
If you look at Huygens, light is a WAVE,
as confirmed by the double-slit experiments.
The Big Bang Theory Season 1, Episode 1

How does this gravity WAVE
breakthrough
The Big Bang Theory Season 7, Episode 20

Guess a WAVE function and vary its parameters
until I found the lowest energy solution.
The Big Bang Theory Season 8, Episode 2

Yes, it's the apparent change
in the frequency of a WAVE...
The Big Bang Theory Season 1, Episode 6

...caused by relative motion between
the source of the WAVE and the observer.
The Big Bang Theory Season 1, Episode 6