The equation for wave velocity, v = λ/T, relates the velocity of a wave (v) to its wavelength (λ) and period (T). The substitution of λ (lambda) for Δd (delta distance) is based on the convention used to represent the different quantities in wave equations.
The symbol Δd typically represents a change in distance, indicating the difference between two positions. In the context of a wave, Δd could be used to represent the change in position of a point on the wave over a given time interval. However, when discussing the wavelength of a wave, we are interested in the distance between two corresponding points on the wave that have the same phase (e.g., two adjacent crests or troughs).
To represent this characteristic distance, the symbol λ (lambda) is commonly used. Lambda specifically denotes the wavelength of a wave, which is the spatial period of the wave, representing the distance it takes for the wave pattern to repeat itself.
Therefore, to relate the wave velocity (v) to the wavelength (λ) and period (T), it is appropriate to use λ as the symbol for wavelength instead of Δd, as λ signifies the specific distance associated with a full wave cycle.