# Translations:Sinusoidal waves/33/en

Now we come to an application of the formula . Instead of watching a periodic wave continuously, we look at it through a shutter that is closed most of the time and opens periodically for short time intervals. Such an instrument is the stroboscope. The first time the shutter opens, we see the wave pattern in a certain position. When the shutter is closed, all the pulses move a distance equal to their velocity multiplied by that time duration. As we look through the shutter while it periodically opens and closes, the pattern usually appears to move. However, if the period of the shutter is the same as the period of the wave motion, then while the shutter is closed, each pulse moves up to the position of the pulse just ahead of it. Consequently, we see the same pattern each time the shutter opens. In other words, we see a stationary pattern from which it is easy to measure the wavelength. In addition, as we stated earlier, the period of the shutter is equal to the period of the wave, which we obtain by simply counting the number of times the shutter is opened each second - that is, by measuring the frequency of the shutter. Now we have both *f* and for the wave, so we can use the formula to determine the velocity of the wave.