Read the passage below and fill in the blanks with the mostsuitable sentence. A pilot cannot fly a plane by sight
Read the passage below and fill in the blanks with the mostsuitable sentence.
A pilot cannot fly a plane by sight alone. (47) Since human eyes are not very good at determining speeds of approaching objects, radar can show a pilot how fast nearby planes are moving. The basic principle of radar is exemplified by what happens when one shouts in a cave. (48) . With radar, however, the waves are radio waves instead of sound waves. Radio waves travel at the speed of light, about 300,000 kilometers in one second. A radar set sends outa short burst of radiation waves. Then it receives the echoes produced when the waves bounce off objects. (49) . The word “radar,” in fact, gets its name from the term “radio detection and ranging.” “Ranging” is the term for detection of the distance between an object and the radar set. (50) .
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