Electromagnetic (EM) radiation is a form of energy that is all around us and takes many forms, such as radio waves, microwaves, X-rays and gamma rays. Sunlight is also a form of EM energy, but visible light is only a small portion of the EM spectrum, which contains a broad range of electromagnetic wavelengths.
Electricity and magnetism were once thought to be separate forces. However, in 1873, Scottish physicist James Clerk Maxwell developed a unified theory of electromagnetism. The study of electromagnetism deals with how electrically charged particles interact with each other and with magnetic fields.
There are four main electromagnetic interactions:
- The force of attraction or repulsion between electric charges is inversely proportional to the square of the distance between them.
- Magnetic poles come in pairs that attract and repel each other, much as electric charges do.
- An electric current in a wire produces a magnetic field whose direction depends on the direction of the current.
- A moving electric field produces a magnetic field, and vice versa.
Maxwell also developed a set of formulas, called Maxwell's equations, to describe these phenomena.
Waves and fields
EM radiation is created when an atomic particle, such as an electron, is accelerated by an electric field, causing it to move. The movement produces oscillating electric and magnetic fields, which travel at right angles to each other in a bundle of light energy called a photon. Photons travel in harmonic waves at the fastest speed possible in the universe: 186,282 miles per second (299,792,458 meters per second) in a vacuum, also known as the speed of light. The waves have certain characteristics, given as frequency, wavelength or energy.
A wavelength is the distance between two consecutive peaks of a wave. This distance is given in meters (m) or fractions thereof. Frequency is the number of waves that form in a given length of time. It is usually measured as the number of wave cycles per second, or hertz (Hz). A short wavelength means that the frequency will be higher because one cycle can pass in a shorter amount of time, according to the University of Wisconsin. Similarly, a longer wavelength has a lower frequency because each cycle takes longer to complete.
The EM spectrum
EM radiation spans an enormous range of wavelengths and frequencies. This range is known as the electromagnetic spectrum. The EM spectrum is generally divided into seven regions, in order of decreasing wavelength and increasing energy and frequency. The common designations are: radio waves, microwaves, infrared (IR), visible light, ultraviolet (UV), X-rays and gamma rays. Typically, lower-energy radiation, such as radio waves, is expressed as frequency; microwaves, infrared, visible and UV light are usually expressed as wavelength; and higher-energy radiation, such as X-rays and gamma rays, is expressed in terms of energy per photon.
Radio waves are at the lowest range of the EM spectrum, with frequencies of up to about 30 billion hertz, or 30 gigahertz (GHz), and wavelengths greater than about 10 millimeters (0.4 inches). Radio is used primarily for communications including voice, data and entertainment media.
Microwaves fall in the range of the EM spectrum between radio and IR. They have frequencies from about 3 GHz up to about 30 trillion hertz, or 30 terahertz (THz), and wavelengths of about 10 mm (0.4 inches) to 100 micrometers (μm), or 0.004 inches. Microwaves are used for high-bandwidth communications, radar and as a heat source for microwave ovens and industrial applications.
Infrared is in the range of the EM spectrum between microwaves and visible light. IR has frequencies from about 30 THz up to about 400 THz and wavelengths of about 100 μm (0.004 inches) to 740 nanometers (nm), or 0.00003 inches. IR light is invisible to human eyes, but we can feel it as heat if the intensity is sufficient.
Visible light is found in the middle of the EM spectrum, between IR and UV. It has frequencies of about 400 THz to 800 THz and wavelengths of about 740 nm (0.00003 inches) to 380 nm (.000015 inches). More generally, visible light is defined as the wavelengths that are visible to most human eyes.
Ultraviolet light is in the range of the EM spectrum between visible light and X-rays. It has frequencies of about 8 × 1014 to 3 × 1016 Hz and wavelengths of about 380 nm (.000015 inches) to about 10 nm (0.0000004 inches). UV light is a component of sunlight; however, it is invisible to the human eye. It has numerous medical and industrial applications, but it can damage living tissue.
X-rays are roughly classified into two types: soft X-rays and hard X-rays. Soft X-rays comprise the range of the EM spectrum between UV and gamma rays. Soft X-rays have frequencies of about 3 × 1016 to about 1018 Hz and wavelengths of about 10 nm (4 × 10−7 inches) to about 100 picometers (pm), or 4 × 10−8 inches. Hard X-rays occupy the same region of the EM spectrum as gamma rays. The only difference between them is their source: X-rays are produced by accelerating electrons, while gamma rays are produced by atomic nuclei.
Gamma-rays are in the range of the spectrum above soft X-rays. Gamma-rays have frequencies greater than about 1018 Hz and wavelengths of less than 100 pm (4 × 10−9 inches). Gamma radiation causes damage to living tissue, which makes it useful for killing cancer cells when applied in carefully measured doses to small regions. Uncontrolled exposure, though, is extremely dangerous to humans.
Science >> Physics for Kids
Types of Electromagnetic Waves Electromagnetic waves are a form of energy waves that have both an electric and magnetic field. Electromagnetic waves are different from mechanical waves in that they can transmit energy and travel through a vacuum.
Electromagnetic waves are classified according to their frequency. The different types of waves have different uses and functions in our everyday lives. The most important of these is visible light, which enables us to see.
Radio waves have the longest wavelengths of all the electromagnetic waves. They range from around a foot long to several miles long. Radio waves are often used to transmit data and have been used for all sorts of applications including radio, satellites, radar, and computer networks.
Microwaves are shorter than radio waves with wavelengths measured in centimeters. We use microwaves to cook food, transmit information, and in radar that helps to predict the weather. Microwaves are useful in communication because they can penetrate clouds, smoke, and light rain. The universe is filled with cosmic microwave background radiation that scientists believe are clues to the origin of the universe they call the Big Bang.
Between microwaves and visible light are infrared waves. Infrared waves are sometimes classified as "near" infrared and "far" infrared. Near infrared waves are the waves that are closer to visible light in wavelength. These are the infrared waves that are used in your TV remote to change channels. Far infrared waves are further away from visible light in wavelength. Far infrared waves are thermal and give off heat. Anything that gives off heat radiates infrared waves. This includes the human body!
The visible light spectrum covers the wavelengths that can be seen by the human eye. This is the range of wavelengths from 390 to 700 nm which corresponds to the frequencies 430-790 THz. You can go here to learn more about the visible spectrum.
Ultraviolet waves have the next shortest wavelength after visible light. It is ultraviolet rays from the Sun that cause sunburns. We are protected from the Sun's ultraviolet rays by the ozone layer. Some insects, such as bumblebees, can see ultraviolet light. Ultraviolet light is used by powerful telescopes like the Hubble Space Telescope to see far away stars.
X-rays have even shorter wavelengths than ultraviolet rays. At this point in the electromagnetic spectrum, scientists begin to think of these rays more as particles than waves. X-rays were discovered by German scientist Wilhelm Roentgen. They can penetrate soft tissue like skin and muscle and are used to take X-ray pictures of bones in medicine.
As the wavelengths of electromagnetic waves get shorter, their energy increases. Gamma rays are the shortest waves in the spectrum and, as a result, have the most energy. Gamma rays are sometimes used in treating cancer and in taking detailed images for diagnostic medicine. Gamma rays are produced in high energy nuclear explosions and supernovas.
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Science >> Physics for Kids