Transmittance:
However, if the concentration is expressed as mol dm 3 and b in cm then it is called molar absorptivity (earlier called molar extinction coefficient) and expressed as ε. Its units are c-1, mol-1 dm3. The modified expression for the Beer-Lambert's law becomes
log P0/P =εbc
The equation log Po/P is called absorbance and is represented as 'A'.
A = log P0/P
After that the expressions for Beer-Lambert's law then becomes,
log P0 /P = A = abc or εbc
The absorbance, A is related to another important term called transmittance which is described as the fraction of the incident radiation transmitted by the absorbing medium. As mathematically,
Transmittance, T = P/ P0
It is commonly expressed as a percentage and is expressed as,
Percentage transmittance, %T =P/P0 ×100%
It is related to absorbance as, A = - log T
In typical measurements the radiant power transmitted by a solution is measured and compared with that observed with solvent (also called a blank). The ratio of transmitted powers through solution and that through blank or solvent is called as transmittance.
It is desirable that you get acquainted with the expressions of the Beer-Lambert's law and learn to use them. Now let us take a few examples illustrating the application of these expressions.