The negative sign represents a decrease in intensity
Integrating equation 4.1 from x = 0 to x = l
Now, the molar concentration of the molecules, c can be given by:
Substituting for n in equation 4.4 and converting natural logarithm, ln into log10 gives:
is defined as the absorbance and 
is defined as the molar
absorption coefficient, denoted
by the Greek alphabet, ε. Therefore, equation 4.6 can be written as:
This equality showing linear relationship between absorbance and the concentration of the absorbing molecule (or chromophore, to be precise) is known as the Beer-Lambert law or Beer's law.
Transmittance is another way of describing the absorption of light. Transmittance (T) is simply the ratio of the intensity of the radiation transmitted through the sample to that of the incident radiation. Transmittance is generally represented as percentage transmittance (%T ):
