[arbitrandomuser]

hydrogen and deuterium also count as different groups when accounting for optical activity

carbon forms 4 bonds , if the 4 things bound to carbon are different then the molecule can be assembled in two ways ( mirror images like your left and right hand , similar but distinct as no rotation of your left hand can make it a right hand) and the two different arrangements rotate the polarized light in different ways . so if you have say CHDClF (carbon bonded to hydrogen , deuterium , chlorine and flourine) it is optically active

it was interesting to learn this , i thought it was a purely electromagnetic phenomena and the mass of the nucleus would not be relevant

but iodine 127 and iodine 131 do not have enough of a difference to show detectable optical activity