Question

Please explain why Speed (V)=Wavelength (λ)× Frequency(v) or how to derive it.

Answer 1

Speed= distance travelled by wave per second.
Frequency is the number of oscillations per second.
Basically the number of wavelengths/cycles passing a point per second.
Wavelength is the distance from on crest to another, or from one trough to another.

Time period, T, is the time taken for one oscillation.
Basically time taken for one cycle/wavelength to pass by from a fixed point.

Time period is the inverse of frequency and vice versa. So time period=1/f and f=1/T.

Speed=distance/time
Speed
=wavelength/time period

Speed=wavelength/ (1/f)

Speed=wavelength X frequency if multiply top and bottom of fraction by f.