A wave has a wavelength of 3 mm and a frequency of 4 Hz. At what speed does the wave travel?
The relationship between the wavelength, frequency and speed of a wave is c = L*F, where c is the wave velocity, L is the wavelength and F is the frequency.
Here the wavelength is given as 3 mm and the frequency is 4 Hz or 4 per second, this gives the wave velocity as
c = 3*10^-3*4 = 12*10^-3 m/s.
The given information can be interpreted as 4 waves with a wavelength of 3 mm can pass in a second.
We see from the equation given earlier that for a constant velocity, waves with a higher wavelength have a lower frequency and those with a higher frequency have a smaller wavelength.