Light waves can move through , but they travel fastest when they move through a(n) .

Water, Vacuum

Explanation:

Light waves travels faster than the sound waves with a speed of 299792458 m/s or 3.0 *10 (power 8) m/s approx in vacuum. Light waves shows two different properties i.e reflection and refraction. When light waves travels from one medium to the other medium it shows the property of refraction and when light reflects back from a surface in the same medium it shows the property of the reflection.

The light waves travels slower in water than vacuum as water is denser medium than the vacuum. The speed of the light becomes slow on moving from rare to denser medium where it bends towards the normal.

On moving in vacuum there is no bending of the light wave so there is no interruption to it .

So,

Light waves can move throughwater , but they travel fastest when they move through a(n) vacuum .