Light’s lack of a longitudinal vibration
(vibration along its direction of travel)
is eventually explained by Albert Einstein—the speed
of light is constant.
Many frequencies are inherent in normal, incoherent light
in the range perceptible to human eyes,
and each of those is a blur of ordinary and extraordinary transverse waves.
Some people see ghosts, some see auras, some say they have second sight.
See also in The book of science:
Readings on wikipedia: