We know that many organic molecules do not absorb electromagnetic radiation in the ultraviolet-visible (UV-VIS) regions (~wavelengths range 190 nm - 800 nm). However, there are some compounds which absorb a portion of the radiation when a radiation passes through them.

There is a relationship between the amount of light absorbed and the distance the light travels through the absorbing medium. This relationship is known as the Lambert law (Bouguer-Lambert law). According to the Lambert law, each layer of equal thickness of the medium absorbs an equal fraction of the light traversing it. In other words, the fraction of light absorption is proportional to the sample path length. Say, a beam of monochromatic radiation of intensity Io, falls on a sample. For a traversal of a small path length dℓ (with unit cross-section, ℓ is the path length) the decrease in the intensity is dI due to the absorption of radiation. Then according to the Lambert law,

\begin{equation} dI_λ / I_λ \propto -dl \end{equation}

or

\begin{equation} dI_λ / I_λ = -K_λ.dl \end{equation}

where I _λ is the intensity in the wavelength interval λ and λ+dλ and k _λ is a constant, practically independent of path length but depends on the wavelength, and is known as the absorption coefficient. Rearrangement and integration yield the following equation:

\begin{equation} I_λ / I_{O,λ} = exp (-K_λ l) \end{equation} where I _λ intensity of the beam of radiation leaving the sample after absorption. For simplicity let us drop the suffix, λ, and express the above equation as follows: \begin{equation} -log_e (I/I_o) = kl \end{equation}

The fraction of the amount of radiation absorbed is known as transmittance, T = I/I ₀ . T is often measured in terms of % transmittance, %T = 100×T. Another parameter, absorbance, A, is defined as, \begin{equation} A = -log_{10} T = log_e (I/I_o) / 2.303 = kl/2.303 \end{equation}

Therefore, A = (constant)× ℓ. The measurement of A at a particular wavelength gives a measure of the absorption capacity and in turn transparency of the medium with respect to the wavelength. By using a spectrophotometer and a few cuvettes of different path lengths, one can verify this law that the fraction of the light absorbed is proportional to the path length. In other words, one can find that the absorbance (A) varies linearly with the path length, ℓ.