Electromagnetic waves are a form of radiation that moves through space, with different wavelengths and frequencies. They have many applications in science and everyday life, but can also be harmful at higher energies. Electromagnetic waves behave like both waves and particles, known as wave-particle duality, and are produced when subatomic particles release energy.
The term electromagnetic wave describes the way electromagnetic radiation (EMR) moves through space. Different forms of EMR are distinguished by their wavelengths, which range from many yards (meters) to less than the diameter of an atomic nucleus. The entire spectrum, in order of decreasing wavelength, runs from radio waves through microwaves, visible light, ultraviolet and X-rays to gamma rays and is known as the electromagnetic spectrum. Electromagnetic waves have many applications, both in science and in everyday life.
Waves of light
In many respects, an electromagnetic wave behaves similar to ripples on water or sound traveling through a medium such as air. For example, if a light is shone onto a screen through a barrier with two narrow slits, a pattern of light and dark stripes appears. This is called an interference pattern: where the crests of waves from one slit meet those of the other, they reinforce each other, forming a bright streak, but where a crest meets a trough, they cancel each other out, leaving a dark streak. Light can also bend around an obstacle, such as ocean breakers around a harbor wall – this is known as diffraction. These phenomena provide evidence of the wave nature of light.
It has long been believed that, like sound, light must travel through some kind of medium. This was given the name “aether,” sometimes spelled “ether,” and was thought to be an invisible material that filled space, but through which solid objects could pass unimpeded. Experiments designed to detect the aether by its effect on the speed of light in different directions failed to find any evidence in favor of it, and the idea was eventually rejected. It was evident that light and other forms of EMR required no medium and could travel through empty space.
Wavelength and frequency
Just like an ocean wave, an electromagnetic wave has peaks and troughs. Wavelength is the distance between two identical points on the wave from cycle to cycle, for example, the distance from one peak, or crest, to the next. EMR can also be defined in terms of frequency, which is the number of crests that pass in a given time interval. All forms of EMR travel at the same speed: the speed of light. Therefore, the frequency depends entirely on the wavelength: the shorter the wavelength, the higher the frequency.
Energy
Shorter wavelength or higher frequency, EMR carries more energy than longer wavelengths or lower frequencies. The energy carried by an electromagnetic wave determines how it affects matter. Low-frequency radio waves disturb atoms and molecules slightly, while microwaves make them move more vigorously: the material heats up. X-rays and gamma rays are much more powerful: They can break chemical bonds and knock electrons off atoms, forming ions. For this reason they are called ionizing radiation.
The origin of electromagnetic waves
The relationship between light and electromagnetism was established by the work of physicist James Clerk Maxwell in the 19th century. This led to the study of electrodynamics, in which electromagnetic waves, such as light, are viewed as disturbances, or “ripples,” in an electromagnetic field, created by the motion of electrically charged particles. Unlike the non-existent aether, the electromagnetic field is simply the sphere of influence of a charged particle, and not a tangible, material thing.
Later work, in the early 20th century, demonstrated that EMR also had particle-like properties. The particles that make up electromagnetic radiation are called photons. Although it seems contradictory, EMR can behave like waves or like particles, depending on the type of experiment being performed. This is known as wave-particle duality. It also applies to subatomic particles, whole atoms, and even fairly large molecules, which can sometimes behave like waves.
The wave-particle duality emerged during the development of quantum theory. According to this theory, the “wave” represents the probability of finding a particle, such as a photon, in a given location. The wave nature of particles and the particle nature of waves have given rise to much scientific debate and some mind-boggling ideas, but no general consensus on what that actually means.
In quantum theory, electromagnetic radiation is produced when subatomic particles release energy. For example, an electron in an atom can absorb energy, but must eventually drop to a lower energy level and release the energy as an EMR. Depending on how this radiation is viewed, it can appear as a particle or an electromagnetic wave.
it is used
Much of modern technology depends on electromagnetic waves. Radio, television, cell phones, and the Internet rely on the transmission of radio frequency EMR via aerial, space, or fiber-optic cables. The lasers used to record and play DVDs and audio CDs use light waves to write to and read from discs. X-ray machines are an essential tool in medicine and airport security. In science, our knowledge of the universe comes largely from analyzing the light, radio waves, and X-rays of distant stars and galaxies.
Dangers
Low-energy electromagnetic waves, such as radio waves, are not thought to be harmful. At higher energies, however, EMR has risks. Ionizing radiation, such as X-rays and gamma rays, can kill or damage living cells. They can also alter DNA, which can lead to cancer. The risk to patients from medical X-rays is considered negligible, but radiology technicians, who are exposed to them on a regular basis, wear lead aprons – which X-rays cannot penetrate – to protect themselves. Ultraviolet light, present in sunlight, can cause sunburn and can even cause skin cancer if exposure is too much.
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