To understand Electromagnetic Spectrum, let us first look at what is Electromagnetic radiation. Electromagnetic radiation is a form of energy that is all around us and takes many forms, such as light, infrared rays, X-rays and gamma rays. Spectrum here denotes the range of frequencies/wavelengths of radiation for example, within the visible light spectrum, the human eye can detect between 380 and 700 nm. So in summary, electromagnetic spectrum is the range of frequencies of electromagnetic radiation with their different wavelengths and varying photon energies.

The electromagnetic spectrum has a frequency range from below 3*109 Hz to above 3*1019 Hz, correspondingly wavelengths of thousands of kilometers down to a fraction of the size of an atom. Although all electromagnetic waves travel at the speed of light in vacuum, they possess different frequencies, wavelengths, and photon energies. This electromagnetic spectrum has been divided into many subranges, commonly referred to as portions or bands, such as visible light or ultraviolet radiation. The different portions have been designed keeping in mind, the differences in the emission, transmission, and absorption of waves and their practical applications.

Different regions of the Electromagnetic Spectrum

1. Gamma radiation: Gamma-rays have the smallest wavelengths and the maximum energy among the electromagnetic waves. These waves are generated by radioactive atoms, nuclear explosions, supernova explosions and black holes. Gamma-rays travel to us across the universe and are absorbed by the Earth's atmosphere. Gamma-rays can kill living cells hence it is used to kill cancerous cells in medicine. Gamma-ray telescope using Compton scattering is used to study these rays.
2. X-ray radiation: As the wavelengths of electromagnetic radiation decrease, they increase in energy. X-rays have smaller wavelengths and therefore higher energy than ultraviolet waves. We usually talk about X-rays in terms of their energy rather than wavelength. X-ray light tends to act more like a particle than a wave. X-ray detectors collect actual photons of X-ray light - which is very different from the radio telescopes that have large dishes designed to focus radio waves. X-rays were discovered in 1895 by Wilhelm Conrad Roentgen who accidentally found these rays while experimenting with vacuum tubes. X-rays are used to capture images of bones in the body and also in telescopes in space that study black holes, sun, neutron stars etc.
3. Ultraviolet radiation: Ultraviolet (UV) light has shorter wavelengths than visible light. Some insects are able to see the UV light. The UV spectrum is split in three regions: near UV, far UV and extreme UV. The near UV is closest to visible light while extreme UV is closest to X-rays. The sun emits a lot of ultraviolet rays which causes sun burns but luckily the ozone in the atmosphere absorbs most of it.
4. Visible light: Visible light waves are the only electromagnetic waves we can see. The different colors of the rainbow fall in this category. Each color has a different wavelength. Red has the longest wavelength and violet has the shortest wavelength. Together all the seven colors of the rainbow make up white light. The cones in our eyes are able to detect tiny visible light waves between the wavelengths of 380 and 700 nm. The sun is a natural source of visible light. The color of an object is the color that is reflected while the other colors are absorbed. The visible light allows us to see the world around us and satellite also use these to study the Earth and the outer space.
5. Infrared radiation: Infrared light lies between the visible and microwave portions of the electromagnetic spectrum. "Near infrared" light is closest in wavelength to visible light and "far infrared" is closer to the microwave region of the electromagnetic spectrum. The wavelengths vary from in size of a pin head to cell's size. Far infrared rays are thermal for example heat from the sunlight, a fire etc. Near Infrared rays are not hot, they are used in TV's remote control. Every object that is hot or cold emits infrared light, which can be captured by infrared camera. It can be used to study anything from cloud structure, geographical features in a place to distant galaxies.
6. Microwave radiation: Microwaves have wavelengths of few centimeters. The microwave oven uses waves of wavelength of a foot or more to heat our food. Microwaves are very good for transmitting information like computer data or telephone calls, from place to place as they can penetrate haze, light rain and snow, clouds, and smoke. This is also why these waves are good for viewing the Earth from space. Microwaves with shorter wavelengths are used for remote sensing. These microwaves are used for radar like the doppler radar used in weather forecasts. Radar acronym for "radio detection and ranging" has many applications like detecting objects and their position. Microwaves, used for radar, are just a few inches long.
7. Radio waves: Radio waves have the longest wavelengths in the electromagnetic spectrum. These waves can be longer than a football field. Radio waves are used to carry signals for radio, television and cellular phones. Antennae or dishes are used to receive the waves broadcasted. Many celestial objects like planets, comets, stars and galaxies emit waves at different wavelengths. Sometimes they are as wide as a mile, hence they fall in radio waves region. Radio telescopes, which are huge metal dishes that focus the radio waves to a point are used to study them.

For most of history, the visible light was the only known electromagnetic radiation. But over the years, scientists discovered the other different regions of the Electromagnetic spectrum. The Electromagnetic radiation is everywhere. They have many uses in our everyday lives. We really cannot envision our lives without them.