Nafea,
I copied this from a google search. Hope it helps!
The spectrophotometer is an instrument which measures the amount of light of a specificed wavelength which passes through a medium. According to Beer's law, the amount of light absorbed by a medium is proportional to the concentration of the absorbing material or solute present.
Go here for more information:
http://biology.clc.uc.edu/fankhauser/Labs/Microbiology/Growth_Curve/Spectrophotometer.htm