What is the basic principle of determining protein concentration by UV absorption?

The basic principle of determining protein concentration using ultraviolet absorption is that the aromatic amino acid residues in protein molecules (mainly tryptophan and tyrosine) and some other structural elements can absorb ultraviolet light (particularly at wavelengths around 280 nm). When ultraviolet light passes through a solution containing protein, the protein molecules absorb part of the ultraviolet light. By measuring the absorbance (A) of the solution, the protein concentration can be evaluated. According to Beer's law (Beer-Lambert law), absorbance is directly proportional to the concentration of the protein.

Specifically, Beer's law is described as: $A = ε \times c \times l$ Where:

$A$ is the absorbance.
$ε$ is the molar absorptivity, indicating the amount of light absorbed per mole of a particular substance at a specific wavelength.
$c$ is the concentration of the substance (moles/liter).
$l$ is the length of the path through which the light passes through the sample (usually 1 centimeter).