65) In the outer space astronauts talk to each other through radios. Why?

Ans: sound waves need a material medium for its propagation. There is no air in space and hence,sound waves cannot travel. However, radio waves can travel through space. The astronauts can see each other because light, like radio waves, can travel through vacuum.

66) Sonar scanners are used by doctors. Explain.

Ans: sonar scanners send out ultrasonic waves, which are reflected by body tissue and organs. From the pattern of reflections, a computer can build up an image of the internal structure which is vital for diagnosis.

67) Why the flash of lightning is seen before the sound of thunder is heard?

Ans: the velocity of light is much greater than that of the sound. Therefore, flash of lightning is seen before the sound of thunder is heard.

68) Bats have poor eyesight but are able to home their prey with great accuracy. Also, dolphins can avoid fishing nets and can detect fish at night. Explain

Ans: Bats emit high frequency (1, 20,000 Hz) sound waves and listen with their sensitive ears for any echoes. From the time taken to hear the echo and from the nature of sound received. Bats are able to estimate the distance and the type of surroundings. This process is known as echolocation. Same process is used by the dolphins.

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