A material's<unk>  shows the fraction of incident electromagnetic radiation absorbed by the material over a range of frequencies.  An<unk>  is, in a sense, the opposite of an emission<unk> .
Every chemical element has<unk>  lines at several particular wavelengths corresponding to the differences between the energy levels of its atomic orbitals. For example, an object that absorbs blue, green and yellow light will appear red when viewed under white light.<unk>  spectra can therefore be used to identify elements present in a gas or liquid. This method is used in deducing the presence of elements in stars and other gaseous objects which cannot be measured directly.
Absorption spectrum
Visible spectrum
Mass versus weight
Electromagnetic spectrum
Extraction ratio
Concentration
Saturation 
Percentage depth dose curve