Final answer:
The wavelength of the sound waves emitted by a moving train decreases as it approaches a stationary observer due to the Doppler effect, resulting in an increased frequency and a higher perceived pitch.
Step-by-step explanation:
When a train is emitting its whistle while moving towards a stationary observer, the wavelength of the sound waves decreases. This phenomenon is explained by the Doppler effect, which describes the change in observed frequency as an observer moves toward or away from a stationary source of sound. When the source of sound moves towards an observer, the sound waves get compressed, resulting in a shorter wavelength and a higher frequency, which is perceived as a higher pitch.
If the source and observers are stationary, they observe the same wavelength and frequency as emitted by the source. However, if the source is moving, the air compressions become closer together in the direction the source is moving, leading to a decreased wavelength and an increased frequency. The stationary observer hears a higher-pitch sound as the train approaches, and once the train passes by, the pitch lowers as the frequency decreases and the wavelength increases.