Check out this interactive stereochemistry tutorial on the ChemEdDL
The optical purity or the enantiomeric excess (ee%) of a sample can be determined as follows:
Optical purity = % enantiomeric excess = % enantiomer1 - % enantiomer2
= 100 [a]mixture / [a]pure sample
where [a] is the specific rotation
ee% = 100 ([R]-[S]) / ([R]+[S])
where [R] = concentration of the R-isomer
[S] = concentration of the S-isomer
Diasteromeric substances can have different rotations both in sign and in magnitude.
Look at some problems like these more in depth.
Consider that (S)-bromobutane has a specific rotation of +23.1o and (R)-bromobutane has a specific rotation of -23.1o