2021 A/L Physics Past Paper · Question 27 (English Medium)

Answer: (2) Only (C) is true.

What the question asks

Find which of the statements (A), (B) and (C) are true for EM waves.

They are about the kind of wave, the need for a medium, and bending in a field.

The plan: test each statement against what an EM wave is.

Relevant theory

Since, electromagnetic waves undergo polarization these waves are seen to behave as transverse waves.

No medium is necessary for the progression of electromagnetic waves, since the fields E and B do not need any medium for their transmission.

The radiation which travels without getting deviated is uncharged and it is an electromagnetic radiation with a very high frequency (γ - rays).

Reading the options

The options are combinations of (A), (B) and (C).

An EM wave is an electric field $E$ and a magnetic field $B$ vibrating at right angles to each other and to the direction of travel.

We test each statement on this picture.

Reasoning and calculation

They can be transverse or longitudinal.

EM waves are transverse only. There is no longitudinal EM wave.

A

They require a medium to propagate.

The fields $E$ and $B$ need no medium. EM waves travel through a vacuum.

B

They are not deflected by electric or magnetic field.

An EM wave carries no charge. So an electric field or a magnetic field does not push it sideways.

C

Only (C) is true.

(A) and (B) are false.

Why the other options fail

(A) is false, because EM waves are only transverse.

(B) is false, because EM waves need no medium.

(B) is false, because EM waves travel through a vacuum.

(A) and (B) are false.

Conclusion

(A) is false, (B) is false, (C) is true.

The combination "only (C)" is option (2).

For EM waves, remember three facts: transverse only, no medium needed, no deflection by fields.

Then check each statement against these three.

Exam tip