Final answer:
The student's question describes the Doppler effect, where the relative motion of a light source and observer causes the perceived distance between wave crests to change, leading to a change in frequency. This concept is illustrated using light and sound waves, with frequencies being higher when the source approaches and lower when it recedes.
Step-by-step explanation:
The student's question seems to be describing a phenomenon related to the Doppler effect in physics. However, there may be some confusion in the terminology used in the question. Given the context provided, the description deals with what happens when a source of light is moving with respect to an observer. As the source moves closer to observer A, the distance between crests is decreased, a phenomenon often described as the crests being 'squeezed together'. This is a key concept in understanding the Doppler effect, which explains how the frequency of waves, including light and sound, changes due to the relative motion of the source and the observer.
To put this in a real-world context, consider the classic example of an ambulance siren: as the ambulance moves towards you, the sound waves are compressed, and the pitch of the siren sounds higher. Conversely, as the ambulance moves away from you, the sound waves are stretched out, and the pitch sounds lower. The same principle applies to light waves; when the source moves towards an observer, the wavelengths appear shorter and the frequency appears higher (known as blue-shifting), and when the source moves away, the wavelengths appear longer and the frequency lower (known as red-shifting).