a prefix occurring in loanwords from Greek, where it meant “two,” “twice,” “double” (diphthong); on this model, freely used in the formation of compound words (dicotyledon; dipolar) and in chemical terms (diatomic; disulfide).
visible
a : capable of being seen
b : situated in the region of the electromagnetic spectrum perceptible to human vision —used of radiation having a wavelength between about 400 nanometers and 700 nanometers
Are you having problems with double vision?
Some di- prefixes are of Greek origin, others of Latin origin...and sometimes an initial di- is not a prefix (divan, divot).