Question

Calculate the period of a strobe light flashing $25$ times in $5.0$ seconds.
(A) $5s$
(B) $0.2s$
(C) $3s$
(D) $0.6s$

Answer 1

Period is generally defined for periodic events (events which repeat themselves) . In the given question, the periodic event is the flashing of the strobe light. Generally, the word period is associated with waves and waveforms and implies the time taken for one waveform to be generated. We can extend that meaning to say that for any periodic event, the time taken to do one instance of that event will be its time period or simply it's period.

Complete step-by-step solution:
We have been told that the time taken for $25$ flashes of the strobe light is $5.0$ seconds.
Now, considering the flashing of the strobe as a periodic event.
using the unitary method as specified below,
Since $25$ flashes of the strobe take $5$ seconds, $1$ flash would take $\dfrac{5}{{25}}$ seconds
Time taken for one flash of the strobe light $(T) = \dfrac{{5s}}{{25}} = 0.2s$
This is the required period of the strobe light.

Note:- An alternative way of tackling this problem is to find the frequency of the strobe light first and then take its inverse as a period of any periodic event is the reciprocal of its frequency. Since the light flashes $25$ times in $5$ seconds, the frequency of flashing (which is the no. of flashes of the strobe light in 1 second) will be $f = \dfrac{{25}}{{5s}} = 5Hz$ . The time period is the reciprocal of frequency (as mentioned above). On taking the reciprocal of $5$ Hz, our answer will again come out to be $0.2$ s.