Question(a) State what is meant by plane polarised light. (1)
(b) Explain why light waves can be polarised but sound waves cannot. (2)
(c) A microwave transmitter and receiver face each other. When the receiver is rotated through 90° about the line joining them, the signal falls to zero. Explain what this shows about the microwaves. (2)Answer(a) Light in which the oscillations (of the electric field) occur in one plane only, a plane that contains the direction of travel. (1)(b) Light is a transverse wave, so its oscillations are perpendicular to the direction of travel and can be in many planes; polarisation restricts them to one plane. (1) Sound is a longitudinal wave: its oscillations are parallel to the direction of travel, so there is only one direction of oscillation and it cannot be restricted to a plane. (1)(c) The microwaves from the transmitter are plane polarised. (1) The receiver only detects oscillations in one plane, so when it is at 90° to the plane of polarisation no signal is detected. (1)Examiner tips- In (a), always say that the oscillations are in one plane. Writing "the light travels in one plane" or "the wave goes in one direction" scores no marks: all light travels in one direction.
- In (b), you must link polarisation to the wave being transverse. "Sound cannot be polarised because it is sound" is not an explanation.
- In (c), rotating the receiver a further 90° (180° in total) brings the signal back to a maximum.
- Polarisation is the key evidence that light, and all electromagnetic waves, are transverse.
(b) Explain why light waves can be polarised but sound waves cannot. (2)
(c) A microwave transmitter and receiver face each other. When the receiver is rotated through 90° about the line joining them, the signal falls to zero. Explain what this shows about the microwaves. (2)Answer(a) Light in which the oscillations (of the electric field) occur in one plane only, a plane that contains the direction of travel. (1)(b) Light is a transverse wave, so its oscillations are perpendicular to the direction of travel and can be in many planes; polarisation restricts them to one plane. (1) Sound is a longitudinal wave: its oscillations are parallel to the direction of travel, so there is only one direction of oscillation and it cannot be restricted to a plane. (1)(c) The microwaves from the transmitter are plane polarised. (1) The receiver only detects oscillations in one plane, so when it is at 90° to the plane of polarisation no signal is detected. (1)Examiner tips- In (a), always say that the oscillations are in one plane. Writing "the light travels in one plane" or "the wave goes in one direction" scores no marks: all light travels in one direction.
- In (b), you must link polarisation to the wave being transverse. "Sound cannot be polarised because it is sound" is not an explanation.
- In (c), rotating the receiver a further 90° (180° in total) brings the signal back to a maximum.
- Polarisation is the key evidence that light, and all electromagnetic waves, are transverse.