2019 A/L Physics Past Paper · Question 07 (English Medium)

Answer: (5) All A, B, and C

What the question asks

We want:

The statements that are incorrect for both ultraviolet and ultrasound.

Ultraviolet is a light wave. Ultrasound is a sound wave.

Relevant theory

Infrared, visible light and ultra violet radiations are all classified as light.

These sounds are called ultrasonics for various technological purposes.

Since sound too is a kind of a mechanical wave, which in general transmits as longitudinal waves, it can be concluded that sound too needs a material medium for transmission.

For a statement to be true of both waves, it must be true for ultraviolet and for ultrasound.

Ultraviolet is a transverse wave of light.

Ultrasound is a longitudinal wave of sound.

If a statement fails for one of them, it is incorrect.

Reading the options

The options list which statements are incorrect.

Judge A, B and C first. Then pick the matching option.

Reasoning and calculation

The energy of both waves depends on the frequency

For ultraviolet light, $E = hf$. The energy goes with the frequency.

For sound, the energy depends on the amplitude and the distance.

So A is not true for ultrasound.

A

Both waves ionize

Ultraviolet photons carry a lot of energy. They can ionize some materials.

Ultrasound only shakes the particles of the medium to and fro.

It is not harmful like X-rays. It is used to see a baby in the womb.

So B is not true for ultrasound.

B

Both waves can be polarized

Light is a transverse wave. It can be polarized.

Sound is a longitudinal wave. It vibrates along its direction of travel.

So it passes a slit in every direction. It cannot be polarized.

So C is not true for ultrasound.

C

All three fail for the same reason.

They are true for ultraviolet. But ultrasound is a longitudinal sound wave.

So A, B and C are all incorrect.

Why the other options fail

B and C are incorrect too.

C is missing.

B is missing.

It takes A to be correct.

Conclusion

A note on statement A.

At a higher level, the intensity of a sound wave has $f$ in it too:

$$I = 2\pi^2 \rho v f^2 A^2$$

But the frequency alone does not decide the energy of sound, as it does for a photon ($E = hf$).

So A is incorrect.

Teacher's note

A, B and C all fail for ultrasound.

So all three are incorrect.

So the answer is option (5).

Taking ultraviolet and ultrasound to be the same kind of wave because both names begin with "ultra".

Ultraviolet is a transverse wave of light. Ultrasound is a longitudinal sound wave of a very high frequency.