Question

define wavelength and frequency and also give its si units

Answer 1

wavelength is the distance between two consecutive crests or two consecutive troughs
its SI unit is metre

frequency is the number of complete waved produced in unit time . its SI unit is hertz


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Answer 2

wavelength is the distance between two consecective trough or creast of a wave. 
S.I unit is Metre

Frequency is the rate per second vibration constituting the wave
S.I unit is hertz