2. The Beer –Lambert Law
• When a monochromatic light of initial intensity Io passes
through a solution in a transparent vessel, some of the light
is absorbed so that the intensity of the transmitted light I
is less than Io .There is some loss of light intensity from
scattering by particles in the solution and reflection at the
interfaces, but mainly from absorption by the solution.
• Therelationship between I and Io depends on the path
length of theabsorbing medium, l, and the concentration of
the absorbing solution,c. These factors are related in the
laws of Lambert and Beer
3.
4. Lambert’s law
• When a ray of monochromatic light
passes throughan absorbing medium
its intensity decreases exponentially
as the length of the absorbing
medium increases.
5. Beer’s law :
• When a monochromatic light passes
through an absorbing medium its
intensity decreases exponentially as
the concentration of the absorbing
medium increases.
6.
7. Transmittance:
• The ratio of intensities is known as the
transmittance (T) and this is usually
expressed as percentage
8. Extinction
• If logarithms are taken of the
equation instead of a ratio then
9. • The expression log10 Io/I is known as the
extinction (E) or absorbance(A). The extinction is
some times referred as optical density.
• Therefore
• A (or) E = k cl
• where k is molar extinction co-efficient for the
absorbing material atwave length l, c = molar
concentration of the absorbing solution,l = path
length in the absorbing material in cm.
10. • If the Beer- Lambert law is obeyed
correctly and l is kept constant, then
a plot of extinction against
concentration gives a straight line
passing through the origin
• Extinction, E=log10 Io/I
