Worksheet on the Doppler Effect illustrating sound wave compression and rarefaction around a moving car, with questions about frequency perception.

Here is the complete solution and explanation for each question on “The Doppler Effect” worksheet:

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1. TRUE or FALSE:
> Ken Fused is standing on a corner when a police car passes by with its siren on. Ken hears a different pitch when the police car is approaching him than when it is past him. This is because the siren on the front of the car is set to a higher pitch than the siren on the back of the car.

✔ Answer: FALSE

Explanation:
This statement is false because the change in pitch Ken hears is not due to different sirens on the front and back of the car. In reality, there is typically only one siren, and the pitch change is caused by the Doppler Effect — a phenomenon where the observed frequency of a wave changes due to the relative motion between the source (the moving police car) and the observer (Ken). As the car approaches, sound waves are compressed, increasing the frequency (higher pitch). As it moves away, waves are stretched, decreasing the frequency (lower pitch).

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2. Describe the real reason Ken Fused observes what he does.

✔ Answer:

Ken hears a higher pitch as the police car approaches him because the motion of the car compresses the sound waves in front of it, decreasing the wavelength and increasing the frequency perceived by Ken. After the car passes him and moves away, the sound waves behind the car are stretched out, increasing the wavelength and decreasing the frequency — so Ken hears a lower pitch. This change in perceived pitch due to relative motion is called the Doppler Effect.

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3. TRUE or FALSE:
> The Doppler shift is a phenomenon that is observed only of sound waves.

✔ Answer: FALSE

Explanation:
The Doppler effect is not limited to sound waves. It occurs with all types of waves, including light (electromagnetic waves), water waves, and even radar waves. For example, astronomers use the Doppler shift in light from stars (redshift and blueshift) to determine whether stars are moving toward or away from Earth. Radar guns used by police also rely on the Doppler effect with radio waves to measure vehicle speed.

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4. TRUE or FALSE:
> As the source of a sound approaches an observer, the loudness of the sound increases. This is an example of the Doppler Shift.

✔ Answer: FALSE

Explanation:
While it’s true that the sound may get louder as the source approaches (due to being closer), loudness (intensity) is not part of the Doppler effect. The Doppler effect specifically refers to the change in frequency (pitch), not amplitude (volume). Loudness increases because the energy of the sound wave is more concentrated at shorter distances — this is a geometric effect, not the Doppler effect.

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5. Towards which person do the sound waves travel the fastest?
a. Jack b. Jill c. Both the same.

✔ Answer: c. Both the same.

Explanation:
Sound waves travel at a fixed speed through a given medium (like air), regardless of the direction or motion of the source or observer. So, whether the waves are going toward Jack or Jill, they travel at the same speed — approximately 343 m/s in air at room temperature. The *wavelength* and *frequency* change due to motion (Doppler effect), but the *wave speed* remains constant in the medium.

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6. Who will hear the highest frequency?
a. Jack b. Jill c. Both the same.

✔ Answer: a. Jack

Explanation:
In the diagram, the car is traveling away from Jill and toward Jack. Since the car is moving toward Jack, the sound waves are compressed in front of the car — meaning the wavelength is shorter and the frequency is higher for Jack. For Jill, who is behind the car, the waves are stretched out — longer wavelength, lower frequency. Therefore, Jack hears the higher frequency.

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7. The Doppler effect can be described as the difference between the frequency at which sound waves are produced and the frequency at which they are observed by the hearer. It occurs when the distance between the source of a sound and the observer is changing. As the source approaches an observer, the observer hears the pitch (or frequency) to be _________ (higher, lower). As the source moves away from an observer, the observer hears the pitch (or frequency) to be _________ (higher, lower).

✔ Answer:

As the source approaches an observer, the observer hears the pitch (or frequency) to be higher.
As the source moves away from an observer, the observer hears the pitch (or frequency) to be lower.

Explanation:
This is the core definition of the Doppler effect for sound:
- Approaching → waves compressed → higher frequency → higher pitch
- Receding → waves stretched → lower frequency → lower pitch

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✔ Final Summary:

| Question | Answer |
|----------|--------|
| 1 | FALSE |
| 2 | Doppler Effect: frequency changes due to relative motion |
| 3 | FALSE |
| 4 | FALSE |
| 5 | c. Both the same |
| 6 | a. Jack |
| 7 | higher; lower |

Let me know if you’d like a diagram or animation to visualize this!