The reason for using different terms, such as wavelength for light and frequency for sound, lies in the way these two forms of energy propagate and interact with their respective mediums.
Wavelength refers to the distance between two consecutive points of a wave that are in phase with each other. In the case of light, which is an electromagnetic wave, it travels through space without requiring a medium. Light waves consist of oscillating electric and magnetic fields that are perpendicular to each other and propagate in a transverse manner. The wavelength of light determines its color or the specific region of the electromagnetic spectrum it belongs to. When we talk about the wavelength of light, we are referring to the characteristic distance between consecutive peaks or troughs of the electromagnetic wave.
On the other hand, frequency relates to the number of oscillations or cycles of a wave occurring in a given time. In the case of sound, it is a mechanical wave that requires a medium (such as air, water, or solids) to travel through. Sound waves consist of compressions and rarefactions of the medium, where particles move back and forth parallel to the direction of wave propagation (longitudinal waves). The frequency of sound determines its pitch or how high or low the sound is perceived. When we discuss the frequency of sound, we are referring to the number of oscillations or cycles occurring per unit of time.
In essence, the distinction between wavelength and frequency arises from the different nature of light and sound waves and how they behave in their respective mediums. While both light and sound are forms of energy, their propagation mechanisms and wave properties differ, leading to the use of distinct terms when discussing them.