In audio signal, an echo (plural echoes) is a reflection of sound, arriving at the listener some time after the direct sound. So if the light starts to be echoed then ____________ what do you think ?
Dear Soudip Sinha Roy,
in my opinion, the first thing to ask is 'Why one should compare light and sound'? Once, it was admitted that a medium was necessary for sound as well as for light propagation. Thus it was quite obvious to compare light and sound propagation. The light-medium was the aether.
Although nowadays the mathematical description as waves is similar in both cases, it is known that -- differently from sound -- light can propagate through interstellar space, which is believed to be void. Thus the analogy between light and sound is probably not quite so strong.
The analogies are stronger when transmission is concerned. For example, to broadcast sound you need a soundbox. Again, to broadcast an electromagnetic signal you need an electrical soundbox (for instance a vacuum tube), besides an aerial (!). As another example, since it is uneasy to broadcast radio waves through water, sonars are deployed. In particular, to survey the depth of water an echo sounding is used, which records the echo returned from the sea bottom. From this viewpoint, a possible answer to you question is that the electrical instrument corresponding to the echo sounding is the radar. The lidar (https://en.wikipedia.org/wiki/Lidar) is the instrument in use for surveying in the optical range.
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