Even through an ordinary transparent material such as glass or water, the speed of light slows down slightly because light interacts with the atoms that make up the material. But in this case, the effect is weak, and any attempt to enhance it will lead to light absorption. Therefore, it is important to slow down the speed of light so that it will not be absorbed. Welch did this through the chamber. This method places rubidium atoms in a very delicate quantum state in which rubidium atoms cannot absorb light.At the same time, the interaction of the two beams produces another beam of light with a long wavelength, which travels much slower than the original two beams.
not part of Welch's experiment?
Slow down the speed of light so that it will not be absorbed