Plane Polarisation

• Polarising Filters: These materials contain long-chain molecules aligned in one direction. They absorb energy from oscillations parallel to the molecules and transmit oscillations perpendicular to them (the transmission axis).

• Reflection: When light reflects off non-metallic surfaces like water or glass, it becomes partially plane polarised. The reflected light is polarised in a plane parallel to the reflecting surface (e.g., horizontal for a lake).

• Refraction: Light passing from one medium to another also undergoes partial polarisation. In this case, the refracted light is polarised in a plane perpendicular to the transmitting surface.

Comparison of Polarisation Methods

• Unpolarised vs. Partially Polarised: Unpolarised light has equal intensity in all planes. Partially polarised light has a dominant plane of oscillation but still contains some energy in other orientations.

• Glare Reduction: Polaroid sunglasses use vertical transmission axes to block horizontally polarised light reflected from roads or water. This selectively removes glare while allowing useful light to pass through.

• Stress Analysis: Transparent plastics become 'optically active' when under mechanical stress, rotating the plane of polarised light. By viewing these materials between crossed polarisers, engineers can see colored patterns that indicate areas of high stress.

• Chemical Analysis: Certain molecules, like sugars, rotate the plane of polarised light. The angle of rotation is directly proportional to the concentration of the substance, allowing for precise measurement of chemical purity.

• Identify Wave Type: Always check if the wave in the question is transverse or longitudinal. If it is longitudinal (like sound or ultrasound), it cannot be polarised, and any answer suggesting otherwise is a trap.

• Crossed Polarisers: Remember that two polarisers with axes at $90^\circ$ to each other will block all light. If a question asks about rotating a second filter, the intensity will be zero at $90^\circ$ and $270^\circ$.

• Reflection Orientation: In multiple-choice questions, remember that reflected light from a horizontal surface is horizontally polarised. This is why sunglasses must be vertically polarised to block it.

• Terminology Precision: Use the term 'plane polarised' rather than just 'polarised' to describe light restricted to a single plane of oscillation.