The Different Types of Waves
There are two types of waves: mechanical waves like sound that must travel through a medium like air, and electromagnetic waves like light that don’t. Both types of waves transfer energy in a wave-like manner, as that is what a wave is, a physical effect of energy being transferred over space and time.
On this page, we explain the different types of waves, the difference between sound waves and light waves, and the difference between mechanical waves and electromagnetic waves. I suggest this video for a visual overview.
Mechanical and Electromagnetic Waves. Sound is a mechanical wave; that means it must travel through a medium. Light is electromagnetic radiation propagating in a wave-like manner; it can travel through empty space or a medium.
Summary of the Difference Between Mechanical Waves and Electromagnetic Waves
- Mechanical waves like sound are pressure traveling from a source, through a solid, liquid, gas, or plasma, medium, distorting that medium’s molecules. Mechanical waves aren’t made out of anything and can’t travel through “empty space.”
- Electromagnetic waves describe the wave-like manner in which photons (and other quanta) travel. Electromagnetic waves can travel through a medium like air, passing through it or reflecting off it’s molecules, but they don’t have to. Electromagnetic waves are made out of energy and can travel unimpeded through empty space.
Other Categorizations of Waves
Waves can also be divided into three categories based on the direction of their movement:
- Transverse waves travel perpendicular to wave motion.
- Longitudinal waves travel parallel to wave motion.
- Surface waves travel in a circular motion on the surface of a medium.
- Classic waves are continuous.
- Quantum waves exist in quantum superposition and quantize in discrete packets.
Beyond this, there are some other ways to categorize waves like a cresting surface wave, but this is enough to understand the basics of waves and wave-like movement. If we consider waves broadly, we can say that all forms of energy are transferred by waves (see forms of energy).
Traveling Waves: Crash Course Physics #17. A video is discussing waves the properties of different types of waves.
FACT: In almost all instances light travels faster than sound, but not always. See instances where sound can travel faster than light.
TIP: The carrier of electromagnetic force is the photon. The photon carries electromagnetic radiation. Light is a specific wavelength in the electromagnetic spectrum, and so are radio waves, X-rays, and some other recognizable waves. The photon is one of the primary building blocks of matter; sound is an effect of objects made out of matter.Aa mechanical wave displaces particles while an electromagnetic wave is itself particles (see the standard model of particle physics for a closer look at photons).
What are Mechanical Waves?
Mechanical waves like sound are pressure traveling from a source, through a solid, liquid, gas, or plasma, medium, distorting that medium’s molecules.
For example, a sound wave traveling through the air displaces air molecules, causing them to vibrate and collide with other air molecules at a given frequency as the wave travels through the medium. The molecules bounce back in place; for every force there is an equal and opposite force. The result is molecules vibrating back and forth parallel to the source. This is true for gasses and liquids, but not for a denser matter, see Longitudinal Waves, Transverse Waves, and Circular Waves section below.
A water molecule doesn’t travel from one place to another with the wave, the wave travels through the water while the molecules bounce back into place. The wave of pressure created from the source of the sound travels through the medium until it reaches our ears, we hear the vibrating of molecules at certain frequencies as sound via our sense of hearing (click that link to understand the human senses, it is vital to understanding waves).
TIP: There is no sound particle, a sound is an effect that displaces molecules made of particles. Most waves that aren’t quantum waves like light are mechanical waves including ocean waves and gravitational waves as far as we know.
TIP: You can use electromagnetic energy to measure all particles, so you can think of electromagnetic waves as being like electricity and mechanical waves as being like gravity. One is a thing; the other is an effect. Both transfer energy.
What are Electromagnetic Waves?
Electromagnetic waves describe the wave-like manner in which photons (and other quanta) travel. Electromagnetic waves can travel through a medium like air, but they don’t have to. When they do, they travel between air molecules and have their travel slowed accordingly. Even though photons can bounce off some mediums or be slowed down, it doesn’t affect the medium directly like the concussive nature of sound.
Electromagnetic waves travel through the vacuum of empty space and are massless. Thus, they don’t directly affect objects with mass. “The speed of light” is a term that describes electromagnetic energy traveling at it’s constant and only speed in a true vacuum although it can slow down in a non-vacuum when it does interact with the medium.
TIP: Only objects with mass can affect objects with mass. A photon can only add to the mass of a system by being absorbed by the system.
TIP: Electromagnetic energy is carried by a particle, the photon. The photon doesn’t displace other molecules to create a wave, the photon itself propagates in a wave-like manner.
FACT: This means that in space no one can hear you scream, but they can see you shine a flashlight.
7.1.3 Mechanical and Electromagnetic Waves.
The Major Differences Between Mechanical Waves and Electromagnetic Waves
Given the above, the differences between mechanical waves like sound and electromagnetic waves like light are:
- Mechanical waves have to travel through something, electromagnetic and waves don’t.
- Mechanical waves aren’t made out of anything. They are the disruption of a medium. Just like a wave in water, the wave is pressure displacing water; it isn’t itself a thing. There is no widget, just an effect. Electromagnetic waves, on the other hand, are made out of something, electromagnetic energy. There are only four forces in the universe. The most important by many measures is electromagnetic energy.
Longitudinal Waves, Transverse Waves, and Surface Waves
Mechanical waves can propagate as longitudinal waves, transverse waves, or surface waves, but electromagnetic waves can only propagate as transverse waves. Understanding the ways in which a wave can travel is important for understanding the difference between sound and light.
Most mechanical waves, including sound, are “longitudinal waves” or a wave in which the particles of the medium move in a direction parallel to the direction that the wave moves. When sound displaces molecules in the air, they push forward in the direction of the wave, bump into the next molecule, and then bounce back into place in the opposite direction because the medium is elastic enough and for every force, there is a equal and opposite force. The result is particles moving back and forth parallel to the direction of the wave.
Light waves are always “transverse waves” or a wave in which particles of the medium move in a direction perpendicular to the direction that the wave moves.
Meanwhile, the waves that appear on the surface of a medium are surface waves, they are sometimes called circular waves because in surface waves only the particles at the surface of the medium undergo circular motion that is a combination of transverse and longitudinal motion. Water has surface waves on its surface, and longitudinal waves in its currents. There is no rigid surface in water, so there are no longitudinal waves. There are also elliptical surface waves called Raleigh surface waves.
Some mechanical waves can act as more than one wave type. Earthquakes are capable of producing both transverse waves that travel through the less dense parts of the Earth, like water and the molten core, and longitudinal waves that travel through the denser parts of the Earth, like rock. Transverse waves require a relatively rigid medium to transmit their energy while longitudinal waves require a more elastic medium.
Physics Lecture – 40 – Transverse and Longitudinal Waves.
The Difference Between Sound Waves and Light Waves
Other important differences between sound waves and light waves include:
- With sound waves, frequency tells you about the pitch, and amplitude gives you information about the volume. With light, amplitude gives you information about intensity, and frequency tells you about wave type or color.
- Sound travels faster in most solids than it does in air, light typically slows down or reflects off solids.
FACT: If we consider the Grand Unified Theory, which unifies every force but gravity at the start of the big bang, we are left with little more than electromagnetic energy to explain the universe. This is why the theory that “we live in the Matrix” is not off the table.
The Similarities Between Mechanical Waves and Electromagnetic Waves
Some similarities between mechanical waves like sound and electromagnetic waves like light are:
- Both wave types are defined by their frequency. The higher the frequency, the less distance between successive crests of a wave (aka wavelength). We can only hear and see waves in a specific frequency; we can only hear and see specific wavelengths.
- Both types of waves can reflect and amplify under the right conditions.
Classic Wave Versus Quantum Waves
A classic wave is a continuous wave; a quantum wave exists in an uncertainty of superpositions (more than one place at once) until it quantizes.
Classic waves are associated with mechanical waves, and travel through something and quantum waves don’t have to.
All the elementary particles exist as quantum fields (which act like quantum waves, learn more about quantum field theory and the photon). Effects like sound and gravitational waves, that travel through mediums made out of Quanta. All matter is made of Quanta. Quanta is another word for quantum particles, quantum particles make up molecules and act like classic waves.
- Mechanical waves resulting from pressure, like gravitational and sound waves, are continuous and consistent. Like if you hit a drum head and see the head ripple.
- Electromagnetic energy is a quantum wave, if we consider a single photon (the carrier of electromagnetic energy) we can say: The photon is an electromagnetic field, propagated in a unidirectional wave-like manner, that exhibits quantized measurable excited energy states we call particles. Until we measure the location of a photon, it exists in uncertainty. Once we measure it, it quantizes to a specific excited state in orders of the Planck constant.