Polarisation
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What type of wave exhibits polarisation?
Polarisation is a phenomenon exhibited by transverse waves only. Longitudinal waves cannot be polarised because their oscillations are parallel to the direction of propagation.
Define polarisation in the context of transverse waves.
Polarisation refers to the restriction of the oscillations of a transverse wave to a single plane. This plane contains the direction of propagation.
State Malus's Law.
Malus's Law states that the intensity (I) of plane-polarised light after passing through a polariser is given by I = I₀cos²θ, where I₀ is the initial intensity and θ is the angle between the polariser's transmission axis and the plane of polarisation of the light.
A polarising filter is rotated from 0° to 90° relative to the plane of polarised light. How does the transmitted intensity change?
As the angle increases from 0° to 90°, the transmitted intensity decreases. At 0°, the intensity is maximum (I₀), and at 90°, the intensity is zero.
Plane-polarised light of intensity 20 W/m² passes through a polarising filter oriented at 30° to the plane of polarisation. What is the intensity of the transmitted light?
Using Malus's Law: I = I₀cos²θ = 20 W/m² * cos²(30°) = 20 W/m² * (√3/2)² = 20 * (3/4) = 15 W/m².
Unpolarised light is incident on two polarising filters. The first filter has its transmission axis vertical, and the second filter has its transmission axis at 45° to the vertical. Describe the light after passing through both filters, with explanation.
The first filter polarizes the light vertically. Then Malus's Law tells us the second reduces the intensity by cos²(45°) = (√2/2)² = 1/2. The light is plane polarised at 45° to vertical, and has intensity I = I₀ * 1/2, where I₀ is the intensity of the polarised light after passing through the first filter.
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