| You are expected to know the difference
between longitudinal and transverse waves, including examples.
You are expected to be aware of polarisation being evidence of the nature
of transverse waves. |
A transverse
wave is one in which the displacement
of the particles is at 90o
to the direction of travel. In
a water wave, the particles move up and down while the wave travels
horizontally. All electromagnetic waves are transverse.
We can show the features of a transverse wave in the diagram below:
In longitudinal waves, the displacement is parallel to the direction of travel of the wave. There are regions of high pressure, compression, and regions of low pressure, rarefaction. In a sound wave the air molecules move forwards and backwards; where they are squashed together, a compression results, where they are forced further apart, there is a rarefaction. Like all mechanical waves, a medium or material is required. The speed of sound in air is 336 m/s, in water 1400 m/s, in steel it is 6000 m/s. Other examples of longitudinal waves include some kinds of earthquake waves (the pressure or P-wave). We can see the features of a longitudinal wave in the diagram below.
| Write down two similarities and two differences between transverse and longitudinal waves. Give one example of a transverse wave and one example of a longitudinal wave. |
Polarisation is a feature of transverse waves only. Longitudinal waves are never polarised. We say that a wave is plane polarised if all the vibrations in the wave are in a single plane, which contains the direction of propagation of the wave. Suppose we have a rope and make waves down it. We could make waves in any direction we liked. But if we made waves through a narrow vertical slit, we would find that the waves would only pass through if they were vertical. This would be a vertically polarised wave.
Light waves are easily polarised using polaroid filters. Light waves, like all electromagnetic waves, consist of an electric field component perpendicular to a magnetic field component, which are always in phase. We normally consider only the electric field component in polarisation, because the electrical effects are those that dominate. The unpolarised waves are normally oriented in any direction.
If two polaroid filters are mounted such that they are parallel, the light will pass through both the first at which point it is vertically polarised, and then through the second.
If the two filters are crossed, so that the transmission planes are at 90o to each other, the vertically polarised light gets blocked, because it cannot pass the horizontal transmission plane. No light passes.
Crossed polaroids are found in liquid crystal displays on calculators and petrol pumps.
| Question 2 |
Radio aerial rods must be in the correct plane, vertical or horizontal in order to work properly, otherwise the signal is weak. Use the information above to explain why this is the case. |
ANSWER |
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