The phenomenon of waves occurs when waves propagate in a medium and encounter other waves, with changes in the environment, borders, gaps and obstacles in general. This causes changes in the shape of the waves and their displacement.
Waves carry energy, it doesn’t matter. If you look closely, when a stone is thrown into a pond, it is the disturbance that propagates through the water, since the liquid molecules briefly move from their equilibrium position and return to it as the disturbance moves away.
Since there is no matter transport, we can expect waves to behave differently than objects would when they interact.
Waves can cross different media and even occupy the same space at the same time, something that massive particles cannot do, at least macroscopically (electrons have mass and can experience wave phenomena).
Among the main wave phenomena that we can observe in nature are reflection, refraction, interference and diffraction.
Both light and sound, so precious to the senses, behave like waves and experience all these phenomena, within the differences existing in their respective natures.
For example, light does not need a material medium to spread, while sound does. Furthermore, light is a transverse wave (the disturbance is perpendicular to the direction the wave is moving), while sound is a longitudinal wave (the disturbance and displacement are parallel).
Types of wave phenomena
Despite their different nature, all waves have the following phenomena in common:
Reflection
When waves travel, they sometimes encounter boundaries that separate one medium from another, for example, a pulse traveling across a string firmly attached to one end.
When the pulse reaches the end of the string, it largely returns but reverses. The pulse is said to undergo reflection, that is, it is reflected at the boundary between the string and the support.
The pulse reversal is due to the reaction exerted by the support on the string, which by law of action and reaction has the same direction and magnitude, but in the opposite direction. For this reason, the pulse is reversed on return.
Another possibility is that the rope has some freedom at the coupled end, for example, it is tied to a ring that can slide on a bar. Therefore, the pulse sent through the string does not return inverted.
Generally speaking, when a wave propagates and reaches the boundary that separates two different media, it experiences a change of direction. The incoming wave is known as the incident wave, the returning wave is the reflected wave, and if one part is transmitted to the other medium, it is known as the refracted wave.
Sound is a wave, which is why you experience reflection when speaking in an empty room. Light is also a wave, and we can see it reflecting in a mirror, on the calm surface of a lake, or on the stained glass in the skyscraper in Figure 1.
Refraction
The phenomenon of refraction occurs when a wave passes from one medium to another, for example, from air to water. A part of the wave is transmitted to the second medium: the refracted wave (see figure 2).
When trying to grab an object submerged in the bottom of a fountain or bucket, it is very likely that you will not reach it, even if your hand is directed towards where it is. And this is because the light rays changed direction when they passed from air to water, that is, they experienced refraction.
Also, the speed at which waves move varies with the medium. In a vacuum, light waves move at a constant speed c = 300,000 km/s, but in water the speed decreases to (3/4) c and in glass even more: a (2/3) c.
The speed of light in a medium depends on its refractive index, defined as the ratio between c and the speed v that light has in the medium:
n = c / v
The phenomenon is analogous to a toy car that rolls over a hard ceramic floor or polished wood and suddenly rolls over a rug. Not only does it change direction, it also slows down.
Absorption
If the wave encounters a different medium, it can happen that all the energy it carries produces and its amplitude becomes zero. The wave is said to have been absorbed.
Two objects do not share their space, however, two or more waves have no problem being at the same point in space at the same time. This behavior is unique to them.
This happens whenever two stones are thrown into the water simultaneously, independent wave patterns are produced that can overlap and generate a resultant wave.
The amplitude of the resulting wave may be larger or smaller than the interfering waves, or they may simply cancel out. In them, the superposition principle is fulfilled.
For waves, the superposition principle states that the resulting wave is equal to the algebraic sum of the interfering wave displacements (may be more than two).
If the waves are in phase, which means that their valleys and ridges are aligned, it results in a wave with twice the amplitude. This is known as constructive interference .
On the other hand, when the crest of one wave overlaps the valley of another, they oppose each other and the amplitude of the resulting wave decreases or becomes zero. This effect is called destructive interference .
After interacting, the waves continue on their way as if nothing has happened.
Diffraction
This phenomenon is typical of waves; in it, the wave is deflected and distorted when it encounters an obstacle in the path of the wave or a space between them. The effect is significant when the size of the obstacle is comparable to the wavelength.
The waves follow Huygens’ principle, which states that each point in the middle behaves, in turn, like a focus that emits waves. Since a medium has an infinite number of points, the overlap of all gives the wavefront.
When it reaches an aperture the size of the wavelength, the foci on the wavefront manage to interfere with each other and the waveform deforms.
The diffraction of sound is easy to appreciate, as its wavelength is comparable to that of the objects around us, while the wavelength of light is much shorter and, consequently, diffraction requires very small obstacles.
In the image below, we have a flat wavefront, which moves vertically down to find an opening in a wall.
On the left, the incident wavelength is much smaller than the aperture size and the wave is hardly deformed. On the other hand, in the figure on the right, the wavelength is comparable in size to the aperture, and as it exits, the wave bends considerably.
Examples of wave phenomena
– Listening to music and talking in another room is due to sound diffraction when you encounter openings such as doors and windows. Low frequencies are better at this than high frequencies, which is why distant thunder rumbles much louder than near thunder, which is perceived more as a brief explosion.
-The mirages are due to the fact that parts of the air have different refractive indices, due to the uneven density.
This makes the sky and distant objects appear to reflect off a nonexistent liquid surface in the desert or on a hot road. The successive refractions of light in the irregular layers of the atmosphere are what create this effect.
-It is not possible to see objects smaller than the wavelength of light with which they are illuminated. For example, viruses are smaller than visible wavelengths and therefore cannot be seen under an ordinary microscope.
-The fraction makes us see the sun just before it rises (or sets). At these moments, the sun’s rays hit the atmosphere obliquely and the change in the medium is responsible for bending and deflecting them.
Thus, we can see the king of stars before he is actually above the horizon or continue to see him just above the horizon when he has actually passed below.
