What are Electromagnetic Waves?

Electromagnetic waves are waves that consist of magnetic field and electric field components perpendicular to each other. These field components are also perpendicular to the direction of propagation of the wave.

What are the Properties of Electromagnetic Waves?

Electromagnetic waves are created by the interaction between the electric and magnetic fields, and these fields are perpendicular to each other. It is represented in the below diagram.

Note: The terms light, electromagnetic waves and electromagnetic radiation all refer to the same phenomenon. Thus, in this article, these terms will be used interchangeably.

Particle vs Wave Nature

The question whether electromagnetic radiation is a wave or particle bears a long and interesting history. Newton believed that electromagnetic radiations have a particle nature as they travel in a straight line and cast shadows of objects in their path. Later, Thomas Young's double-slit experiment established the idea that light is a wave. Then later, Maxwell developed the theory of electromagnetic waves in 1860, which could predict the speed of electromagnetic radiation. These events convincingly established that light is a wave phenomenon.

The "wave vs. particle" debate sparked again when wave theory failed to explain the photoelectric emission of electrons from a metal surface. In this experiment, the kinetic energy of ejected electrons was observed to be independent of the intensity of the incident light. Later, Albert Einstein gave the theory of the dual nature of light, meaning that light can behave both as particles and waves depending upon its interaction with surroundings.

In waves on a string, the displacement of a particle from the mean position changes with time. The same instance of time-displacement of different particles is different. Similarly, in the case of sound waves, pressure at a point oscillates with time. The speed of propagation of sound waves depends on the properties of the propagation material. Lightwave also shows some such properties, which are listed below.

• Electromagnetic waves do not need any medium to travel.
• Electromagnetic waves are transverse and thus, can be polarized.
• The speed of electromagnetic waves in a vacuum is constant and is nearly equal to $3\times {10}^8 {\mathrm{ms}}^{-1}$(denoted by c).
• When light travels through a medium of refractive index $\mu$, the speed of light decreases by a factor $\mu$. For example, if the refractive index of glass is 1.5, the speed of light in glass will be $\frac{3\times {10}^8 {\mathrm{ms}}^{-1}}{1.5}=2\times {10}^8 {\mathrm{ms}}^{-1}$.
• Electromagnetic waves travel in a straight line unless the medium of propagation is changed or reflected by a surface.
• When the reflection of the light angle made by the incident and the reflected ray with the normal to the incident surface are equal.
• When the light goes from one medium to another, light may deviate from its path if refractive indices of the mediums are different. This phenomenon is called the refraction of light.

Electromagnetic Spectrum and its Components

The electromagnetic spectrum consists of different types of electromagnetic radiation. For example, the radio waves emitted by radio stations, the visible light and high-energy X-rays used to detect any fractures in bones.  The categorization of electromagnetic radiations is done based on their wavelength and frequency. The below figure shows different types of electromagnetic waves on the spectrum and where one might have encountered them.

Radio Waves

Radio waves consist of electromagnetic waves of wavelength in the range of 100000 km to 1 mm. Radio and television use radio waves for transmission. Radio waves are generated by transmitters and received using antennas. Different types of radio waves are used depending upon the medium it needs to travel through.  For example, long waves can diffract around large objects like mountains following the contours of the earth, and these waves are called ground waves.

Microwave

Microwave radiation consists of electromagnetic waves of wavelengths in the range of 0.3 m to 3 mm. This type of radiation helps to cook food easily (used in microwave ovens). It is also used by astronomers to study and learn about nearby galaxies.

Infrared Waves

Infrared radiation consists of electromagnetic waves of a wavelength in the range of 1 mm to 700 nm. Black-body radiation emitted by the objects around room temperature falls in the range of infrared waves. These waves find application in military, medical and night vision devices. Infrared thermal imaging cameras are used to detect any heat loss occurring in an isolated system.

Visible Waves

Visible light is part of the electromagnetic spectrum detected using eyes. It consists of wavelengths ranging from 380 nm to 700 nm. This part contains all the pure colors or spectral colors.

Ultraviolet Waves

The range of wavelengths for ultraviolet (UV) rays is from 10 nm to 400 nm. Ultraviolet radiation constitutes about 10% of the total radiation output from the sun. For humans, suntan and sunburn are the most common effects of exposure to UV light, and long exposure can increase the risk of skin cancer. Ultraviolet radiation does not have enough energy to ionize an atom and thus is considered to be nonionizing radiation. However, ultraviolet rays can induce chemical reactions and can cause many substances to glow.

X-rays

The range of wavelength in which X-rays fall is from 10 nm to 10 pm. X-rays carry enough energy to ionize atoms and break molecular bonds. This makes X-rays ionizing electromagnetic radiation, and thus exposure to this type of radiation can cause damage to tissues and organs. X-rays are mostly used to check for fractures in bones and get images to diagnose problems related to soft tissues. X-rays are also used in security scanners at airports.

Gamma Rays

Gamma rays consist of electromagnetic waves of a wavelength less than 100 pm. These are very high-energy electromagnetic radiations that occur mostly as a result of radioactive decay reactions. Gamma rays are ionizing and are biologically hazardous. They can damage internal organs, and thus nuclear reactors require a high amount of shielding to protect from radiation exposure.

Equations Related to Electromagnetic Waves

• The velocity of electromagnetic radiation in a vacuum is $c=\frac{1}{{\mu }_0{\epsilon }_0}$, where ${\mu }_0$ is the permeability of free space, and ${\epsilon }_0$ is the permittivity of free space.
• The equation of energy of electromagnetic wave of wavelength λ is $\frac{hc}{\lambda }$. Here, h is Planck's constant, and c is the speed of light.
• The relation between wavelength and frequency of light is λν=c, where c is the speed of light.
• The solution for electric field and magnetic field using Maxwell's equations is as follows:

$$\begin{gathered} {\nabla }^{2}E={\mu }_0{\epsilon }_0\frac{{\partial }^{2}E}{\partial t^2} \\ {\nabla }^{2}B={\mu }_0{\epsilon }_0\frac{{\partial }^{2}B}{\partial t^2} \end{gathered}$$

Here, ∇ is the nabla symbol, and it denotes the three-dimensional gradient operator del.

Common Mistakes

The dual nature of light is a difficult concept. Thus, one might use the concept of light as a wave in situations where one needs to consider light as a particle (in photoelectric effect problems). Preferentially an individual might use concepts of particle nature of light where wave nature has to be considered (as in diffraction of light).

Context and Application

This topic finds its application in university courses related to electrodynamic waves and particle physics.
It is significant for the following programs:
Bachelor of Science in Physics
Master of Science in Physics
Doctor of Philosophy in Physics

Related Concepts

Control of Electromagnetic Radiation

Electromagnetic Pulse

Electromagnetic Radiation and Health

Maxwell's Equation

Gravitational Wave

Practice Problems

Q.1 Which electromagnetic wave has the shortest wavelength in the electromagnetic spectrum?

(a) X-ray

(b) Radio waves

(c) Gamma Rays

(d) Ultraviolet

Correct option: (c)

Q.2 Which electromagnetic waves are used in thermal imaging?

(a) Visible light

(b) Infrared

(c) Gamma Rays

(d) X-rays

Correct option: (b)

Q.3 How are the directions of oscillation of electric field and magnetic fields related in electromagnetic waves?

(a) Both are perpendicular to the direction of motion of electromagnetic wave.

(b) Both are parallel to the direction of propagation of electromagnetic wave.

(c) Direction of propagation of electromagnetic wave is perpendicular to the direction of oscillation of electric field and is parallel to the direction of oscillation of magnetic field.

(d) Direction of propagation of electromagnetic wave is perpendicular to the direction of oscillation of magnetic field and is parallel to the direction of oscillation of electric field.

Correct option: (a)

Q.4 Do humans emit electromagnetic radiation?

(a) Yes

(b) No

(c) Depends on the temperature

(d) Depends on some other parameters

Correct option: (a)

Q.5 Which of the following is possible value of wavelength of a radio wave?

(a) 1 km

(b) 1 micrometre

(c) 1 nanometre

(d) 1 angstrom

Correct option: (a)