Electromagnetic waves are a method of energy transfer. They are formed by a varying magnetic field that induces a varying electric field. Electromagnetic waves consist of these induced oscillating electric and magnetic fields, which are perpendicular to each other.
Unlike mechanical waves, electromagnetic waves do not require a medium in order to transmit. Therefore, electromagnetic waves can travel through a vacuum where there is no medium. Electromagnetic waves include radio waves, microwaves, infrared waves, visible light, ultraviolet light, X-rays, and gamma rays.
Just so you know
Mechanical waves are caused by a vibration in matter, like solids, gases, and liquids. Mechanical waves pass through a medium via small collisions between particles that transfer energy from one particle to another. Therefore, mechanical waves can only travel through a medium. Some examples of mechanical waves are sound waves and water waves.
Discovery of electromagnetic waves
In 1801, Thomas Young performed an experiment called the double-slit experiment during which he discovered the wave-like behaviour of light. This experiment involved directing light from two small holes onto a plain surface, which resulted in an interference pattern. Young also suggested that light is a transverse wave rather than a longitudinal wave.
Later on, James Clerk Maxwell studied the behaviour of electromagnetic waves. He summarised the relationship between magnetic and electrical waves in equations known as Maxwell’s equations.
Hertz’s experiment
Between 1886 and 1889, Heinrich Hertz used Maxwell’s equations to study the behaviour of radio waves. He discovered that radio waves are a form of light.
Hertz used two rods, a spark gap as a receiver (connected to a circuit), and an antenna (see the basic outline below). When waves were observed, a spark was created in the spark gap. These signals were found to have the same properties as electromagnetic waves. The experiment proved that the velocity of radio waves is equal to the velocity of light (but they have different wavelengths and frequencies).
A basic outline of Hertz's experiment. A is the switch, B is the transformer, C is the metal plates, D is the spark gap, and E is the receiver. Wikimedia Commons.
In the equation below, you can see that frequency and wavelength are related to the speed of light, where c is the speed of light measured in metres per second (m/s), f is the frequency measured in Hertz (Hz), and λ is the wavelength of the wave measured in metres (m). The speed of light is constant in a vacuum and has a value of approximately 3 ⋅ 108m/s. If a wave has a higher frequency, it will have a smaller wavelength and vice versa.
\[c = f \cdot \lambda\]
As electromagnetic waves were found to possess properties similar to mechanical waves, they were thought of as only waves. However, at times, electromagnetic waves also exhibit particle-like behaviour, which is the concept of wave-particle duality. The shorter the wavelength, the more particle-like behaviour and vice versa. Electromagnetic radiation (and, by extension, light) has both wave-like and particle-like behaviour.
The properties of electromagnetic waves
Electromagnetic waves display both wave and particle properties. These are their properties:
- Electromagnetic waves are transverse waves.
- Electromagnetic waves can be reflected, refracted, diffracted, and produce interference patterns (wave-like behaviour).
- Electromagnetic radiation consists of energised particles creating waves of energy with no mass (particle-like behaviour).
- Electromagnetic waves travel at the same speed in a vacuum, which is the same speed as the speed of light (3 ⋅ 108 m/s).
- Electromagnetic waves can travel in a vacuum; therefore, they don’t need a medium to transmit.
- Polarisation: the waves can be constant or rotate with each cycle.
What is the electromagnetic spectrum?
The electromagnetic spectrum is the entire spectrum of electromagnetic radiation made up of different types of electromagnetic waves. It is arranged according to frequency and wavelength: the left-hand side of the spectrum has the longest wavelength and lowest frequency, and the right-hand side has the shortest wavelength and highest frequency.
You can see the different types of electromagnetic waves that make up the entire electromagnetic radiation below.
The electromagnetic spectrum showing wavelength and frequency, Wikimedia Commons
Types of electromagnetic waves
There are different types of electromagnetic waves in the entire electromagnetic radiation spectrum, which you can see in the following table.
Types | Wavelength [m] | Frequency [Hz] |
Radio waves | 106 – 10-4 | 100 – 1012 |
Microwaves | 10 – 10-4 | 108 – 1012 |
Infrared | 10-2 – 10-6 | 1011 – 1014 |
Visible light | 4 · 10-7 – 7 · 10-7 | 4 · 1014 – 7.5 · 1014 |
Ultraviolet | 10-7 – 10-9 | 1015 – 1017 |
X-rays | 10-8 – 10-12 | 1017– 1020 |
Gamma rays | <10-10 | >1018 |
Electromagnetic waves are used in technology depending on the properties of each wave type. Some of the electromagnetic waves have harmful effects on living organisms. In particular, microwaves, X-rays, and gamma rays can be dangerous under certain circumstances.
Radio waves
Radio waves have the longest wavelength and the smallest frequency. They can be easily transmitted through the air and do not cause damage to human cells when they are absorbed. Since they have the longest wavelength, they can travel long distances, making them ideal for communication purposes.
Radio waves transmit coded information through long distances, which is then decoded once the radio waves are received. The image below shows an antenna working as a transmitter, which generates radio waves. An antenna transmits and receives radio waves over a specific range of frequencies.
An example of an antenna
Microwaves
Microwaves are electromagnetic waves with wavelengths ranging from 10m to centimetres. They are shorter than a radio wave but longer than infrared radiation. Microwaves are well transmitted through the atmosphere. Here are some applications of microwaves:
- Heating food at high intensities. High-energy microwaves have frequencies that are easily absorbed by water molecules. Microwaves heat food using a magnetron that generates microwaves, which reach the food compartment and cause the water molecules in the food to vibrate. This increases friction between molecules, resulting in increased heat.
- Communication, such as WIFI and satellites. Due to their high frequency and easy transmission through the atmosphere, microwaves can carry lots of information and transmit this information from Earth to different satellites.
High-intensity microwaves can be harmful to living organisms and, more specifically, to internal organs as water molecules absorb microwaves more easily.
Infrared
Infrared radiation is part of the electromagnetic spectrum. It has wavelengths that range from millimetres to micrometres. Infrared radiation is also known as infrared light, and it has a longer wavelength than visible light (so it is not visible to the human eye). Thermal radiation in the form of infrared electromagnetic waves is emitted by all matter with a temperature greater than absolute zero.
Infrared waves can be transmitted through the atmosphere, so they are also used for communication. Infrared radiation is also used in fibre optics, sensors (like remote controls), infrared thermal imaging to make medical diagnoses (like arthritis), thermal cameras, and heating.
Visible light
Visible light is the part of the electromagnetic spectrum that is visible to the human eye. Visible light is not absorbed by the Earth’s atmosphere, but the light that passes through is scattered due to gas and dust, which creates different colours in the sky.
In the image below, you can see a laser emitting visible light. The beam of light contains waves with similar wavelengths and concentrates its energy on a small spot. Due to this concentrated energy over a small area, lasers can travel long distances and are used in applications that require high precision.
Some applications of visible light waves include fibre optic communication, photography, and TV and smartphones.
Lasers are an example of the application of visible light
Ultraviolet light
Ultraviolet light is a part of the electromagnetic spectrum between visible light and X-rays. When ultraviolet light illuminates any object that contains phosphorus, visible light is emitted that seems to glow. This type of light is used to cure or harden some materials and detect structural defects.
Ultraviolet radiation can cause sunburn. Long-term and high-intensity ultraviolet radiation exposure can potentially harm living cells and cause premature ageing of the skin and skin cancer.
Some applications of ultraviolet light include sun tanning, fluorescent light for hardening materials and detection, and sterilisation.
X-rays
X-rays are highly energetic waves that can penetrate matter. They are a type of ionising radiation. Ionising radiation is the type of radiation that can displace electrons from the shells of atoms and convert them into ions. This type of ionising radiation causes DNA mutations in living cells at high energies, which can lead to cancer.
X-rays emitted from objects in space are mostly absorbed by the Earth’s atmosphere, so they can only be observed using X-ray telescopes in orbit. X-rays are also used in medical and industrial imaging due to their penetrative characteristic.
Gamma rays
Gamma rays are the highest energy waves that are created from the radioactive decay of an atomic nucleus. Gamma rays have the shortest wavelength and highest energy, so they can also penetrate matter. Gamma rays are also a form of ionising radiation, which can damage living cells at high energies. Like X-rays, gamma rays emitted from objects in space are mostly absorbed by the Earth’s atmosphere and can be detected using gamma-ray telescopes.
Due to their penetrating abilities, gamma rays are used in various applications, such as
- medical treatments where gamma rays are used for radiotherapy or medical sterilisation,
- nuclear studies or nuclear reactors,
- security, like smoke detection or food sterilisation, and
- astronomy.
A region of the sky centred on the pulsar Geminga. On the left is the total number of gamma rays detected by Fermi’s Large Area Telescope. The brighter the colours, the higher the number of gamma rays. The right shows the pulsar’s gamma-ray halo.
Check out our explanation on Alpha, Beta, and Gamma Radiation and Radioactive Decay for more info on gamma rays.
Electromagnetic Waves - Key takeaways
Electromagnetic waves consist of oscillating electric and magnetic fields that are perpendicular to each other.
Electromagnetic waves can travel through a vacuum at the speed of light.
Electromagnetic waves can be reflected, refracted, polarised, and produce interference patterns. This demonstrates the wave-like behaviour of electromagnetic waves.
Electromagnetic waves also possess particle properties.
Electromagnetic waves are used for a variety of purposes, such as communication, heating, medical imaging and diagnostics, and food and medical sterilisation.
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