The electromagnetic spectrum

The electromagnetic spectrum is the full range of electromagnetic radiation and includes radio waves, heat and light rays, X rays, and gamma rays.

Electromagnetic radiation is a form of energy that travels at the velocity of light. 186.000 mi/sec (300.000km/sec). As it travels, its energy switches back and forth between electric and magnetic fields. As one field increases in strength, the other decreases. The rate at which this exchange happens is called the frequency of the radiation. Different types of electromagnetic radiation have different frequencies. Radio waves have lower frequencies than light rays, for example, and blue light has a higher frequency than red. The frequency of electromagnetic radiation, measured in hertz (Hz), is the number 61 times in one second that the electrical field reaches its maximum value.

Scientists say that electromagnetic radiation travels in waves. This is because the strengths oi the electric and magnetic fields vary continually as they travel through space. The wavelength is the distance the wave travels in the time it takes the electric field to fall from its maximum value to its minimum value and then rise back to its maximum value. Because of this, the wavelength is the speed of light divided by the frequency. The signal from a radio station whose broadcast frequency is 1,200 kilohertz, or 1,200,000 Hz, has a wavelength of around 820 feet (250m), for example.

RADIO AND MICROWAVE

Radio stations broadcast using frequencies in a range from 150,000 Hz to around 20.000,000 Hz. Each station uses a particular frequency, so receivers tunc to a given station by only accepting waves at the correct
frequency for that station. Land-based television transmitters send signals between about 70 MHz and 800 MHz. (One megahertz is one million hertz.)
Satellite TV works at even higher frequencies. These electromagnetic waves arc captured by dish-shaped antennae that point toward the satellite.
Radars bounce radio waves off planes, ships, and clouds to show their positions, which can be many miles away. They use wavelengths of about 1 inch (2.5 cm). Doppler radar measures the speed of moving objects from the minute change in the frequency of the reflected waves.
Microwave ovens use wavelengths of a few millimeters, which correspond to frequencies of billions of hertz. The radiation heats food by causing water molecules to vibrate.

INFRARED LIGHT AND BEYOND

Infrared radiation has frequencies just lower than those of visible light. Its wavelength ranges from 1 millimeter to 750 nanometers. A nanometer, or 1 nm,
is one billionth of a meter, or 1/25,000,000 of an
inch. Hot objects give off infrared radiation, which is felt as heat. Visible light is the tiny part of the electromagnetic spectrum that human eyes can sense.The spectrum of colors stretches from red light at 770 nm to violet light at 400 nm.

The energy of electromagnetic radiation increases as the wavelength becomes shorter. Invisible ultraviolet rays cause sunburn and have shorter wavelengths (100-400 nm) than visible light.
X rays have even shorter wavelengths, usually less than the diameter of an atom (0.1 nm) .They penetrate flesh and bone can be used to produce images of cracks deep inside pieces of metal.
Reference : The Kingfisher Science Encyclopedia De Charles Taylor