The wavelength changes during the Doppler effect because of the relative motion between the source of the wave and the observer. The Doppler effect is the observed change in frequency and wavelength of a wave when there is relative motion between the source of the wave and the observer.
When the source of the wave and the observer are moving towards each other, the waves get compressed, resulting in a shorter wavelength. This leads to an increase in frequency since frequency is inversely proportional to wavelength according to the wave equation (v = fλ), where v is the velocity of the wave. As a result, the observed frequency is higher than the emitted frequency.
Conversely, when the source and the observer are moving away from each other, the waves get stretched, resulting in a longer wavelength. This leads to a decrease in frequency, as the frequency becomes lower when the wavelength increases. The observed frequency is lower than the emitted frequency in this case.
In summary, the relative motion between the source and the observer affects the apparent wavelength of the wave due to compression or stretching, resulting in a change in frequency observed during the Doppler effect.