Sonar, radar
Why is sonar and not radar used in water for tracking objects at greater depths? Can sonar be used for tracking objects in air just as radar does?
Gaurav Awasthi, New Delhi
Radar uses high frequency radio waves and microwaves for communication whereas sonar uses high frequency sound waves. Though the processes involved in both the cases are same, there is a fundamental difference. Radio waves are electromagnetic waves whereas sound waves are mechanical waves. Energy loss of any wave in a medium arises from two important factors namely, scattering and absorption by the particles (atoms or molecules) present in the medium.
In air, radar system sends radio waves and upon reflection from the target, the reflected radio waves will be received. Since density of air is less, the absorption of radio waves by air molecules is very less. But the absorption of radio waves by sea water is very high especially at high frequencies. This is because water molecules and electrically charged ions effectively absorb radio waves. For example, sea water can absorb almost 99 per cent of high frequency radio waves within one metre length. Typically, submarines are inside the sea by several kilometers which makes it very difficult to communicate to them through radio waves. Even if one can try to communicate by low frequency (longer wavelength) radio waves, then the length of the antenna would be of the order of few thousand kilometres, which is practically not possible. Also, usage of low frequency radio waves results in slower data transfer.
However being mechanical waves, sound waves can propagate few hundreds of kilometers and water molecules and ions cannot absorb much. For example, 10 kHz radio wave can propagate only 6m inside sea whereas for the same frequency a sound wave can travel 38 km. However, the absorption of sound waves in air is very high when compared to radio waves. For example, the intensity of sound waves is 3,000 times reduced when compared with sea water. Hence usage of sound waves in air is limited only to short distances.
Dr. K. Saravanakumar, Assistant Professor of Physics, Department of Physical Sciences, Bannari Amman Institute of Technology, Sathyamangalam, Tamil Nadu
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