The phenomenon you're referring to is known as the Doppler effect. The Doppler effect describes the change in frequency or wavelength of a wave when there is relative motion between the source of the wave and an observer. It applies to various types of waves, such as sound waves and light waves.
When a wave source is moving towards an observer, the wavelengths of the waves appear shorter, resulting in a higher frequency. Conversely, when the source is moving away from the observer, the wavelengths appear longer, resulting in a lower frequency. This change in wavelength or frequency occurs because the relative motion affects the number of wave crests that reach the observer per unit of time.
To understand why wavelengths change with speed, it's essential to consider that the speed of a wave is determined by its frequency and wavelength. The speed of a wave is given by the equation:
v = λf
where "v" represents the velocity or speed of the wave, "λ" denotes the wavelength, and "f" represents the frequency. This equation indicates that the wavelength and frequency of a wave are inversely proportional when the speed remains constant.
When an observer moves towards a wave source, they experience an increase in the number of wave crests passing them per unit of time, which leads to a higher frequency. However, since the speed of the wave remains constant, the wavelength must decrease to compensate for the increased frequency.
Similarly, when an observer moves away from a wave source, they experience a decrease in the number of wave crests passing them per unit of time, resulting in a lower frequency. Again, to maintain a constant wave speed, the wavelength must increase.
In summary, the change in wavelength with speed is a consequence of the Doppler effect, which occurs due to the relative motion between the wave source and the observer. As the speed changes, the wavelengths adjust to ensure a constant wave speed in accordance with the equation v = λf.