Wave Effects at Interfaces

What Happens at a Boundary?

Have you ever looked at a glass window and seen both the outside world and your own faint reflection? This happens because waves rarely do just one thing when they meet an interface (boundary) between two different materials.

When a wave hits a boundary, its energy is divided into three different processes: reflection, transmission, and absorption.
The total energy must be conserved, meaning the percentages of energy shared between these three processes will always add up to $100\%$ .
A crucial rule for all waves is that their frequency remains entirely constant across any boundary. The frequency is set by the wave source and does not change, no matter what happens to the wave's speed or direction.

Worked Example

A beam of light shines onto a dark, tinted glass window. $18\%$ of the wave's energy is reflected back into the air, and $55\%$ of the energy is absorbed by the dark glass. Calculate the percentage of the wave's energy that is transmitted through the window.

Step 1: State the principle of energy conservation.

Total energy = Reflected + Absorbed + Transmitted
Total energy = $100\%$

Step 2: Substitute the known values.

$100\% = 18\% + 55\% + \text{Transmitted}$

Step 3: Rearrange and calculate the transmitted energy.

$\text{Transmitted} = 100\% - (18\% + 55\%)$
$\text{Transmitted} = 100\% - 73\% = 27\%$

Reflection (Bouncing Back)

Reflection occurs when a wave hits a boundary and bounces back into the original medium. Because the wave stays in the same material, its speed, wavelength, and frequency all remain identical.

To describe or measure reflection, you must first draw a Normal line—an imaginary dashed line exactly perpendicular ( $90^\circ$ ) to the boundary.
All angles are measured between the ray and this Normal line.

Refraction (Changing Direction)

Refraction is a change in the wave's direction that is caused entirely by a change in speed as it enters a new medium. Different materials have different levels of optical density, which dictates how fast light can travel through them.

If a wave enters a denser medium (like moving from air to glass), it slows down and bends towards the Normal.
If a wave enters a less dense medium (like moving from glass to air), it speeds up and bends away from the Normal.
Because speed ( $v$ ) changes but frequency ( $f$ ) stays the same, the wave equation ( $v = f \lambda$ ) dictates that the wavelength ( $\lambda$ ) must also change proportionally.

If you are studying Higher Tier, you must understand refraction in terms of wavefronts.

First, a wavefront approaches a boundary at an angle.
Then, one edge of the wavefront crosses the boundary and changes speed immediately.
The rest of the wavefront is still in the old medium, traveling at the original speed.
Finally, this staggered change in speed causes the entire wave to pivot, resulting in a change of direction.

Transmission and Absorption

If a wave does not reflect or refract away, it will interact with the inside of the new material through transmission or absorption.

Transmission is the process where a wave successfully passes all the way through a material and emerges out the other side. Because some energy is usually lost to reflection or absorption at the boundary, a transmitted wave typically has a lower amplitude than the original incident wave.
Absorption occurs when the wave is stopped by the material and its energy is taken in.

When a wave is absorbed, you must describe the mechanism step-by-step:

The wave strikes the material interface.
The particles at the boundary absorb the wave's energy.
This energy is transferred into the material's internal energy stores.
The particles vibrate more vigorously, gaining kinetic energy.
This increase in internal energy causes a measurable rise in the material's temperature.

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Exam Tips

Common Mistake
Students often think the frequency of a wave changes when it slows down in a new medium. Frequency is determined entirely by the wave's source and remains constant; it is the wavelength that changes.
2
In 6-mark questions asking you to describe absorption, examiners will specifically award marks for the exact phrase 'transfer of energy to internal energy stores'.
3
When drawing ray diagrams for reflection or refraction, always draw the Normal line first as a dashed line at 90° to the boundary. Make sure you measure all your angles from this Normal, not from the surface of the material.

Key Terms(13)

Interface (boundary): The exact plane or surface where two different materials or media meet.
Reflection: The process where a wave hits a surface and bounces back into the medium it came from.
Refraction: The change in direction of a wave crossing a boundary, caused by a change in its speed.
Transmission: The process by which a wave passes through a material and emerges from the other side.
Absorption: The process where a wave's energy is taken up by a material, transferring energy to its internal energy stores.
Internal energy stores: The total kinetic and potential energy stored by the particles that make up a system.
Normal: An imaginary dashed line drawn exactly perpendicular (at 90 degrees) to a surface or boundary.
Optical density: A measure of how much a material slows down light traveling through it.
Wavefronts: Imaginary lines representing corresponding points of a wave that vibrate in unison, often drawn as the crests of waves.
Amplitude: The maximum displacement of a wave from its undisturbed (rest) position.
Frequency: The number of wave cycles passing a point per second, measured in Hertz (Hz).
Wavelength: The distance between two corresponding points on adjacent waves (e.g. crest to crest).
Kinetic energy: The energy an object or particle has due to its motion.

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Key Terms(13)

Interface (boundary): The exact plane or surface where two different materials or media meet.
Reflection: The process where a wave hits a surface and bounces back into the medium it came from.
Refraction: The change in direction of a wave crossing a boundary, caused by a change in its speed.
Transmission: The process by which a wave passes through a material and emerges from the other side.
Absorption: The process where a wave's energy is taken up by a material, transferring energy to its internal energy stores.
Internal energy stores: The total kinetic and potential energy stored by the particles that make up a system.
Normal: An imaginary dashed line drawn exactly perpendicular (at 90 degrees) to a surface or boundary.
Optical density: A measure of how much a material slows down light traveling through it.
Wavefronts: Imaginary lines representing corresponding points of a wave that vibrate in unison, often drawn as the crests of waves.
Amplitude: The maximum displacement of a wave from its undisturbed (rest) position.
Frequency: The number of wave cycles passing a point per second, measured in Hertz (Hz).
Wavelength: The distance between two corresponding points on adjacent waves (e.g. crest to crest).
Kinetic energy: The energy an object or particle has due to its motion.

Wave Effects at Interfaces

What Happens at a Boundary?

When a wave hits a boundary, its energy is divided into three different processes: reflection, transmission, and absorption.
The total energy must be conserved, meaning the percentages of energy shared between these three processes will always add up to $100\%$ .
A crucial rule for all waves is that their frequency remains entirely constant across any boundary. The frequency is set by the wave source and does not change, no matter what happens to the wave's speed or direction.

Worked Example

Step 1: State the principle of energy conservation.

Total energy = Reflected + Absorbed + Transmitted
Total energy = $100\%$

Step 2: Substitute the known values.

$100\% = 18\% + 55\% + \text{Transmitted}$

Step 3: Rearrange and calculate the transmitted energy.

$\text{Transmitted} = 100\% - (18\% + 55\%)$
$\text{Transmitted} = 100\% - 73\% = 27\%$

Reflection (Bouncing Back)

To describe or measure reflection, you must first draw a Normal line—an imaginary dashed line exactly perpendicular ( $90^\circ$ ) to the boundary.
All angles are measured between the ray and this Normal line.

Refraction (Changing Direction)

If a wave enters a denser medium (like moving from air to glass), it slows down and bends towards the Normal.
If a wave enters a less dense medium (like moving from glass to air), it speeds up and bends away from the Normal.
Because speed ( $v$ ) changes but frequency ( $f$ ) stays the same, the wave equation ( $v = f \lambda$ ) dictates that the wavelength ( $\lambda$ ) must also change proportionally.

If you are studying Higher Tier, you must understand refraction in terms of wavefronts.

First, a wavefront approaches a boundary at an angle.
Then, one edge of the wavefront crosses the boundary and changes speed immediately.
The rest of the wavefront is still in the old medium, traveling at the original speed.
Finally, this staggered change in speed causes the entire wave to pivot, resulting in a change of direction.

Transmission and Absorption

If a wave does not reflect or refract away, it will interact with the inside of the new material through transmission or absorption.

Transmission is the process where a wave successfully passes all the way through a material and emerges out the other side. Because some energy is usually lost to reflection or absorption at the boundary, a transmitted wave typically has a lower amplitude than the original incident wave.
Absorption occurs when the wave is stopped by the material and its energy is taken in.

When a wave is absorbed, you must describe the mechanism step-by-step:

The wave strikes the material interface.
The particles at the boundary absorb the wave's energy.
This energy is transferred into the material's internal energy stores.
The particles vibrate more vigorously, gaining kinetic energy.
This increase in internal energy causes a measurable rise in the material's temperature.

Sign up to continue reading

Get full access to revision notes, key terms, and exam tips for every subtopic.

Exam Tips

Common Mistake
Students often think the frequency of a wave changes when it slows down in a new medium. Frequency is determined entirely by the wave's source and remains constant; it is the wavelength that changes.
2
In 6-mark questions asking you to describe absorption, examiners will specifically award marks for the exact phrase 'transfer of energy to internal energy stores'.
3
When drawing ray diagrams for reflection or refraction, always draw the Normal line first as a dashed line at 90° to the boundary. Make sure you measure all your angles from this Normal, not from the surface of the material.

Key Terms(13)

Interface (boundary): The exact plane or surface where two different materials or media meet.
Reflection: The process where a wave hits a surface and bounces back into the medium it came from.
Refraction: The change in direction of a wave crossing a boundary, caused by a change in its speed.
Transmission: The process by which a wave passes through a material and emerges from the other side.
Absorption: The process where a wave's energy is taken up by a material, transferring energy to its internal energy stores.
Internal energy stores: The total kinetic and potential energy stored by the particles that make up a system.
Normal: An imaginary dashed line drawn exactly perpendicular (at 90 degrees) to a surface or boundary.
Optical density: A measure of how much a material slows down light traveling through it.
Wavefronts: Imaginary lines representing corresponding points of a wave that vibrate in unison, often drawn as the crests of waves.
Amplitude: The maximum displacement of a wave from its undisturbed (rest) position.
Frequency: The number of wave cycles passing a point per second, measured in Hertz (Hz).
Wavelength: The distance between two corresponding points on adjacent waves (e.g. crest to crest).
Kinetic energy: The energy an object or particle has due to its motion.

Previous Subtopic: Depth CalculationDepth Calculation Next Subtopic: Refraction at BoundariesRefraction at Boundaries

Back to Wave Effects at Interfaces

Put your knowledge into practice — try past paper questions for Physics

Key Terms(13)

Interface (boundary): The exact plane or surface where two different materials or media meet.
Reflection: The process where a wave hits a surface and bounces back into the medium it came from.
Refraction: The change in direction of a wave crossing a boundary, caused by a change in its speed.
Transmission: The process by which a wave passes through a material and emerges from the other side.
Absorption: The process where a wave's energy is taken up by a material, transferring energy to its internal energy stores.
Internal energy stores: The total kinetic and potential energy stored by the particles that make up a system.
Normal: An imaginary dashed line drawn exactly perpendicular (at 90 degrees) to a surface or boundary.
Optical density: A measure of how much a material slows down light traveling through it.
Wavefronts: Imaginary lines representing corresponding points of a wave that vibrate in unison, often drawn as the crests of waves.
Amplitude: The maximum displacement of a wave from its undisturbed (rest) position.
Frequency: The number of wave cycles passing a point per second, measured in Hertz (Hz).
Wavelength: The distance between two corresponding points on adjacent waves (e.g. crest to crest).
Kinetic energy: The energy an object or particle has due to its motion.