Determination of Frequency By Stroboscope

Determination of Frequency By Stroboscope:

The frequency of a vibrating object can be determined easily with the help of a stroboscope. A stroboscope is a device that can make an oscillating or rotating object appear at rest. By its use, we can find the number of vibrations done by a vibrating body in a certain interval of time. Conversely, by knowing the number of vibrations, time can be measured.

To understand the principle of stroboscopic vision, suppose a camera taking 16 pictures a second is focussed on a spoked wheel. If the wheel speed is adjusted such that one spoke moves into the position occupied by its neighbor in 1/16th of a second, the film will appear to show the wheel at rest.

In practice, a tuning fork F with light metal strips A, B attached to each prong is used. The strips have slits S, S in them. These slits overlap each other when the fork is not vibrating. Behind the slit is placed the object under examination. The object is seen only when the fork is passing through its rest position. It is, therefore, seen after every T/2 second, where T is the time period of the tuning fork.

Suppose we are seeing a disc D containing m equidistant dots. Let the disc appear at rest when it is rotating at the rate of p revolutions per second. Since the wheel completes one rotation in 1/p second, we can write

In place of a vibrating tuning fork, we may use a neon lamp. If it is operating on ordinary mains, 50 c/s, the light will be dimmed 100 times each second. This gives an intermittent view of the object. Electronic circuits are now available by which the rate of intermittent flashes of light can be adjusted and thereby rotating wheels can be seen stationary.

By using the principle of stroboscopic vision we can find the frequency of oscillation or speed of rotation without the use of the clock. In other words, a calibrated stroboscope can be used to measure time intervals.