Why does a light wave travel more slowly through a gas than a vacuum?

Question

For my daughter science quiz.

Answer 1

Through any transparent or translucent material medium, like glass or air, it has a lower speed than in a vacuum; the ratio of c to this slower speed is called the refractive index of the medium (the actual speed is not affected, the speed of light is constant, the fact that the light is reflected around inside the glass will make a greater amount of time necessary for it to reach the next point). Changes of gravity, however, warp the space the light has to travel through,

Answer 2

Presently there is no absolute determination of the nature of light, be it wave or energy. It takes on the nature of both. However, it has been determined that it is infinitesimally affected by objects and medium. We currently do not know exactly why this should be so, since it seems to partake of 2 natures rather than 1.
Hoever, it does seem that even the tiny, distantly spaced molecules in gas are sufficient to have an effect upon light.

Answer 3

It's due to the refraction of the light through a medium. In a vacum, there is no refection. In air or water or whatever, the liught is bent and the speed is slowed.