Прочитайте текст и выполните задания. 1. A radar system has a transmitter that emits radio waves called radar signals in predetermined directions

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1. A radar system has a transmitter that emits radio waves called radar signals in predetermined directions. When these come into contact with an object they are usually reflected and/or scattered in many directions. Radar signals are reflected especially well by materials of considerable electrical conductivity – especially by most metals, by seawater, by wet land, and by wetlands. The radar signals that are reflected back towards the transmitter are the desirable ones that make radar work. If the object is moving either closer or farther away, there is a slight change in the frequency of the radio waves, due to the Doppler effect.

2. Radar receivers are usually, but not always, in the same location as the transmitter. Although the reflected radar signals captured by the receiving antenna are usually very weak, these signals can be strengthened by the electronic amplifiers that all radar sets contain. The weak absorption of radio waves by the medium through which it passes is what enables radar sets to detect objects at relatively-long ranges – ranges at which other electromagnetic wavelengths, such as visible light, infrared light, and ultraviolet light, are too strongly attenuated. In particular, there are weather conditions under which radar works well regardless of the weather. Such things as fog, clouds, rain, falling snow, and sleet that block visible light are usually transparent to radio waves.

3. Finally, radar relies on its own transmissions, rather than light from the Sun or the Moon, or from electromagnetic waves emitted by the objects themselves, such as infrared wavelengths (heat). This process of directing artificial radio waves towards objects is called illumination, regardless of the fact that radio waves are completely invisible to the human eye or cameras.

Определите, какое утверждение соответствует содержанию текста.

	Radar can detect an object even without radio signals being reflected back towards the transmitter.
	Radar signals are transmitted equally both by its own transmissions and electromagnetic waves emitted by the objects themselves.
	Bad weather conditions such as fog, clouds, rain, falling snow, and sleet can upset the work of the radar.
	The reflection capacity of radar signals depends on the physical properties of the objects they are reflected from.

2. Завершите утверждение согласно содержанию текста.
Unlike visible light radio waves …

	are not usually influenced by weather conditions
	can be reflected from any object
	can reflect from moving objects
	are not detectible by human eyes

3.Ответьте на вопрос:

What enables radio waves to detect objects being far enough from radio transmitter?

	It depends upon weather conditions.
	It depends upon the frequency of electromagnetic waves.
	It depends upon the property of the medium they go through.
	It depends upon the intensity of radio waves’ reflection.

Определите основную идею текста.

	Radar systems possess important properties enabling people to detect moving or still objects being far away from radio transmitter regardless of weather conditions.
	People use radar systems which are provided with a transmitter emitting radio waves which are reflected and/or scattered in many directions when coming into contact with an object.
	Unlike visible light radio waves don’t depend much on the medium they go through while detecting objects at relatively-long distances.
	Radar relies on its own transmissions, rather than light from the Sun or the Moon, or from electromagnetic waves emitted by the objects themselves.

Text 18

Radio Systems

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