What Is a Medium in Waves

Short answer

• In wave physics, a medium is the substance through which a mechanical wave travels. For a water wave, the medium is water. For most sound waves you hear, the medium is air. For a seismic wave, the medium is rock, soil, or another part of Earth. The medium is the stuff that gets disturbed, not the disturbance itself. A mechanical wave cannot travel without one, because it is a pattern of motion passing from particle to particle. Electromagnetic waves are the major exception. Light, radio waves, and X-rays are oscillations in electric and magnetic fields, so they can travel through empty space where no material medium exists at all.

What a Medium Means in Wave Physics

A wave is a disturbance that moves from the place where it was created. That definition alone does not say much until you ask what is doing the moving. In a mechanical wave, the answer is the medium. OpenStax University Physics defines a medium as the substance mechanical waves propagate through, and notes that the medium produces an elastic restoring force when it is deformed. That restoring force is the key idea. When one part of the medium is pushed out of place, a force pulls it back, and in returning it disturbs its neighbor. The disturbance is handed along, particle by particle, while each particle stays close to its own starting position.

A useful picture is a rope tied at one end. Shake the free end up and down and a wave travels along the rope. No piece of rope travels to the far end. Each section of rope moves up and down near where it started, and the pattern moves forward. The rope is the medium. The same logic applies to a pond. Drop a stone and ripples spread outward, but the water itself does not travel outward with them. A floating leaf bobs up and down as each crest passes and ends up near where it started. OpenStax makes the same point with a seagull bobbing on a surface wave: the medium moves up and down while the disturbance moves horizontally through it.

That distinction, between the motion of the medium and the motion of the wave, clears up the most common confusion about this topic. The medium carries energy and momentum from one place to another without carrying mass along with the wave. If the medium itself flowed with the wave, a pond would empty toward the shore every time a ripple crossed it, and a spoken sentence would blow the air out of a room. Neither happens, because the particles oscillate around fixed average positions while the energy moves on.

The word medium is singular. The plural is media. You will see both in textbooks, and older writing sometimes uses mediums, but media is the standard plural in physics. A single wave normally has one medium at a time, though a wave can pass from one medium into another, as sound does when it leaves air and enters water, or as light does when it leaves air and enters glass.

Mechanical Waves Need a Medium, Electromagnetic Waves Do Not

The first split to learn is between mechanical waves and electromagnetic waves, because it decides whether a medium is required at all.

Mechanical waves are governed by the laws of motion for matter, and they require a medium. Sound waves, water waves, and seismic waves are the standard examples given by OpenStax. A sound wave in air is a traveling pattern of compressions and rarefactions, which are regions of slightly higher and slightly lower pressure. A water wave is a traveling pattern of raised crests and lowered troughs on a surface, with more complicated motion below. A seismic wave is a traveling pattern of shaking in Earth itself. Remove the medium in each case and there is nothing left to disturb, so the wave cannot exist.

Electromagnetic waves work differently. OpenStax describes them as oscillations in electric and magnetic fields that do not require a medium. Gamma rays, X-rays, ultraviolet waves, visible light, infrared waves, microwaves, and radio waves all belong to this family. A changing electric field generates a changing magnetic field, and a changing magnetic field generates a changing electric field, so the pair sustains itself as it moves forward. NASA states the same conclusion in plain terms: unlike sound and water waves, light does not need any matter to carry its energy, so it can travel through a vacuum, like space. That is why sunlight reaches Earth across about 150 million kilometers of near empty space, and why starlight reaches us across vastly greater distances still.

This difference also explains a famous fact about space. Space is not perfectly empty, since it contains a very thin scattering of particles, but it is close enough to a vacuum that sound has effectively no medium to travel through. A shout in open space would have no air to carry it, so no one nearby could hear it. Astronauts do not shout across a vacuum. They speak into microphones, their voices are carried as radio waves, which need no medium, and speakers in their helmets turn those radio waves back into sound in the air inside the helmet. The lesson is not that space is silent because sound is weak there. It is silent because the medium sound needs is missing.

There is a third family, matter waves, which belong to quantum mechanics and are associated with particles such as electrons. They are not the subject of this article, and they should not be confused with mechanical waves in an ordinary material medium. For everyday wave questions about sound, water, light, and earthquakes, the mechanical versus electromagnetic split covers what you need.

How a Medium Carries a Wave, Step by Step

A mechanical wave travels because the medium has two properties working together: its particles can be displaced, and a restoring force pulls them back. Think of neighbors in a row holding hands. If you push the first person sideways, that person pulls on the next, the next pulls on the one after, and a visible wave of sideways motion travels down the row. No one walks down the row. The motion passes from person to person because each person is connected to the next.

In a gas such as air, the connection is made by collisions and pressure. A vibrating speaker cone pushes nearby air molecules together, creating a compression. Those molecules push into the region ahead, which then pushes further ahead, while the region behind relaxes into a rarefaction. In a liquid such as water, the connection is made by the forces between molecules and by gravity pulling a raised surface back toward level. In a solid such as steel, the connection is made by the strong bonds between atoms, which resist being stretched or sheared and snap back firmly. The mechanism changes with the state of matter, but the pattern is the same: displace, restore, pass on.

The direction of that particle motion gives the two basic wave shapes. In a transverse wave, the medium moves perpendicular to the direction the wave travels. The rope example is transverse: the rope moves up and down while the wave moves along the rope. In a longitudinal wave, also called a compressional wave, the medium moves parallel to the direction the wave travels. Sound in air is longitudinal: air molecules shuffle back and forth along the same line the sound travels. Some waves combine both motions. Surface water waves and seismic surface waves have particles that trace small looping paths, with both up and down and back and forth components. Knowing which motion applies matters when you ask what a given medium can and cannot carry, as the seismic section below shows.

Frequency and wavelength describe the wave, not the medium, but both are measured in the medium. Frequency is the number of cycles that pass a fixed point in one second, measured in hertz. Wavelength is the distance between matching points on neighboring cycles, such as crest to crest. Wave speed ties them together through the relation speed equals frequency times wavelength. If you want to work through that relation in detail, with examples for sound, water, and light, see how wave speed is calculated. The important point here is that the medium sets the speed, and frequency and wavelength then adjust to fit it. Change the medium and the speed changes, which is why the same sound has a different wavelength in water than in air even though its frequency stays the same.

The Medium by State of Matter: Solids, Liquids, and Gases

Almost any material can serve as a medium for some wave, but different states of matter carry waves in different ways.

Solids are usually the most effective media for mechanical waves. Their particles are held tightly together, so a disturbance is passed on quickly and firmly. Sound travels through steel at 5,960 meters per second according to the OpenStax table of sound speeds, far faster than in air. Solids can also carry both longitudinal and transverse waves, because they resist both compression and shearing. That double ability is why an earthquake produces several distinct wave types in rock, a point taken up in the seismic section below. A guitar string is another solid medium, and the string itself is the medium for the wave on the string, while the air around it becomes the medium for the sound wave that reaches your ear. One event, two media, two waves.

Liquids carry mechanical waves well, but with a limitation. A liquid resists compression, so it carries longitudinal waves such as sound efficiently. Sound travels through fresh water at 1,480 meters per second at 20 degrees Celsius, more than four times faster than in air at room temperature. A liquid does not, however, hold a shear shape the way a solid does, so it does not carry transverse waves through its interior in the same way. Surface waves on water are a special case: they live at the boundary between water and air, where gravity and surface forces provide the restoring force, rather than deep inside the liquid alone.

Gases are the softest ordinary media. Their particles are far apart and weakly connected, so a disturbance spreads more slowly and the restoring force is weaker. Sound travels through air at 331 meters per second at 0 degrees Celsius and 343 meters per second at 20 degrees Celsius. Those two numbers also show that a medium is not just a material name. Air at a different temperature is, for wave purposes, a medium in a different state, and the speed changes with it. The same applies to a gas at a different pressure or composition: helium carries sound at 965 meters per second at 0 degrees Celsius, because its light molecules move faster, while carbon dioxide carries it at only 259 meters per second under the same conditions.

A fourth state of matter, plasma, also supports waves. Plasma is ionized gas, in which some electrons have been stripped from their atoms, and it is the state of matter found in stars and in lightning channels. Waves in plasma involve both particle motion and electric and magnetic fields, so they sit between the simple mechanical picture and the electromagnetic picture. They are an advanced topic, and they do not change the basic rule: a mechanical disturbance still needs something to be disturbed, and a plasma provides charged particles that can carry it.

How Medium Properties Set Wave Speed

For a mechanical wave, speed is set by the medium, not by how hard you start the wave. OpenStax gives the general rule as a square root: wave speed equals the square root of an elastic property divided by an inertial property. In plain language, a stiffer medium transmits a wave faster, and a denser medium transmits it more slowly, all else being equal. For sound in a fluid, the elastic property is the bulk modulus, which measures resistance to compression, and the inertial property is density. For sound in a solid rod, the elastic property is Young’s modulus, which measures resistance to stretching, again divided by density. For a wave on a string, tension plays the role of the elastic property and mass per unit length plays the role of the inertial property.

The two factors pull in opposite directions, which is why a simple rule like denser means slower is only half true. Density alone would slow a wave, because heavier particles are harder to accelerate. Stiffness alone speeds a wave up, because a stronger restoring force snaps particles back faster and hands the disturbance on sooner. Real materials differ in both at once, so the winner depends on the balance. Steel is much denser than air, yet sound travels far faster in steel, because steel is enormously stiffer. Lead is denser than aluminum, yet sound travels at only 1,960 meters per second in lead compared with 5,120 meters per second in aluminum, because lead is much less stiff for its weight. Anyone who says denser always means slower is ignoring the stiffness half of the rule, and anyone who says stiffer always means faster is ignoring the density half.

Temperature changes speed because it changes both the state of the medium and how fast its particles move. In a gas, warming the gas makes its molecules move faster, and the speed of sound rises with the square root of absolute temperature. OpenStax notes that sound in air rises from 331 meters per second at 0 degrees Celsius to 343 meters per second at 20 degrees Celsius, an increase of less than 4 percent over that range. The change is modest but real, and it is one reason careful sound measurements always state the temperature. In solids and liquids, temperature effects are more complicated, because warming can change stiffness and density in different directions, but the principle stands: describe the medium fully, including its temperature, or your speed value is incomplete.

Frequency does not set the speed of sound in air for ordinary audible sounds. OpenStax notes that the speed of sound in open air is nearly independent of frequency across the audible range. If high notes traveled faster than low notes, music heard from a distance would fall apart, with the parts arriving out of step. That does not happen, because all audible frequencies travel at nearly the same speed in the same air under the same conditions. There are media where this breaks down, and they are covered in the dispersion section below, but for ordinary sound in air you can treat speed as a property of the medium and the conditions, not of the note being played.

A worked comparison shows how the same wave changes when its medium changes. Take a sound with a frequency of 440 hertz, the standard tuning note A. In air at 20 degrees Celsius, where sound travels at 343 meters per second, its wavelength is 343 divided by 440, which is 0.78 meters. In fresh water at 20 degrees Celsius, where sound travels at 1,480 meters per second, the same 440 hertz sound has a wavelength of 1,480 divided by 440, which is 3.36 meters. The frequency is unchanged, because it is set by the source. The speed is set by the medium. The wavelength stretches to fit the new speed. This is the same relation used in wave speed calculations, applied here to show what a change of medium does.

What Happens When a Wave Meets a New Medium

A wave rarely stays in one medium forever. Sound leaves a speaker in air, hits a wall, and partly enters the wall. Light leaves the Sun, crosses space, enters the atmosphere, and then may enter water or glass. At each boundary, the wave splits: part reflects back into the old medium, and part transmits into the new one. The transmitted part changes speed, because the new medium sets a new speed. Its frequency normally stays the same, because frequency is fixed by the source and by continuity at the boundary, so its wavelength must change to match the new speed.

That speed change at a boundary is what bends a wave, an effect called refraction. Light bending as it enters water is the familiar example, and it is why a straw in a glass looks broken at the surface. The full explanation, with the rule that links the angle change to the speed change, is covered in what refraction is. A related quantity is the index of refraction, which compares the speed of light in a vacuum with its speed in a given material. Water has an index of about 1.33, meaning light travels about 1.33 times slower in water than in a vacuum, and the details for water specifically are covered in the index of refraction of water. The key point for this article is that refraction is a medium effect: no change of medium, no speed change, no bending.

Reflection is also a medium effect, in a different way. When a wave meets a boundary, how much reflects and how much transmits depends on how different the two media are in the properties that carry the wave. Sound reflects strongly at an air to solid boundary partly because air and a solid differ so greatly in stiffness and density. That is why a shout echoes off a cliff face, and why a room with bare walls sounds lively while a room full of soft furnishings sounds dull. The furnishings absorb energy rather than reflecting it cleanly, which leads to the next medium effect: loss.

No real medium is perfectly elastic. As a wave travels, some of its energy is converted to heat through internal friction, and its amplitude falls with distance. This is attenuation. A distant sound fades not only because it spreads out, but because the air itself absorbs a little energy with every cycle. Low frequencies generally travel farther than high frequencies in the same medium, because they lose less energy per unit distance, which is one reason distant thunder sounds like a low rumble while nearby thunder cracks sharply. Attenuation is also why a long rope, a thick wall, or a deep body of water eventually swallows a wave entirely. The medium gives the wave its path, and it also slowly takes the wave’s energy away.

Dispersion: When One Medium Gives Different Speeds to Different Frequencies

In many media, all frequencies travel at the same speed. Such a medium is called non-dispersive. Air is very nearly non-dispersive for audible sound, which is why music survives a journey across a field intact. Some media are dispersive, meaning different frequencies travel at different speeds in the same material. A pulse made of many frequencies then spreads out and changes shape as it travels, because its parts separate.

The classic example is a glass prism. White light entering glass slows down, and it slows by slightly different amounts for different colors, because each color has a different frequency. The colors therefore bend by different amounts and fan out into a spectrum. Glass is dispersive for visible light, and that dispersion is the whole reason a prism works. Ocean waves are also dispersive in deep water: longer wavelengths travel faster than shorter ones, so the first swells to arrive from a distant storm are the long, rolling waves, with shorter chop arriving later. Dispersion is a reminder that wave speed is a property of a medium plus a frequency, not of a medium alone, and in dispersive media you must state which frequency your speed value belongs to.

Dispersion matters beyond prisms and surf forecasts. In optical fibers, which carry signals as light pulses through glass, dispersion spreads each pulse and can blur one pulse into the next, limiting how fast data can be sent over a long fiber. Engineers manage this by choosing fiber designs and wavelengths that minimize the spread. The same principle in a simpler form explains why a sharp handclap sounds crisp nearby but blurred at a distance in a dispersive environment: the frequencies that made the clap sharp no longer arrive together.

Waves Meeting in the Same Medium: Overlap and Interference

Two waves can occupy the same medium at the same time, and when they do, the medium responds to both at once. At any point, the total displacement of the medium is the sum of the displacements each wave would cause on its own. When crest meets crest, the displacements add and the result is larger. When crest meets trough, they partly or fully cancel. This is interference, and it happens in the medium, not in empty space between waves. The medium is the shared stage on which the overlap plays out.

Interference is why noise cancelling headphones work: they produce a sound wave in the air of your ear canal that cancels the unwanted wave there. It is also why some spots in a room sound loud and others sound quiet for the same note, as reflected waves add and cancel at different positions. The detailed patterns of adding and cancelling, including standing waves, are covered in constructive and destructive interference. For this article, the point is simpler: a medium does not carry one wave at a time. It carries whatever combination of waves is present, and its particles move with the sum of all of them.

Seismic Waves: One Planet, Several Media, Several Wave Types

Earthquakes provide the most dramatic demonstration that the medium decides what kind of wave can travel. An earthquake releases energy in rock, and that energy travels outward as seismic waves through the solids and liquids that form Earth. OpenStax notes that seismic waves travel through the solids and liquids of Earth, and that sound in solids can have both longitudinal and transverse components, as in a seismic wave.

The two main body waves are P-waves and S-waves. P-waves, or primary waves, are compressional waves, with particle motion parallel to the direction of travel, like sound in air. They travel through solids, liquids, and gases, and they are the fastest seismic waves, arriving first at a distant station. OpenStax gives their speed in granite as 4 to 7 kilometers per second. S-waves, or secondary waves, are shear waves, with particle motion perpendicular to the direction of travel. They travel only through solids, because a liquid or gas cannot support the shearing force they need. OpenStax gives their speed in granite as 2 to 5 kilometers per second, always slower than P-waves in the same rock.

That difference is not a curiosity. It is evidence about Earth’s interior. Because S-waves do not pass through the liquid outer core, seismologists observe a shadow zone where S-waves from a distant earthquake never arrive, and that absence is part of the evidence that the outer core is liquid. A medium can therefore reveal itself by the waves it refuses to carry, as surely as by the waves it does carry. If you want the broader story of how seismic monitoring works, see US Geological Survey seismic activity.

Surface waves add a third group. Love waves shake the ground side to side along the surface, and Rayleigh waves roll the ground in a looping motion like an ocean wave. They travel more slowly than body waves but often cause the most damage, because their energy stays near the surface instead of spreading through Earth’s volume. All of them, body and surface alike, are waves in a medium. Change the medium from solid rock to loose sediment and the waves slow down and grow larger in amplitude, which is one reason earthquake shaking is often stronger on soft ground than on bedrock.

Light in a Medium: Slower, But Still Medium Free in Principle

Light deserves a section of its own, because it confuses the medium question more than any other wave. Light travels through a vacuum at 299,792,458 meters per second, the speed usually written as c. When light enters a material medium such as air, water, or glass, it slows down. It does not slow because it suddenly needs a medium. It slows because its electric and magnetic fields interact with the charged particles in the material, which absorb and re-emit the energy in a way that delays the overall progress of the wave. The light is still an electromagnetic wave, and the material is a medium it passes through, not a medium it requires.

NASA makes the same distinction for space flight: electromagnetic radiation does not require a medium for transmission, and it slows as it passes through a medium such as air, water, or glass. That single sentence carries both halves of the idea. No medium is needed for the wave to exist, and a medium that is present still affects the wave’s speed. The full family of electromagnetic waves, from radio waves to gamma rays, is described in what the electromagnetic spectrum is, and the visible slice of it, with its wavelengths, is covered in the wavelength of visible light.

A related point concerns heat transfer. Radiation carries energy through empty space as electromagnetic waves, which is how sunlight warms Earth. Conduction and convection, by contrast, need a material medium: conduction passes heat through direct contact between particles, and convection carries it in moving fluid. That contrast is explained with examples in radiation, convection, and conduction examples. It is a useful cross check on the medium idea. If a process needs particles to bump into each other, it needs a medium. If it is carried by fields, it does not.

The Old Ether Idea, and Why Physicists Dropped It

For centuries, physicists assumed light must have a medium too. If sound needs air and water waves need water, surely light needs something to wave in. They called that hypothetical substance the luminiferous ether, imagined as an invisible, weightless material filling all of space. The idea was reasonable, and it was tested seriously. The most famous test, the Michelson-Morley experiment of 1887, tried to detect Earth’s motion through the ether by comparing light speeds in different directions. It found no difference. Later work, including Einstein’s theory of special relativity, showed that light needs no ether at all, because its speed in a vacuum is the same for all observers and its fields sustain themselves.

The ether story is worth knowing because it shows how the medium concept was refined, not abandoned. Physicists did not conclude that media do not exist. They concluded that a medium is required only where the wave is a disturbance in matter. For electromagnetic waves, the fields themselves are the disturbance, and empty space is a legitimate place for fields to exist. The modern rule is therefore sharper than the old one: ask what is waving. If matter is waving, name the matter. If fields are waving, no material medium is needed, though a material medium in the path will still slow, bend, absorb, or reflect the wave.

Common Misconceptions About Media

Several misunderstandings recur whenever this topic is taught, and each comes from mixing up the wave with its medium.

First, the medium does not travel with the wave. Water in a pond, air in a room, and rock under a city all stay roughly where they are while waves pass through them. A surfer is carried toward shore by a breaking wave, but that is a special case where the wave itself breaks and collapses, not ordinary wave travel. In an ordinary traveling wave, particles oscillate and the pattern advances.

Second, a vacuum is not a medium. It is the absence of a medium. Saying that light travels through the medium of a vacuum confuses the issue. Light travels through a vacuum precisely because it does not need a medium. Space is sometimes described as a medium for light in loose popular writing, but in physics teaching that wording is misleading and best avoided.

Third, louder does not mean faster, and brighter does not mean faster. Amplitude measures how large a disturbance is, not how fast it moves. A louder sound in the same air travels at the same speed as a quiet one. A brighter light in the same glass travels at the same speed as a dim one. Speed is set by the medium and, in dispersive media, by frequency, not by amplitude.

Fourth, a medium is not always a single pure substance. Air is a mixture of gases, seawater is salty water, and rock is a mixture of minerals, yet each acts as a medium with measurable wave speeds. What matters is not purity but the combination of elastic and inertial properties over the region the wave crosses. A wave crossing from fresh water into salt water, or from one rock layer into another, meets a new medium and changes speed accordingly, even though both sides are mixtures.

Fifth, sound does not travel at one fixed speed. Students often memorize a single number for the speed of sound, then apply it everywhere. The OpenStax table alone gives 331 meters per second in air at 0 degrees Celsius, 343 at 20 degrees Celsius, 1,480 in fresh water at 20 degrees Celsius, 1,540 in seawater, and 5,960 in steel. A speed value without its medium and conditions is an incomplete answer, and a good habit is to state all three: the wave, the medium, and the conditions.

How to Identify the Medium in Any Wave Question

When you meet a new wave, work through four questions in order. First, what type of wave is it? If it is mechanical, it has a medium, and your next job is to name it. If it is electromagnetic, it needs no medium, though it may be passing through one. Second, what is the medium made of, and in what state? Air, water, steel, and rock are different media with different speeds, even for the same wave type. Third, what are the conditions? Temperature, pressure, and composition all shift the speed, so a careful answer names them. Fourth, what is the wave doing at any boundary? Reflection, refraction, absorption, and interference all depend on the media on both sides, not on one side alone.

A worked habit helps. Suppose a question asks why you hear a distant train sooner through a rail than through the air. Name the media: steel rail and air. Compare their properties: steel is far stiffer than air, so sound travels far faster in steel despite steel being denser. The sound reaches you through the rail first, even though it started at the same time in both media. Suppose instead a question asks why you can see lightning before you hear thunder. Name the waves: light is electromagnetic and needs no medium, traveling at about 300 million meters per second, while thunder is sound in air traveling at about 343 meters per second. The light wins by an enormous margin, and the delay between flash and sound lets you estimate the storm’s distance.

These habits also protect against a subtle error: naming the source as the medium. A speaker is not the medium for sound. The air between the speaker and your ear is. A dropped stone is not the medium for a ripple. The water is. An earthquake fault is not the medium for a seismic wave. The rock is. The source creates the disturbance, the medium carries it, and the receiver detects it. Keeping those three roles separate makes almost every wave question easier.

Frequently Asked Questions

What is a medium in waves in simple terms?

A medium is the substance a wave travels through. Air is the medium for most sounds, water is the medium for ripples, and rock is the medium for seismic waves. The medium’s particles vibrate or shift as the wave passes, but they do not travel along with the wave. Electromagnetic waves such as light are the exception, because they travel as oscillating electric and magnetic fields and need no material medium.

What are examples of a medium for waves?

Common examples are air for sound, water for water waves, steel or another solid for vibrations in a structure, and rock and soil for seismic waves. A guitar string is the medium for the wave on the string, and the air around the guitar is the medium for the sound that reaches a listener. Each example pairs one wave with the substance that carries it.

Do all waves need a medium?

No. Mechanical waves need a medium, because they are disturbances in matter. Electromagnetic waves do not need a medium, because they are disturbances in electric and magnetic fields. Light, radio waves, microwaves, infrared waves, ultraviolet waves, X-rays, and gamma rays can all travel through a vacuum, which is why sunlight crosses space to reach Earth.

Can sound travel in space?

No, not as sound. Space is close to a vacuum, so there is effectively no air or other material for a sound wave to travel through. Astronauts communicate by radio waves, which need no medium, and those waves are converted back into sound inside their helmets. Sound can, however, travel through the solid material of a spacecraft or a spacesuit, because a solid is a medium.

How does a medium affect wave speed?

A medium sets wave speed through its stiffness and its density. Greater stiffness speeds a wave up, because the restoring force is stronger. Greater density slows a wave down, because heavier particles are harder to move. The balance of the two decides the result, which is why sound is faster in steel than in air even though steel is denser, and slower in lead than in aluminum even though both are metals.

Does light slow down in a medium?

Yes. Light travels fastest in a vacuum, at 299,792,458 meters per second. In air it travels very slightly slower, in water it travels about 1.33 times slower, and in glass it travels slower still. The slowing happens because light interacts with the charged particles in the material. It does not mean light needs the material: light can travel through a vacuum, and a material in its path only changes its speed and direction.

What is the difference between a medium and a vacuum?

A medium is matter that a mechanical wave travels through, such as air, water, or rock. A vacuum is the absence of matter. A mechanical wave cannot travel through a vacuum because there is nothing to disturb. An electromagnetic wave can, because its disturbance is in fields rather than in matter. That difference is the cleanest test of whether you have understood the medium concept.

Why does sound travel faster in water than in air?

Sound travels faster in water mainly because water is much stiffer, meaning much harder to compress, than air. Water is also denser than air, which on its own would slow sound down, but the stiffness advantage is far larger than the density disadvantage. The measured result is 1,480 meters per second in fresh water at 20 degrees Celsius, compared with 343 meters per second in air at the same temperature.

Sources & references

• OpenStax, University Physics Volume 1, Section 16.1, Traveling Waves: definition of a medium as the substance mechanical waves propagate through, mechanical versus electromagnetic waves, transverse and longitudinal motion, and the relation speed equals frequency times wavelength. https://openstax.org/books/university-physics-volume-1/pages/16-1-traveling-waves
• OpenStax, University Physics Volume 1, Section 17.2, Speed of Sound: the square root rule for wave speed from elastic and inertial properties, the table of sound speeds in gases, liquids, and solids, the temperature dependence of sound in air, P-wave and S-wave speeds in granite, and the fact that S-waves do not pass through the liquid core. https://openstax.org/books/university-physics-volume-1/pages/17-2-speed-of-sound
• NASA Science, Big Idea 2.3, Intermediate Level Guiding Question: light does not need matter to carry its energy and can travel through a vacuum, like space. https://science.nasa.gov/big-idea-2-3-intermediate-level-guiding-question/
• NASA Science, Basics of Space Flight, Chapter 6, Electromagnetics: electromagnetic radiation does not require a medium for transmission and slows as it passes through a medium such as air, water, or glass. https://science.nasa.gov/learn/basics-of-space-flight/chapter6-1/
• US Geological Survey, Body Waves Inside the Earth: P-waves as compressional waves and S-waves as shear waves, their relative speeds, and their behavior in rock. https://earthquake.usgs.gov/earthquakes/events/1906calif/18april/earthwaves.php