Main Difference – Electromagnetic vs. Mechanical Waves
A wave is made of some type of disturbance that travels. Depending on their properties, we can classify waves into several different types. One of the distinctions is based on whether disturbances in a wave need a medium to travel through. In this way, we classify waves into electromagnetic and mechanical waves. The main difference between mechanical and electromagnetic waves is that electromagnetic waves do not require a medium to propagate whereas mechanical waves require a medium in order to propagate.
What are Mechanical Waves
Mechanical waves consist of disturbances that can only travel through a medium. You can create a simple wave by wiggling a rope up and down, and this is a mechanical wave. When one molecule moves up, it makes the neighbouring molecules also move up, and in this way the disturbance moves all the way along the rope. Here, the rope is the medium of the wave because it is the motion of “rope molecules” that carries the disturbance along.
Another very good example of a mechanical wave is a sound wave. Sound waves are made of oscillating molecules. Typically, when we hear sound, our ear is detecting the back-and-forth motion of air molecules (you can check that sound really is made of vibrating air molecules by placing a candle in front of a speaker, see the video below). The brain interprets this back-and-forth motion of air molecules as “sound”. We can also hear sound through vibrations of other types of molecules: you can hear under water because of the vibrations in water molecules, for instance.
What makes sound waves mechanical is that the sound would not be able to propagate if there was no medium. For instance, imagine putting a ringing bell inside a vacuum jar and slowly letting the air out. As air moves out of the jar, the sound would become fainter. When there is no air inside the jar, the bell is still vibrating but there is nothing in between to carry the sound outside, so the sound stops. If you slowly let air in, you can start to hear the sound again. This experiment is shown in the video below:
What are Electromagnetic Waves
Electromagnetic waves consist of disturbances that can travel without a medium. For example, the light from the Sun is an electromagnetic wave that travels through the vacuum between the Earth and the Sun. Electromagnetic waves can do this because they do not rely on vibrating molecules. Instead, what is oscillating in an electromagnetic wave is an electric field. There is a magnetic field as well, which is oscillating in phase with the electric field, at 90o to the electric field. The vibrations are conducted in a direction that is at 90o to both the oscillations in the electric field and the magnetic field. A short animation of how these fields vibrate is shown in the video below:
In vacuums, all electromagnetic waves travel at a speed of about 3×108 m s-1. This is often called the speed of light in vacuum. Depending on their wavelength, they can be classified into several different types. The diagram blow shows these different types, starting from larger wavelengths (left) to smaller wavelengths (right).
Since the oscillations are at right angles to the direction of propagation, electromagnetic waves are transverse waves. This means that electromagnetic waves can be polarized.
Difference Between Mechanical and Electromagnetic Waves
Method of Propagation
Mechanical waves require a medium to propagate.
Electromagnetic waves do not require a medium to propagate. They can propagate in a vacuum.
Mechanical waves travel slower than electromagnetic waves.
Electromagnetic waves travel at 3×108 m s-1 in vacuums. When they travel through other media, they slow down a little.
Type of Waves
Mechanical waves could be transverse or longitudinal. When they are longitudinal, they cannot be polarized.
Electromagnetic waves are transverse waves, so they can be polarized.
“A diagram of the Milton spectrum, showing the type, wavelength (with examples), frequency, the black body emission temperature…” by Inductiveload, NASA (self-made, information by NASA) [CC BY-SA 3.0], via Wikimedia Commons