2023 A/L Physics Past Paper · Question 05 (English Medium)

Answer: (5) polarization.

What the question asks

We want: the one property that longitudinal waves do not show.

The five options are properties of waves. Four of them are shown by longitudinal waves. One is not.

Relevant theory

If the vibrations are along the direction of the travel of the wave, they are called longitudinal waves. Thus the particles of the medium oscillate along the direction of motion of the wave.

Sound waves and compression waves in springs are longitudinal.

All known wave motions share four common properties. They can be reflected, refracted, diffracted and also exhibited interference.

Polarization of transverse waves

If slit D is rotated, the string vibrations do not pass through D until both slits become parallel to each other.

Reading the options

For example, think of a string that passes through a tall, narrow slit between two upright boards.

Shake the string up and down. It passes through the slit.

Shake it from side to side. The boards stop it.

Turn the boards on their side, and now the up and down shake is stopped.

So the slit picks out one direction of shaking. This is polarization.

Now think of a spring toy that you push forward and back along its length. Its coils move forward and back, along the way the wave goes.

There is no sideways shake for a slit to pick out.

The quotes say the same. In a longitudinal wave the particles oscillate along the direction of motion of the wave. Polarization is taught for transverse waves, with a string and two slits. The other four properties are shared by known wave motions, and sound is one of them.

Test each option against the quotes.

(1) Reflection: it is one of the four shared properties. A longitudinal wave shows it.

(2) Refraction: it is one of the four shared properties. A longitudinal wave shows it.

(3) Interference: it is one of the four shared properties. A longitudinal wave shows it.

(4) Diffraction: it is one of the four shared properties. A longitudinal wave shows it.

(5) Polarization: it picks out one plane of shaking, and a longitudinal wave shakes along its way, not across it. A longitudinal wave does not show it.

Only option (5) is left.

Why the other options fail

Reflection is one of the four properties that known wave motions share. A longitudinal wave such as sound shows it.

Refraction is one of the four properties that known wave motions share. A longitudinal wave such as sound shows it.

Interference is one of the four properties that known wave motions share. A longitudinal wave such as sound shows it.

Diffraction is one of the four properties that known wave motions share. A longitudinal wave such as sound shows it.

Conclusion

Longitudinal waves do not show polarization.

The answer is (5).

Polarization needs a sideways direction of shaking to pick out.

A longitudinal wave shakes along the way it travels, so it has no sideways shake. Polarization belongs to transverse waves.

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