Boundary Behavior of Waves and Polarization
Introduced| English | Meaning |
|---|---|
| polariser/ˈpəʊlɑːraɪzə/ | A device that selectively transmits light in particular polarisation states. |
Why polarised sunglasses kill the glare off water
- Slip on polarised sunglasses and the harsh glare off a lake or road just melts away.
- They work by blocking light that vibrates in one particular direction.
- That trick only works on transverse waves — and it reveals what happens when waves meet a boundary.
- Boundaries reflect, transmit and filter waves in useful ways.
What happens at a boundary
- When a wave hits a boundary between two media, part is reflected and part is transmitted.
- The transmitted part refracts (bends) as its speed changes, as we saw with light.
- A wave reflecting off a denser medium can flip (invert) — important for standing waves.
- How much reflects vs transmits depends on how different the two media are.
When a wave meets a boundary between two media, it generally:
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Part of the wave reflects and part transmits (refracting) at a boundary.
Polarisation
- A transverse wave vibrates in a plane; ordinary light vibrates in all directions at once (unpolarised).
- A polariser 偏振片 lets through only the vibrations in one direction — polarised light.
- Cross two polarisers at right angles and no light gets through.
- Reflected glare is partly polarised, which is why polarised lenses can block it.
A transverse wave to polarise
See a transverse wave vibrating in a plane — the kind a polariser can filter.
Two polarisers are crossed at right angles. How much light passes through both?
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The second polariser blocks the direction the first passed, so almost no light gets through.
Select all true statements about polarisation and boundaries.
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Polarisers filter one direction (transverse only), and boundaries split waves. Sound (longitudinal) cannot be polarised.
Only transverse waves polarise
- Because sound is longitudinal (it vibrates along its travel), it cannot be polarised.
- The fact that light can be polarised is direct proof that light is a transverse wave.
- Polarisation is used in sunglasses, LCD screens and stress-testing materials.
- It is one of the clearest ways to tell a transverse wave from a longitudinal one.
Which type of wave can be polarised?
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Only transverse waves vibrate in a plane, so only they can be polarised.
Sound cannot be polarised because it is a longitudinal wave.
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Sound vibrates along its travel direction, so there is no cross-direction to filter.
The fact that light can be polarised proves light is a ____ wave.
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Only transverse waves can be polarised, so light must be transverse.
Only transverse waves can be polarised — sound cannot, because its vibration is along the direction of travel, not across it. If a wave can be polarised, that alone proves it is transverse.
Unpolarised light passes through one vertical polariser, then a second polariser turned to horizontal. How much light emerges?
- The first makes the light vertically polarised.
- The second (horizontal) blocks vertical vibrations, so almost no light gets through.
At a boundary, a wave partly reflects and partly transmits (refracting). A polariser passes only one vibration direction, giving polarised light — possible only for transverse waves. That light polarises (and sound does not) proves light is transverse.
Write from a transmitted intensity of 0
- Two ideal polarisers are crossed at 90 degrees.
- Say what intensity gets through the second one, and why a sound wave is different.
Light passes through an ideal polariser, then through a second ideal polariser whose axis is at 90 degrees to the first. What fraction of the light intensity gets through the second polariser, and can a sound wave be polarised in the same way?
Crossed ideal polarisers block the remaining transverse light.
What intensity gets through the second polariser?
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Axes at 90 degrees leave no transverse component. Other options — The full intensity (as if the second filter did nothing); Half the intensity (the result for two polarisers at 45 degrees).