Seven rays, one family

"Isn't it a small world." You have probably heard this exclamation many times. People often say it when they find that acquaintances they had met at different times and places, and whom they never connected with each other, turn out to be related to each other. Scientists often have a similar ex­perience with occurrences in nature. Things or events that at first seem to have nothing to do with each other turn out to be related after all. We shall repeat this experience with seven kinds of rays. We find them in different places, and use them in different ways, but they are close relatives. They are members of one family, the family of electromagnetic waves.

The kind of ray that mankind has known for the longest time is light. It

helps us see the objects that surround us, when the objects reflect the light into

our eyes. Because our eyes can detect light, we call it a visible ray. The other

rays are invisible.

We find three types of invisible rays in use in our homes. When we

listen to a radio programme, we are using the rays that are called radio waves. When we cook a meal on an electric cooker, we are using infrared rays, sometimes referred to as heat rays. When we sit under a sun-tan lamp, we are using ultraviolet rays. We meet the other three types of rays outside the home. Inside the hospital we shall find X-rays, produced by X-rays ma­chines, and used for taking pictures of the insides of our bodies. At airports everywhere we shall find microwaves used with radar equipment to detect planes in the air, or guide them in to land. Also in hospitals we find gamma rays used as invisible bullets to kill cancer cells.

These seven types of rays resemble each other in that they are all elec­tromagnetic waves. What makes them different from each other is their fre­quency or their wavelength. The distance that the wave moves during the time it takes for one complete cycle of vibration is called the wavelength of the wave. The frequency is the number of cycles in a second. Notice that ra­dio waves are the longest of the electromagnetic waves and have the lowest frequency.

Radio waves

During the last few decades, a subtle change has occurred which none of our senses can register. Radio waves flow ceaselessly around us, through us and above us. We can only hear and see them if we convert them to other waves to which our ears and eyes are receptive.

Radio waves are the longest members of the family of electromagnetic waves. In the spectrum, in which the waves are arranged in order of increas­ing wavelength, they lie beyond the infrared waves. Their wavelengths range from about three hundredths of a centimetre to about 300 kilometres. Radio broadcasts today are made by two different methods known as AM (ampli­tude modulation) and FM (frequency modulation). The frequencies of the waves used are expressed in kilocycles or megacycles. The vibrating current is fed into an antenna from which the radio waves are broadcast into space.

Microwaves arc the smallest radio waves. In the spectrum of electro­magnetic waves they lie between infrared rays and the long radio waves. The shortest micro- waves have a wavelength of about three hundredths of a cen­timetre and a frequency of one million megacycles. The longest microwaves have a wavelength of about three metres and a frequency of one hundred megacycles.

The first microwaves made by man were the two-foot waves produced by

Heinrich Hertz. It is interesting that they were the last to be put to a prac­tical use. Long waves were easier to produce and send out over long dis­tances. Scientists had to return to the use of short waves in order to solve a problem that came up during World War П. The problem was "How can you detect an approaching enemy plane white it is still far away?" A possible an­swer to the problem was to send a beam of radio waves. Long radio waves could not be used for this purpose because they fan out too quickly from the broadcasting antenna. Very short waves were necessary to make the radar system work. So new transmitters and receivers were designed to make and use microwaves.

RADIO WAVES IN THE IONOSPHERE

When a radio wave leaves a transmitting antenna, it travels in all direction. One part of the signal travels along the ground and is called the ground wave. The second part of the signal travels through the lower atmos­phere in a direction parallel to the ground. Another part travels at an angle to the ground. The part travelling through the lower atmosphere is called the tropospheric wave; part travelling at an angle to the ground - the ionospheric wave.

In the high-frequency part of the radio spectrum - 3-30 megacycles (mc) - the ground and tropospheric wave components travel short distances, not more than 25 or 30 miles. The ionospheric wave component of the sig­nal can travel to great distance, making long-distance short-wave communi­cation possible.

While leaving the transmitting antenna, the ionospheric wave starts travelling and reaches a region called the ionosphere. This region begins at a height of about 60 miles above the ground. On reaching the ionosphere, radio waves supply the free electrons of the region with electromagnetic energy. As a result, the free electrons start vibrating and radiate this energy.

The ionosphere is formed primarily by ultraviolet radiation reaching it from the Sun. As this radiation interacts with the gases in the upper atmos­phere, these gases, which consist mainly of neutral molecules, absorb the ul­traviolet energy, and in the process lose an electron. This process leaves free electrons and positively charged gas molecules, which are called ions. The formation of ions is called ionization.

The ionosphere has the property of bending radio waves and returning them to the ground at considerable distances from the point of transmission. Bending may change the direction of the wave and it is returned to the ground at some distant point.

The ionosphere is not a single region; it consists of several layers and the properties of these layers vary, depending on many factors. One of the layers, the F- layer, is of primary importance. Most of the world's long ­distance high-frequency communication takes place by means of reflection of radio waves from this region. The F-layer is at the height of about 175miles.

Radio signals from the F-layer may be returned to the ground at dis­- tances as great as 2,000 miles from the transmitter.