Electromagnetic 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).

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.

\[c = f \cdot \lambda\]

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.

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.

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.

• 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.

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.