The correct examples of the Doppler effect from the options provided are:
- Approaching a stationary sound source and the volume increases.
- Hearing the pitch of an ice cream truck's music get lower as it moves away.
The Doppler effect refers to the change in frequency or wavelength of a wave in relation to an observer who is moving relative to the wave source. When you approach a stationary sound source, the sound waves are compressed, leading to an increase in frequency and volume. Conversely, as a source of sound moves away from you, the sound waves are stretched, leading to a decrease in frequency and pitch.
The first option about light wave diffraction is related to wave behavior, not the Doppler effect. The third option about increasing the intensity of a sound wave refers to the loudness rather than a change in frequency or pitch, which is not characteristic of the Doppler effect.