Polarisation
Waves - OCR A-Level Physics
- The electromagnetic spectrumThe full range of electromagnetic waves ordered by frequency/wavelength: radio, microwave, infrared, visible, UV, X-ray, gamma. (in order of increasing frequencyThe number of complete oscillations passing a point per unit time. Measured in hertz (Hz).): radio, microwave, infrared, visible, ultraviolet, X-ray, gamma ray.
- All EM waves travel at c = 3.00 × 10⁸ m s⁻¹ in a vacuum and are transverse.
- EM waves are oscillating electric and magnetic fields, perpendicular to each other and to the direction of propagation.
- All EM waves can be polarised.
Key Definition
Polarisation
Restriction of the oscillations of a transverse wave to a single plane. Only transverse waves can be polarised - longitudinal waves cannot.
Restriction of the oscillations of a transverse wave to a single plane. Only transverse waves can be polarised - longitudinal waves cannot.
- Unpolarised light oscillates in all planes perpendicular to the direction of travel.
- A polarising filterA filter that only allows oscillations in one plane to pass through, producing plane-polarised light. (Polaroid) transmits oscillations in one plane only, called the transmission axis.
- If two polarising filters are crossed at 90°, no light is transmitted.
- PolarisationThe restriction of oscillations of a transverse wave to a single plane. Only transverse waves can be polarised. is evidence that light is a transverse wave.
- Applications: polarising sunglasses reduce glare from reflected light (which is partially polarised).
Key Definition
Intensity
The power transmitted per unit cross-sectional area, measured in W m⁻².
The power transmitted per unit cross-sectional area, measured in W m⁻².
$$I \propto A^2$$
$$I \propto A^{2}$$
- Doubling the amplitudeThe maximum displacement of a point on a wave from its equilibrium (rest) position. Measured in metres (m). quadruples the intensityThe powerThe rate of energy transfer. Measured in watts (W). transmitted per unit area perpendicular to the wave direction. Measured in W m⁻². Proportional to amplitude squared..
- Halving the amplitudeThe maximum displacement of a point on a wave from its equilibrium (rest) position. Measured in metres (m). reduces intensityThe powerThe rate of energy transfer. Measured in watts (W). transmitted per unit area perpendicular to the wave direction. Measured in W m⁻². Proportional to amplitude squared. to one quarter.
- This relationship applies to all progressive waves.
Examiner Tips and Tricks
- If asked to 'explain how polarisationThe restriction of oscillations of a transverse wave to a single plane. Only transverse waves can be polarised. demonstrates that light is transverse', state that only transverse waves can be polarised because their oscillations are perpendicular to the direction of travel.
- Longitudinal oscillations along the direction of travel cannot be restricted to one plane.