Physicopoeia

φυσικοποιΐα
THINGS I WISH I'D KNOWN WHEN I STARTED TEACHING PHYSICS

Polarisation - the danger of the picket fence analogy

A very common analogy for explaining how waves can be polarised is to imagine transverse waves travelling on a rope with the rope threaded through the upright sections of a picket fence (here's a Google image search which shows how prevalent it is). It's a nice enough idea on the face of it, but suffers from the problem that, in reality, electromagnetic waves behave in exactly the opposite way as they pass through actual polaroids!

This is easily observable with the slightly older 3cm microwave kit that some schools still have. I made a video with Lewis Matheson to show the whole thing - skip ahead to 3:40 if you don't need an explanation of what the kit is and how it works.

First, set up the kit as usual to emit vertically polarised waves and get a strong signal at the receiver:

It's only when the polaroid bars are horizontal that the vertically polarised waves get through, at almost full-strength (but the picket-fence analogy predicts that this is when nothing should get through):

This is because the electrons in the vertical bars are free to oscillate sympathetically with the incoming vertically polarised waves, which effectively absorbs their energy. When the bars are horizontal, the electrons are much less free to vibrate so can barely absorb any of the incoming energy.

WHY DON'T THE OSCILLATING ELECTRONS JUST RE-EMIT THE WAVES?

One slight confusion which might come from this is why the sympathetically vibrating electrons in the vertical bars don't seem to re-emit the e-m waves. The answer is that they do, but in all directions, not just the direction the waves were received in. The percentage of re-emitted waves detected at the receiver is therefore almost zero.