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(Not to be that guy, but if we're giving you the words, they're not your own words, are they?)
because space is very large and the speed of light is the max. speed we know this time so we use light and use terms like light years etc.......:)
the light is to find the distance between the planets called light year
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well put on the light and you can see whats occupying the space ... (THESE WORDS ARE MY OWN)
because scientist believe space is vaccum and only light can travel in vaccum and light is EM wave. other types of wave that travels in space is Gravitational wave. but GW wave has very less amplitude when travels too difficult to measure. however, visible light spectrum is Em wave. so east to carry out space experiments .