Polarising Filters: These are the most common tools for achieving polarisation. They contain long-chain molecules aligned in a specific direction; the filter absorbs energy from oscillations parallel to the molecules and transmits oscillations perpendicular to them (the transmission axis).
Reflection: When unpolarised light reflects off a non-metallic surface (like water or glass), it becomes partially or fully plane polarised. The reflected light is usually polarised parallel to the surface, which is why polarised sunglasses are designed with vertical transmission axes to block horizontal glare.
Two-Filter Systems: By placing a second filter (the analyser) after the first (the polariser), one can control the intensity of the transmitted light. If the transmission axes are parallel, light passes through; if they are crossed at , the light is blocked completely.
| Feature | Transverse Waves | Longitudinal Waves |
|---|---|---|
| Oscillation Direction | Perpendicular to travel | Parallel to travel |
| Polarisation | Possible | Impossible |
| Examples | Light, Radio, S-waves | Sound, Ultrasound, P-waves |
Identify Wave Type First: If a question asks why a specific wave (like sound) cannot be polarised, the answer must always state that it is a longitudinal wave and that polarisation only applies to transverse waves.
Filter Orientation Logic: Remember that if two polarising filters are 'crossed' (at to each other), the intensity of the light transmitted will be zero. Always check the relative angle between the polariser and the analyser.
Intensity Relationships: Be prepared to explain that a single polariser reduces unpolarised light intensity by . If the light is already polarised and hits a filter at , the intensity drops to .
Visual Cues: In diagrams, unpolarised light is often shown as a 'star' of arrows, while polarised light is shown as a single double-headed arrow. Ensure your descriptions match these standard representations.
