Question

How do you measure the wavelength?

Answer 1

To measure wavelength we have to know what is wavelength, wavelength is the property of a wave in which the distance between the identical points between the two successive waves are calculated.

Complete solution:
To measure wavelength first let's understand what is wavelength.
The wavelength is the distance between either one crest or trough at one wave and the next wave is known as wavelength.
Now let’s understand one transverse wave example.

Wavelength is denoted by the Greek word lambda and the symbol of the lambda is $\lambda$
Now as we understand from the definition that the distance between the two adjacent troughs is determined as wavelength.
Now crest is the point which shows maximum amount of the positive upward displacement from the rest position,
In the figure point A, C, and E are the crests of the wave and trough at a wave is a point on the medium that shows the maximum amount of negative or downwards displacement from the rest position.
Therefore, wavelength $\lambda$is measured by measuring the distance between two adjacent crests or two adjacent crests or two adjacent troughs, as shown in the figure.

Note:
We can get confused when we see the structure of the longitudinal wave.

The longitudinal; wave is shown in the figure. But there is no crest or trough to measure wavelength.
In order to measure wavelength in longitudinal waves we will measure distance between two adjacent compression or two adjacent rare- fractions.