**Beer-Lambert law: When light is passed through a medium
and comes out at the other side, its intensity will be reduced, because part of
the light will be absorbed by the medium. The actual amount of light absorbed
by the medium is governed by the Beer-Lambert law. The derivation of the law is
as follows:**** **

*Lambert’s law*: (This
law was actually observed by Pierre Bouguer in 1729. Its origin is wrongly
attributed to Johann Heinrich Lambert, who only quoted from Bouguer’s work in
1760). This law says that the decrease in intensity (dI) when light passes
through a medium of thickness ‘dx’ is directly proportional to (a) the
intensity of the incident radiation ‘I’ and (b) the thickness of the medium.
This may be mathematically expressed as:** **

**-dI a I.dx
or -dI =
k**_{1}.I.dx

**where ‘k’ is the proportionality
constant. The minus sign indicates that the intensity decreases as light passes
through the medium.**

*Beers Law*: (August Beer, 1852): This law says
that the decrease in intensity (dI) when light passes through a solution of
concentration ‘c’ is directly proportional to the concentration. This may be
mathematically expressed as:

**-dI a c or -dI =
k**_{2}.c

** ****Combining the two relationships, we get:**

**-dI
a
I.c.dx or -dI
= k.I.c.dx or **

**
Samples in spectroscopy are usually in the form of
solutions.**