There are many compounds which absorbs visible as well as ultraviolet light. If for example there is a radiation which has power Q0, and is directing to a sample and if the beam emitted by the sample is Q then that means absorption has occurred. There are many ways in which we can measure the amount of radiation being absorbed.
A = log10 Q0 / Q and A is called as absorbance and its unit less because radiation is divided and units cancels out. In this way we can say if there is no absorption then value of absorbance is zero and percentage of transmittance is hundred percent, that is no change in radiation. But if there is no light coming from the sample that means whole light is absorbed and we can say there is zero value of transmittance and infinite value of absorbance.
We express absorbance in The Beer-Lambert Law and its very simple.
Here A is absorbance which has no unit, e? is the molar absorption and b is the length of the path of the sample placed, c is the concentration of the matter which is in the sample.