What happens when waves reflect and refract?

What are the key points about the reflection and refraction of waves?

Wavefronts.
Reflection wavefront diagrams.
Refraction wavefront diagrams.

How are wavefronts observed in a ripple tank?

A ripple tank can be used to investigate the , and the speed of water waves.

A ripple tank is a transparent shallow tray of water with a light shining down through it onto a white card below.

The light allows you to see the motion of the ripples created on the water's surface more easily.

Ripples can be made by hand but to generate regular ripples it is better to use a motor.

A ripple tank on a table, with surrounding equipment including a lamp, motor, power supply, meter ruler, wooden rod, white card.

Wavefronts

When drawing wave diagrams, it is easier to draw wavefronts rather than and .

Each wavefront is drawn at right angles to the wave direction.

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1. A wooden bar vibrating up and down creates waves on the surface of the water.
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2. The wavefronts are lines joining the crests of the same wave.
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3. The wavefronts are perpendicular to the direction of travel of the waves.
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4. The wavelength is the distance between two consecutive wavefronts.
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5. Wave diagram showing the wavelength, wavefront and direction of wave travel.

How are waves reflected?

Waves - including water waves, sound and light - can be reflected at the boundary between two different materials.

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1. Incident wave before hitting the barrier.
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2. Incident wave strikes the solid barrier.
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3. The angle of incidence is the angle between the incident wave direction and the normal. The wavelength is the distance between two consecutive wavefronts.
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4. The wave reflects from the solid barrier.
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5. The angle of reflection is the angle between the reflected wave direction and the normal.
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6. Waves, such as water waves, obey the law of reflection that states: angle of incidence i = angle of reflection r. The diagram shows that when water waves are reflected their direction changes, but their wavelength and frequency remain unchanged.

Key points

When waves are reflected:

angle of incidence i = angle of reflection r
wavelength remains unchanged
frequency remains unchanged

How are waves refracted?

Refraction is the change in direction of a wave as it travels from one medium to another.

For instance, when light travels from air to glass or water waves travel from deep to shallow water.

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1. A water wave approaches a boundary between deep and shallow water. The normal is an imaginary line perpendicular to the boundary. Notice the normal extends across - on both sides of the boundary)
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2. The water wave travels fast in deep water. As the wave crosses the boundary between deep water into shallow water, the speed of the wave changes.
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3. The wave travels slower in the shallow water (slower = towards).
$The wavelength of the refracted wave is now decreased as the speed decreases.$
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4. The wavelength of the refracted wave has decreased as the speed has decreased. The refracted wave now travels in a direction closer to the normal.
$The wave has <slows down and refracted towards the normal).$
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5. Remember: Angles are always measured between the direction of the wave and the normal. Going from deep to shallow, the angle of refraction will be smaller than the angle of incidence. The wave has slowed down and refracted towards the normal).

As the waves slow down the waves change direction towards the normal.

The of the waves does not change because the source of the waves continues to vibrate at the same frequency.

Since v = f λ

If the speed, v, decreases and the frequency, f, remains unchanged, then the , λ, must also decrease.

$\lambda$shallow is shorter than $\lambda$deep

Wave moving from shallow to deeper water

Key point

FAST is a useful way of remembering the speed and direction changes of waves during refraction:

If waves get Faster, they bend Away from the normal
If waves get Slower, they bend Towards the normal

Letter	Meaning
F	Faster
A	Away
S	Slower
T	Towards

What happens to the speed and wavelength when waves are refracted?

If waves are incident along the normal when passing from one medium to another, their speed and change but their direction does not change.

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Speed and wavelength
Waves incident along the normal and passing from deep to shallow water...
Image caption,
Speed and wavelength
and from shallow to deep water.

The diagram below shows the refraction of waves passing from air to water (the equivalent of from deep to shallow).

As the waves travel from air to water, they slow down – water is denser than air.

They refract towards the normal and wavelength decreases.

The frequency of the waves remains unchanged.

Property	Reflection	Refraction: deep to shallow	Refraction: shallow to deep
Speed v	Unchanged	Decreases (Slower)	Increases (Faster)
Direction	i = r	Towards the normal	Away from the normal
Frequency f	Unchanged	Unchanged	Unchanged
Wavelength λ	Unchanged	Decreases	Increases

Reflection and refraction of waves - CCEA

What are the key points about the reflection and refraction of waves?

How are wavefronts observed in a ripple tank?

Wavefronts

How are waves reflected?

Key points

How are waves refracted?

Key point

What happens to the speed and wavelength when waves are refracted?

How much do you know about the reflection and refraction of waves?

More on Unit 2: Waves