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
'There comes a time when the mind takes a higher plane of knowledge but can never prove how it got there. All great discoveries have involved such a leap. The important thing is not to stop questioning.'