if photons have the same speed in all reference frames according to GR, why do slomo films of light beams exist?

Question

if photons have the same speed in all reference frames according to GR, why do slomo films of light beams exist?

In: Physics

Answer 1

Slow motion videos have nothing to do with reference frames or how time behaves.
You just take pictures very quickly and play them much slower later. That work with any process, as it is just a playback thing, no physical process.

And as a side note: light also actually slows down if it passes through a material. So inside glass light is slower than in vacuum (thats the reason for refraction and is the reason lenses work).