21+ How To Find Frequency When Given Wavelength Today. The formula for finding frequency when time period is. We know that the speed of the light = 3 × 10 8 m/s. How to Calculate Wavelength 11 Steps (with Pictures) wikiHow from www.wikihow.com 100hz has a wavelength of 3000km. C(speed of light) =frequency(v)× wavelength(lamda) therefore, …