enters the medium and z is its linear
absorption coefficient. These laws were first stated (for light) by Johann H. Lambert.
Answered by: Dan Summons, Physics Undergrad Student, UOS, Souhampton
'Arrows of hate have been shot at me too, but they have never hit me, because somehow they belonged to another world with which I have no connection whatsoever.'