TOEFL Reading Comprehension Lesson 4
Đ c đo n văn sau và tr l i các câu h i:ọ ạ ả ờ ỏ
A pilot cannot fly a plane by sight alone. In many conditions, such as flying at night and
landing in dense fog, a pilot must use radar, an alternative way of navigating. 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.
The echo of the sounds against the walls helps a person determine the size of the cave.
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 out
a short burst of radiation waves. Then it receives the echoes produced when the waves
bounce off objects. By determining the time it takes for the echoes to return to the radar
set, a trained technician can determine the distance between the radar set and other
objects. 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. Besides being of critical importance to pilots, radar is essential for air traffic
control, tracking ships at sea, and for tracking weather systems and storms.
1. What is the main topic of this passage?
A. the nature of radar
B. types of ranging
C. alternatives to radar
D. history of radar
2. In line 2, the word “dense” could be replaced by
3. According to the passage, what can radar detect besides location of objects?