  We measured photon absorption in dielectric media and proposed the
photon-version Beer-Lambert's law to quantify the absorption. We used a
Hong-Ou-Mandel interferometer and 810 nm twin-photons. We found that the depth
ratio of the null point in the interference patterns of the interferometer
agreed with the classical transmittance of the samples. We established a
statistical model of the photon absorption process and proposed an information
entropy interpretation to understand the meaning of the Beer-Lambert law.
Comparisons of the results of the photon absorption experiments with classical
experiments demonstrate the validity of our model and interpretation.
