(iii) techniques and procedures used to observe polarising effects using…

The idea

Polarisation shows that transverse waves oscillate in one plane. For microwaves, use a transmitter and receiver aligned so the signal is strong, then rotate the receiver about the axis of travel; the detected intensity falls to a minimum when the receiver is at 90° to the transmitter, showing the wave is polarised. A metal grille placed between them also transmits or blocks the wave depending on whether its bars are parallel or perpendicular to the electric field. For light, pass unpolarised light through a polarising filter, then through a second filter. Rotating the second filter changes the transmitted intensity, with a minimum when the transmission axes are perpendicular. These procedures demonstrate that microwaves and light are transverse waves.

What earns the marks

• Microwaves are emitted by a transmitter and detected by a receiver; rotating the receiver about the direction of travel changes the detected intensity.
• A minimum signal occurs when the receiver is rotated 90° relative to the transmitter, showing the microwave is plane polarised.
• A metal grille with parallel bars transmits microwaves when the bars are perpendicular to the electric field and blocks them when parallel.
• Light is polarised by passing it through a polarising filter; a second filter rotated to 90° reduces the transmitted intensity to a minimum.
• The procedures demonstrate that both microwaves and light are transverse waves because their oscillations can be restricted to one plane.

Common mistakes and how to put them right

• Believing that polarisation can be demonstrated with longitudinal waves.

  Put it right: Only transverse waves can be polarised, so the observation supports the transverse nature of light and microwaves.

• Thinking the receiver or filter must be moved along the beam to change intensity.

  Put it right: The change comes from rotating it about the direction of travel.

• Confusing the orientation of a metal grille.

  Put it right: The grille blocks microwaves when its bars are parallel to the electric field, not when they are perpendicular.

Exam tips

• State clearly that the receiver or analyser is rotated about the axis of propagation, not moved sideways.
• Describe the observation at the minimum, for example the detected signal or transmitted light is at its lowest.
• Use the term transverse wave when explaining what polarisation proves about the nature of the radiation.