Chapter 25, Problem 45GP

To determine

The explanation for deriving the equation, $c=\frac{4LN}{T}$, to find the speed of light, using an apparatus including the source, $S$, from where the light goes to an interrupter, $K$, and is then reflected by a mirror, $M$, onto a rotating wheel. If the light incident on the wheel passes between the teeth of the wheel, it can be observed and if light, on the way back, hits the tooth of wheel, it cannot be observed. The speed of light can be measured by evaluating the time interval between teeth crossing the beam and distance light travel in this time interval. Given that, $L$ is the distance between the wheel and the mirror, $T$ is the period of rotation of the wheel and $N$ is the number of teeth in the wheel (the width of one tooth is equal to the width of the gap between the teeth).