Boundary Behavior of Waves and Polarization
| English | Chinese | Pinyin |
|---|---|---|
| polariser | 偏振片 | piān zhèn piàn |
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:
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?
The second polariser blocks the direction the first passed, so almost no light gets through.
Select all true statements about polarisation and boundaries.
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?
Only transverse waves vibrate in a plane, so only they can be polarised.
Sound cannot be polarised because it is a longitudinal wave.
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.
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.