The optical density   is generated from equation:
:Optical density= Log (Intensity of incident light / Intensity of<br /> Transmitted light)
In practical terms, a sample that contains no DNA or RNA should not<br /> absorb any of the ultraviolet light and therefore produce an OD of 0
Optical density= Log (100/100)=0
When using spectrophotometric analysis to determine the concentration of DNA or RNA,  the Beer–Lambert law is used to determine unknown concentrations without the need for standard curves. In essence, the Beer Lambert Law makes it possible to relate the amount of light absorbed to the concentration of the absorbing molecule. The following absorbance units to nucleic acid concentration conversion factors are used to convert OD to concentration of unknown nucleic acid samples:
:A260 dsDNA = 50 µg/ml 
:A260 ssDNA = 33 µg/ml
:A260 ssRNA = 40 µg/ml