Is the speed of light photons always constant?
Yes, but only in a vacuum.
1750
No, it depends on the facility.
1567
A speed of light is always constant.
2287
The speed at which light propagates through transparent materials, such as glass or air, is less than c. However, this "slowing of the light" is only due to the fact of photos being absorbed and re-emitted after a slight delay between atoms. When a photon is emitted, it's speed is always c. In exotic materials like Bose–Einstein condensates near absolute zero, the effective speed of light may be only a few metres per second.