Answer:
Speed.
Their speeds are different; Have you ever heard the metaphor the speed of light? For this case, we must turn that into a reality. So, people refer to light speed as the fastest travelling thing in the world, and that is right, unless there is a thing faster than light that was never classified. Unlikely the case. The speed of sound through air travels a lot slower than light. Look at the diagrams below to see what I mean by that.
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As shown in the diagrams, sound speed is WAY slower than light speed!