Imagine you are sitting next to a pond and you watch the waves going past you. First one peak arrives, then a trough, and then another peak. Suppose you measure the time taken between one peak arriving and then the next. This time will be the same for any two successive peaks passing you. We call this
time the period, and it is a characteristic of the wave.
The symbol TT is used to represent the period. The period is measured in seconds (ss).
 Definition 6: Period (TT)
The period (TT) is the time taken for two successive peaks (or troughs) to pass a fixed point.
Imagine the pond again. Just as a peak passes you, you start your stopwatch and count each peak going past. After 1 second you stop the clock and stop counting. The number of peaks that you have counted in the 1 second is the frequency of the wave.
 Definition 7: Frequency
The frequency is the number of successive peaks (or troughs) passing a given point in 1 second.
The frequency and the period are related to each other. As the period is the time taken for 1 peak to pass, then the number of peaks passing the point in 1 second is 1T1T. But this is the frequency. So
or alternatively,
For example, if the time between two consecutive peaks passing a fixed point is 12s12s, then the period of the wave is 12s12s. Therefore, the frequency of the wave is:
f
=
1
T
=
1
1
2
s
=
2
s

1
f
=
1
T
=
1
1
2
s
=
2
s

1
(4)The unit of frequency is the Hertz (HzHz) or s1s1.
What is the period of a wave of frequency 10Hz10Hz?
 Step 1. Determine what is given and what is required :
We are required to calculate the period of a 10Hz10Hz wave.
 Step 2. Determine how to approach the problem :
We know that:
 Step 3. Solve the problem :
T
=
1
f
=
1
10
Hz
=
0
,
1
s
T
=
1
f
=
1
10
Hz
=
0
,
1
s
(6)
 Step 4. Write the answer :
The period of a 10Hz10Hz wave is 0,1s0,1s.