Electromagnetic energy is familiar to most people as light and heat, but it can take many other forms, such as radio waves and X-rays. These are all types of radiation originating from the electromagnetic force, which is responsible for all electrical and magnetic phenomena. The radiation travels at the speed of light in a manner resembling waves.
Unlike sound waves, electromagnetic waves do not require a medium through which to move and can travel across empty space. The length of the wave can vary from hundreds of yards (meters) down to subatomic scales. The full range of wavelengths is known as the electromagnetic spectrum, of which visible light forms only a small part. Despite the observed wave-like character of electromagnetic radiation (EMR), it can also behave as if it's composed of tiny particles, known as photons.
Light, Electricity, and Magnetism
The connection between light and electromagnetism was revealed in the 19th century by physicist James Clerk Maxwell's work on electric and magnetic fields. Using equations he developed, he found that the speed at which the fields move through space was exactly the speed of light and concluded that light was a disturbance of these fields, traveling in the form of waves. His equations also showed that other forms of EMR with longer and shorter wavelengths were possible; these were later identified. Maxwell's findings gave rise to the study of electrodynamics, according to which EMR consists of oscillating electric and magnetic fields at right angles to one another and to the direction of motion. This explained the wave-like nature of light, as observed in many experiments.
Wavelength, Frequency and Energy
Electromagnetic radiation can be described in terms of its wavelength — the distance between the crests of the waves — or its frequency — the number of crests that pass by a fixed point during a fixed time interval. When moving through a vacuum, EMR always travels at light speed; therefore, the rate at which the crests travel does not vary and the frequency depends only on the length of the wave. A shorter wavelength indicates a higher frequency and a higher energy. This means that high energy gamma rays do not travel any faster than low energy radio waves; instead, they have much shorter wavelengths and much higher frequencies.
The Wave-Particle Duality
Electrodynamics was very successful in describing electromagnetic energy in terms of fields and waves, but early in the 20th century, Albert Einstein's investigation of the photoelectric effect, in which light dislodges electrons from a metal surface, raised a problem. He found that the energy of the electrons was entirely dependent on the frequency, and not the intensity, of the light. An increase in frequency produced higher energy electrons, but an increase in brightness made no difference. The results could only be explained if the light consisted of discrete particles — later named photons — that transferred their energy to the electrons. This created a puzzle: observed over large scales, EMR behaves as waves, but its interactions with matter at the smallest scales can only be explained in terms of particles.
This is known as the wave-particle duality. It emerged during the development of quantum theory and applies to everything at the subatomic scale; electrons, for example, can behave as waves as well as particles. There is no overall consensus among scientists as to what this duality actually means about the nature of electromagnetic energy.
A new theory, known as quantum electrodynamics (QED), eventually emerged to explain the particle-like behavior of EMR. According to QED, photons are the particles that carry the electromagnetic force, and the interactions of electrically charged objects are explained in terms of the production and absorption of these particles, which themselves carry no charge. QED is considered one of the most successful theories ever developed.
How Electromagnetic Energy is Produced
Classical electrodynamics described the production of EMR in terms of the movement of electrical charges, but a more modern explanation — in line with quantum theory — is based on the idea that the subatomic particles of which matter is composed can only occupy certain fixed energy levels. Electromagnetic radiation is released by the change from a higher to a lower energy state. Left to itself, matter will always try to reach its lowest level of energy.
EMR can be produced when matter temporarily absorbs energy — for example, when it is heated — then releases it to drop to a lower level. A lower energy state can also be achieved when atoms or molecules combine with one another in a chemical reaction. Combustion is a familiar example: typically, a molecule combines with oxygen from the air, forming products that collectively have less energy than the original molecule. This causes electromagnetic energy to be released in the form of flame.
In the Sun's core, four hydrogen nuclei combine, in a series of steps, to form a helium nucleus that has slightly less mass, and therefore less energy. This process is known as nuclear fusion. The excess energy is released as high frequency gamma rays that are absorbed by matter further out, which then emits this energy, mostly in the form of visible light and heat.
Electromagnetic Energy, Life, and Technology
Energy from the Sun is crucial to life on Earth. Sunlight heats the Earth's surface, which in turn heats the atmosphere, maintaining temperatures suitable for life and driving the planet's weather systems. Plants make use of the Sun's electromagnetic energy for photosynthesis, the method by which they produce food. Solar energy is converted to chemical energy that powers the processes that allow plants to make the glucose they need to survive from carbon dioxide and water. The by-product of this reaction is oxygen, so photosynthesis is responsible for maintaining the planet's oxygen levels.
Most forms of technology rely largely on electromagnetic energy. The Industrial Revolution was powered by heat generated by the combustion of fossil fuels, and more recently, solar radiation has been used directly to provide "clean" and renewable power. Modern communication, broadcasting, and the Internet depend heavily on radio waves and on light channeled through fiber optic cables. Laser technology uses light to read from and write to CDs and DVDs. Most of what scientists know about the universe comes from the analysis of EMR of various wavelengths from distant stars and galaxies.
Effects on Health
High frequency EMR, such as gamma rays, X-rays, and ultraviolet light, carries enough energy to cause chemical changes in biological molecules. It may break chemical bonds or remove electrons from atoms, forming ions. This can damage cells and alter DNA, increasing the risk of cancer. Concerns have also been expressed about the health effects of lower frequency EMR, such as the radio waves and microwaves used by cell phones and other communication devices. Although these forms of radiation appear to have no direct effect on the chemistry of life, they may cause tissue to be heated in localized areas with prolonged exposure. There does not, so far, appear to be any conclusive evidence that this can make people ill.