**IBDP Physics SL 2025 – C.5 Doppler Effect SL Paper 1 Exam Style Questions **

## IBDP Physics 2025 SL Paper 1 – All Chapters

## Topic: C.5 Doppler Effect SL Paper 1

Moving Source & Moving Observer Doppler Effects

### Question- C.5 Doppler Effect SL Paper 1

A standing wave is formed between two loudspeakers that emit sound waves of frequency f.

A student walking between the two loudspeakers finds that the distance between two consecutive sound maxima is \(1.5 \mathrm{~m}\). The speed of sound is \(300 \mathrm{~m} \mathrm{~s}^{-1}\).

What is \(f\) ?

A. \(400 \mathrm{~Hz}\)

B. \(200 \mathrm{~Hz}\)

C. \(100 \mathrm{~Hz}\)

D. \(50 \mathrm{~Hz}\)

**▶️Answer/Explanation**

Ans:C

In a standing wave, the distance between consecutive maxima (or minima) is half of the wavelength (\(\lambda/2\)). Given that the distance between two consecutive sound maxima is \(1.5 \, \text{m}\), we can find the wavelength (\(\lambda\)):

\(\lambda/2 = 1.5 \, \text{m}\)

\(\lambda = 2 \cdot 1.5 \, \text{m} = 3 \, \text{m}\)

Now, we can use the formula for the speed of a wave:

\[v = f \cdot \lambda\]

Let’s solve for \(f\):

\[300 \, \text{m/s} = f \cdot 3 \, \text{m}\]

Now, divide both sides by 3:

\[f = \frac{300 \, \text{m/s}}{3 \, \text{m}} = 100 \, \text{Hz}\]

So, the frequency (\(f\)) of the sound waves is \(100 \, \text{Hz}\).

#### Question

Light of wavelength *λ*_{0} is emitted from a nearby galaxy. The light is received on Earth and the wavelength is measured to be *λ* where *λ*<*λ*_{0}. Which of the following correctly describes the speed and direction of motion of the galaxy?

**Answer/Explanation**

## Markscheme

A

## Examiners report

The vast majority of candidates understood that the fractional change in the wavelength was needed and hence discounted B or D. But it would seem that they did not read the stem carefully, where it is clear that the wavelength has decreased – indicating that it is travelling towards Earth.

#### Question

A fire engine with its siren sounding approaches and passes a stationary observer. The frequency of the sound emitted by the siren is \({f_s}\). The frequency of the sound measured by the observer is \({f_o}\). Which of the following describes the relationship between \({f_o}\) and \({f_s}\)?

**Answer/Explanation**

Answer – A

If the observer moves toward the stationary source,

The observed frequency is higher than the source frequency. If the observer is moving away from the stationary source, the observed frequency is lower than the source frequency

### Question

An ambulance emits a sound of frequency *f* as it travels along a straight road between stationary observers A and B.

Which of the following shows how the frequency of the sound heard by each observer compares with *f*?

**Answer/Explanation**

Answer – C

For observer B ambulance is approaching towards him so fobs is greater then f.

If the observer moves toward the stationary source,

The observed frequency is higher than the source frequency

And for observer A source is moving away from him so fobs is lesser then f

If the observer is moving away from the stationary source, the observed frequency is lower than the source frequency

### Question

A point source of sound is moving to the right at constant speed. The source emits sound waves of constant frequency. The speed of the source is less than the speed of sound. Which diagram correctly shows the wavefronts of the sound?

**Answer/Explanation**

Answer – A

Towards right wavelength gets compressed and towards left wavelength gets expanded