When you use the above equation, be careful to express velocity and wavelength in the proper units
of length. For example, in the English system, if the velocity (expressed in feet per second) is divided by
the frequency (expressed in cycles per second, or Hz), the wavelength is given in feet per cycle. If the
metric system is used and the velocity (expressed in meters per second) is divided by the frequency
(expressed in cycles per second), the wavelength is given in meters per cycle. Be sure to express both the
wavelength and the frequency in the same units. (Feet per cycle and meters per cycle are normally
abbreviated as feet or meters because one wavelength indicates one cycle.) Because this equation holds
true for both transverse and longitudinal waves, it is used in the study of both electromagnetic waves and
Consider the following example. Two cycles of a wave pass a fixed point every second, and the
velocity of the wave train is 4 feet per second. What is the wavelength? The formula for determining
wavelength is as follows:
NOTE: In problems of this kind, be sure NOT to confuse wave velocity with frequency.
FREQUENCY is the number of cycles per unit of time (Hz). WAVE VELOCITY is the speed with which
a wave train passes a fixed point.