A light year is the DISTANCE light travels through vacuum in 1 year.
If light is traveling through vacuum, then it's traveling at the speed of light in vacuum. If a student at home at the beginning of the trip is holding the clock, then ...
Traveling 1 light year takes 1 year.
Traveling 2 light years takes 2 years.
Traveling 3 light years takes 3 years.
Traveling 10 light years takes 10 years.
If the light is traveling through some other substance, or if the clock is traveling along with the light, then these numbers all change.
YOU cannot travel at the speed of light. We have to just leave it at that
