If a light wave moved from air into helium, what would happen to its speed

The light would most likely move faster in the helium than in the air.

That's not absolutely positively definite, because the speed of light

in air AND in helium depends on their pressure and temperature.

At sea-level pressure and 0°C temperature:

- - the light would be traveling at 299,704,645 meters per second in the air

and it would speed up to

- - 299,781,993 meters per second in the helium.

That's a gain of 77.348 kilometers per second, or 0.026 percent faster.

Woo hoo!

(using n=1.000293 in air and n=1.00003491 in helium)