Polarisation and diffractionEdexcel International A Level Physics: Revision notes
Section 1
Plane polarisation
A wave is plane polarised when its oscillations (for light, the electric field) are in one plane only, which is perpendicular to the direction of travel. A normal lamp gives unpolarised light, with oscillations in all directions perpendicular to the beam.
Only transverse waves can be polarised, because their oscillations can be confined to one plane perpendicular to the direction of travel. Longitudinal waves such as sound oscillate along the direction of travel, so there is no plane to select. Polarisation is therefore evidence that light is transverse.
Diffraction and refraction happen to all waves, so they are not evidence for a transverse wave. Only polarisation is.
Section 2
Producing and detecting polarised waves
A polarising filter (Polaroid) has a transmission axis: it passes the oscillations along this axis and absorbs the rest. Light from the first filter is plane polarised.
A second filter, the analyser, rotated in its plane, makes the transmitted light vary from maximum (axes parallel) to almost zero (axes at 90°, called crossed). This happens twice in each 360° rotation.
The same idea applies to a rope passing through a slot, or a TV aerial made of rods: signal is greatest when the rods are parallel to the plane of oscillation and nearly zero when they are perpendicular.
Section 3
Diffraction
Diffraction is the spreading of a wave as it passes through a gap or round an obstacle. It is a property of all waves.
The amount of diffraction depends on the gap or obstacle width compared with the wavelength λ:
- gap about the same as λ (or smaller): most spreading, almost semicircular wavefronts
- gap much wider than λ: very little spreading, a nearly straight beam with sharp shadows
The wavelength, frequency and speed of the wave are unchanged by diffraction. This is why long-wavelength radio waves spread round hills and buildings, while light does not noticeably bend round everyday objects.
A narrower gap gives more spreading, but a smaller amount of energy passes through, so the intensity falls.
Section 4
Huygens' construction
Huygens' construction explains diffraction. Every point on a wavefront acts as a source of secondary circular wavelets, which spread out at the wave speed. After a short time the new wavefront is the envelope (the common tangent) of all the wavelets.
For a wide gap, the wavelets from the middle of the gap combine to give a straight wavefront; only at the edges do wavelets spread sideways, so the beam spreads slightly.
For a narrow gap, there are only a few sources, so the new wavefront is almost semicircular and the wave spreads widely.
Section 5
Diffraction at an obstacle
At an obstacle, the wavefront is blocked by the obstacle, but wavelets from points beside its edges spread sideways into the region behind it. The new wavefront bends round the edge, so some wave energy reaches the geometric shadow, though more weakly than in the direct beam.
Diffraction round an obstacle is greatest when the obstacle is similar in size to the wavelength. For example, television waves of wavelength 0.50 m are diffracted less by a 30 m building than radio waves of wavelength 3.0 m.
In exam answers, state Huygens' idea of secondary wavelets, then say where the wavelets spread (into the shadow) and why it depends on the wavelength.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Polarisation and diffraction
- A student shines light from a lamp through a sheet of polarising filter and then through a second sheet of the same material placed directly behind it. She looks at the light that emerges from the second sheet while rotating that sheet slowly in its own plane.Explain why the light emerging from the first sheet is plane polarised.2 marks
- A student shakes the free end of a long rope randomly in all directions in the plane at right angles to the rope. The rope passes through a narrow vertical slot cut in a wooden board and then, further along, through a second board with a narrow slot that can be turned to any orientation.The student repeats the demonstration with a long spring, pushing and pulling its end along the spring. Explain why a pair of slots has no effect on this wave however they are oriented.2 marks
- In a ripple tank, straight water waves of wavelength 3.0 cm travel at 0.12 m s⁻¹ towards a barrier. The barrier has a gap whose width can be adjusted by moving two metal bars.Describe how the waves behave after the barrier when the gap is 3.0 cm wide and when it is 30 cm wide.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).