Absorption is the method which energy of a photon is used by matter. Electromagnetic energy can be converted into internal energy of the absorbing matter. The absorption of light waves will not depend on the intensity of the light waves. However, in certain situations the medium changes its transparency dependency on the intensity of waves.
Absorbance indicates how much of the incident light was absorbed. Measurements of absorbance allow the identification of a substance via absorption spectroscopy. This is where the sample is illuminated from one side and the intensity of the light that exits from the sample at every direction is measured. Same examples of absorption spectroscopy in different arts of the spectrum are ultraviolet-visible spectroscopy, infrared spectroscopy and X-ray absorption spectroscopy.