Under proper conditions, sound waves will be reflected from a hillside or other such obstruction. Sound travels at the rate of about one-fifth of a mile per second. If the hill is eleven hundred feet away, it takes two seconds for the sound to travel to the hill and back. Thus, by timing the interval between a sound and its reflection(the echo), you van estimate the distance to an obstruction.
During World War II the British used a practical application of this principle to detect German planes on their way to bomb London long before the enemy was near the target. They used radio waves instead of sound waves, since radio waves can penetrate fog and clouds. The outnumbered Royal Air Force (RAF) always seemed to the puzzled Germans to be lying in wait at the right time and never to be surprised. It was radio echoes more than anything else that won the Battle of Britain.
Since the radio weaves were used to tell the direction in which to send the RAF planes and the distance to send them (their range of flight, in other words), the device was called radio detecting and ranging, and from the initials the word radar was coined.
81. Sound waves reflected from a hill can be used to estimate the_______.
A. intensity of sound B. distance to the hill
C. speed of sound D. height of the hill
82. Which of the following can be resulted from by practical application of this principle?
A. a radio directing and ranging device.
B. an electronic detecting device
C. new electronic instruments for planes
D. new radio sets for RAF bombers
83. Radar enabled the English to_____
A. confuse German bomber pilots
B. direct the outnumbered RAF planes effectively
C. detect German planes on their way to London.
D. both A and B
84. Why did the British use radio waves?
A. Because they were more exact than sound waves.
B. Because they could not be detected.
C. Because they were easier to use than sound waves.
D. Because they could penetrate fog and clouds.
85. What can you infer from the passage?
A. Radar was a practical application of a well-known principle.
B. Light waves could be used in a device similar to radar.
C. Radar greatly increased the effectiveness of the REAF.
D. Sound waves are reflected from a hillside under all conditions.
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