7 junio, 2024

Wave phenomena: what it is, characteristics, types, examples

What are wave phenomena?

The wave phenomena they take place when waves propagate in a medium and meet other waves, with changes in the medium, boundaries, gaps and obstacles in general. This causes alterations to the shape of the waves and their displacement.

Waves carry energy, not matter. If we look closely, when a stone is thrown into a pond, what is propagated in the water is the disturbance, since the liquid molecules move briefly from their equilibrium position and return to it as soon as the disturbance moves away.

Since there is no transport of matter, we can expect waves to behave differently than objects would when they interact.

Waves manage to cross different media and even to occupy the same space at the same time, something that particles with mass cannot do, at least at a macroscopic level (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 existing differences in their respective natures.

For example, light does not need a material medium to propagate, while sound does. Furthermore, light is a transverse wave (the disturbance is perpendicular to the direction in which the wave travels), 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 wave phenomena in common:

Reflection

When waves travel they sometimes encounter boundaries that separate one medium from another, for example a pulse traveling through a string firmly attached to one end.

Once the pulse reaches the end of the string, it goes back for the most part, but it does reverse. It is then said that the pulse undergoes reflection, that is, it is reflected at the limit between the string and the support.

The inversion of the pulse is due to the reaction exerted by the support on the rope, 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 when it travels back.

Another possibility is that the rope has some freedom at the attached end, for example it is tied to a ring that you can slide over a bar. So the pulse sent through the string does not return inverted.

In general terms, when a wave propagates and reaches the limit that separates two different media, it undergoes a change of direction. The arriving wave is known as the incident wave, the one that returns is the reflected wave and if a part is transmitted to the other medium, it is known as a refracted wave.

Sound is a wave, so you experience reflection when speaking in an empty room. Light is also a wave and we can see it reflecting in the mirror, on the calm surface of a pond or in the glass of a skyscraper.

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.

When trying to grab an object submerged at the bottom of a fountain or a bucket, it is very likely that you will not reach it, even if the hand goes towards where the object is. And that is because the light rays have changed their direction when they passed from the air to the water, that is, they experienced refraction.

In addition, the speed with which the waves move varies according to the medium. In a vacuum, light waves move with a constant speed c = 300,000 km/s, but in water the speed decreases to (3/4) c and in glass even more: to (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/w

The phenomenon is analogous to a toy car rolling on a hard ceramic or highly polished wood floor and suddenly rolling on a carpet. It not only changes its direction, but also decreases its speed.

Absorption

If the wave meets a different medium, it may happen that all the energy it carries is lost and its amplitude becomes zero. The wave is then said to have been absorbed.

Interference

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.

It happens every time two stones are thrown into the water simultaneously, independent wave patterns are produced that can overlap and give a resulting wave.

The amplitude of the resulting wave may be greater or less than that of the interfering waves, or they may simply cancel each other out. In them the superposition principle.

For waves, the superposition principle establishes that the resulting wave is equal to the algebraic sum of the displacements of the interfering waves (they can be more than two).

If the waves are in phase, which means that their troughs and crests are aligned, the result is a wave with twice the amplitude. This is known as constructive interference.

On the other hand, when the crest of one wave overlaps with the trough of another, they cancel each other out 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 had happened.

Diffraction

This phenomenon is typical of waves; in it the wave is diverted and distorted when encountering an obstacle placed in the path of the wave or a gap in the middle. The effect is significant when the size of the obstacle is comparable to that of the wavelength.

The waves attend to Huygens’ principle, which establishes that every point in the middle behaves in turn as a focus that emits waves. Since a medium has an infinite number of points, by superimposing them all the wave front is obtained.

When it reaches an opening the size of the wavelength, the foci in the wavefront manage to interfere with each other and the wave is deformed.

The diffraction of sound is easy to appreciate, since its wavelength is comparable to that of the objects that surround us, while the wavelength of light is much shorter and consequently diffraction requires very small obstacles.

In the following image we have a flat wavefront, which moves vertically downwards to meet an opening in a wall.

On the left, the length of the incident wave is much smaller than the size of the aperture, and the wave is hardly deformed. On the other hand, in the figure on the right, the wavelength is comparable in size to that of the opening and when emerging from it, the wave curves appreciably.

Examples of wave phenomena

-Hearing the music and conversations in another room is due to the diffraction of sound when it meets openings such as doors and windows. Low frequencies are better at this than high frequencies, which is why distant thunder rumbles much more than near ones, which are perceived more as brief booms.

-Mirages are due to parts of the air having different refractive indices, due to unequal density.

This makes the sky and distant objects appear to reflect off a non-existent liquid surface in the desert or a hot highway. The successive refractions of light in the uneven layers of the atmosphere are what create this effect.

-It is not possible to see objects smaller than the wavelength of the light with which they are illuminated. For example, viruses are smaller than the visible wavelengths, so they cannot be seen with an ordinary microscope.

-Refraction means that we can see the Sun shortly before it rises (or sets). At those times the sun’s rays hit the atmosphere obliquely and the change in the environment is responsible for bending and diverting them.

That is why we can see the king sun before it is actually above the horizon or continue to see it just above the horizon when it has actually passed below.

References

Bikos, K. What is refraction of light? Retrieved from: timeanddate.com.
Figueroa, D. 2005. Series: Physics for Science and Engineering. Volume 7. Waves and Quantum Physics. Edited by Douglas Figueroa (USB).
Hewitt, Paul. 2012. Conceptual Physical Science. 5th. Ed. Pearson.
hyperphysics. Refraction. Retrieved from: hyperphysics.phy-astr.gsu.edu.
Rex, A. 2011. Fundamentals of Physics. pearson.
Sears, Zemansky. 2016. University Physics with Modern Physics. 14th. Ed.Volume1.
Wikipedia. Refraction atmospheric. Retrieved from: fr.wikipedia.org.

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