Suppose we have a rope with one end fixed to a wall and stretched by a person. If we make the movement of the hand going up and down, we will be producing a disturbance in the rope, which will propagate along the rope to the fixed end. We give the name to the disturbance that propagates in the middle of pulse.
If we mark a colored point on the string and then again produce a disturbance on it, we will see that the marked point remains in the same place, but the pulse propagates until it reaches the other end. Through this disturbance we can see that there is only energy transfer along the string.
Thus, we can define a wave as the propagation of energy from one point to another, without matter being transported between them.
When we study a wave, we can see that there are sources that constantly produce waves. These waves produced constantly are called periodic waves, that is, they are waves that are repeated with equal time intervals. In the figure below we have a basic example of a periodic wave propagating on a stretched string.
According to the figure above we can see some important elements that are associated with a wave. Let's see:
Crests: We give this name to the highest points of the pulses. In the figure above the letters THE, B and Ç represent the crests that oscillate in phase agreement.
Valleys: are the lowest points of the pulses (points D, AND and F) and, similarly to crests, these points oscillate in phase agreement.
Period: is the time interval in which each point in the medium where a pulse propagates performs a complete oscillation. The period is represented by the letter T.
Frequency: is the number of complete oscillations that each point in the medium in which the wave propagates performs per unit of time. The frequency is represented by the letter f, this being inversely proportional to the period. So we have:
In the international system of units (SI), the unit of frequency is the hertz (Hz), worth 1 Hz = s-1.
Wavelength: represents the distance traveled by the wave in a period of time. The wavelength is represented by the letter (λ). We can see in the figure that the wavelength corresponds to the distance between two consecutive crests or valleys.
The propagation speed of a periodic in a homogeneous medium is considered uniform and is determined by the following equation:
Since, in one period, the wave shifts the equivalent of one wavelength, then we can write in place of Δs, λ; and in place of Δt, T. Rewriting the equation above we have:
By Domitiano Marques
Graduated in Physics
Source: Brazil School - https://brasilescola.uol.com.br/fisica/ondas-periodicas.htm