Question

A laser signal is sent towards the moon with a speed of light c and returns after a time t seconds. The distance of the moon from the observer is?(a) ct (b) (c t)/2 (c) ct-2 (d) ct-1​

Answer 1

A laser signal sent towards the moon returns after 't' seconds.

So,

Time taken to reach moon= Time / 2

= t/2

Therefore , Distance = Speed×Time

= c × t/2

= ct/t

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Answer 2

Concept

Revise the relation which will use Distance = Speed×Time.

Given

Speed of the laser beam is c and time taken by the laser beam to travel =is t sec.

To find

The distance of the moon from the observer.

Solution

The time taken by the laser beam to return to the observer is t

then, the time is taken by the laser beam to reach the moon = t/2

Now we know Distance = Speed×Time

In this case, Speed = c and Time = t

So, distance = (c×t)/2

Hence, the correct answer is an option (b) (ct)/2.