What is optical density?
Optical density is standardly defined as the measure of the transmittance of an optical medium, defined as any medium through which light can pass, at a certain set wavelength. This is very important in a number of fields such as biology and chemistry.
One example is in biology, where optical density measures can be used to create a standardized curve that will allow an experimenter to determine the amount of protein present in a sample. This kind of measurement is known as a bradford protein assay, and utilizes a spectrophotometer to record the necessary measurements.
Another example is the use of optical density to determine how much bacteria is present in a sample when conducting microbiology experiments. Similarly to measuring protein concentrations, a standardized curve can be generated using specific techniques. This curve can then also be used to determine how much bacteria is present similar to a bradford assay. The importance of this is it allows experimenters to both use and record optimal amounts of bacteria for various studies, or genetic manipulations, increasing replicability. Hope this helps!