Question

The apparent frequency of a siren increases as the source approaches a stationary observer. This phenomenon is known as

• A. Doppler Effect
• B. Resonance
• C. Interference
• D. Diffraction

Hint:

Remember that for an approaching source, the observed frequency increases due to the compression of wavefronts. For a receding source, the observed frequency decreases because the wavefronts are stretched out.

Answer 1

The Correct Option is A

Step 1: Read the situation.
A siren moves toward a person standing still, and the person hears a higher pitch than the real one. We must name this effect.

Step 2: Name the effect.
This is the Doppler Effect. It is the change in the heard frequency when the source and the listener move relative to each other.

Step 3: Explain why pitch rises.
As the source comes closer, each new sound wave is sent from a nearer spot. The wave crests bunch up, so more crests reach the ear each second. More crests per second means a higher pitch.

Step 4: Rule out resonance.
Resonance is when a natural frequency matches a driving frequency, which is not what happens here.

Step 5: Rule out interference and diffraction.
Interference is the adding of two waves, and diffraction is the bending of waves around edges. Neither explains the rise in pitch from a moving source.

Step 6: State the answer.
The rising pitch of an approaching siren is the Doppler Effect. \[ \boxed{\text{Doppler Effect}} \]