REFLECTION OF LIGHT: ELEMENTS, LAWS AND APPLICATIONS - PHYSICAL - 2025

The reflection of light is the optical phenomenon by which a ray of light changes direction and is reflected when it hits the separation surface of two media, without being able to cross said surface.

It is a basic phenomenon that occurs in nature and that was already studied in classical Greece. However, the laws governing the reflection of light were not enunciated until well into the seventeenth century. It was the Dutch scientist W. Snell who defined the laws of both reflection and refraction. In this way, these laws were called Snell's Laws.

Reflection of light in the water. Pixabay

Reflection itself is a general phenomenon that affects all waves, although the case of light is the most representative. Every time light falls on a body, a reflection occurs. Depending on which colors of the spectrum the body absorbs and which ones it reflects, we see the body of one color or another.

Reflection is also present in our day-to-day life in matters as everyday as the formation of images in mirrors. These images can be explained from the laws of reflection. It can also be seen in the images reflected on the surface of the water, although in this case the phenomenon of refraction also occurs.

Elements

When studying the reflection of light, the following elements must be taken into account: light, the two media, the separation surface of the media, the incident ray, the reflected ray and the normal to the separation surface.

In physics, the term light includes the entire field of radiation included in the electromagnetic spectrum, with the term visible light being reserved for the part of the spectrum that the human eye perceives.

In reflection, two means must be distinguished. The first is the medium through which the wave travels. The second either does not cross it or if it does, the refraction of the wave occurs. Between the two media is what is called media separation.

The normal is a line perpendicular to the plane of separation of the media. The incident ray is called the beam of light that reaches the separation surface through the first medium. For its part, the reflected ray is the one that is reflected after the incident ray collides with this surface.

Laws of reflection

Although Euclid was the first to publish laws of reflection in the 3rd century BC, the truth is that it was in 1621, with the Dutch astronomer and mathematician Willebrord Snell van Royen, when the current laws of reflection and refraction were established..

The two laws of reflection are discussed below.

First law

The first law is summarized in the following statement: the incident ray, the normal (or perpendicular to the plane) and the reflected ray are in the same plane of space.

Angles of light reflection. No machine-readable author provided. Arvelius assumed (based on copyright claims).

Second law

The second law of reflection states that the angle of reflection is exactly the same as the angle of incidence.

Fermat's principle

Both the previous two laws of reflection, as well as the laws of refraction, can be deduced from Fermat's principle. This principle states that the path that a ray of light follows between two places in space is always the one that takes the shortest possible time.

Total internal reflection of light

Total internal reflection of light occurs when light meets a medium with an index of refraction, n ₂, less than that of the medium in which it is found, n ₁. In this case, the light is not able to pass through the separation surface of both media and is completely reflected.

Of course, this only occurs for angles of incidence greater than those called the critical angle.

Total internal reflection is the cause of the sparkles that can be seen in a cut diamond.

Types of reflection

Reflection can be of several types: specular, diffuse or mixed. Whether one type of reflection or another takes place will depend mainly on the type of surface in its path.

Specular reflection

When light falls on a smooth, polished surface, specular reflection occurs.

Diffuse reflection

Instead, when light falls on an unpolished surface, reflection occurs in all directions in space. It is said, then, that there has been a diffuse reflection.

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Mixed reflection

As its name implies, mixed reflection occurs when a combination of the previous two occurs.

Applications

The reflection of light has different applications. Thus, for example, the phenomenon of total reflection is used in what is known as the Porro prism, which is used in the manufacture of binoculars.

Total reflection is also used for the propagation of light in fiber optic cables. In this way, if your home has a fiber optic internet connection, you should know that part of the responsibility for enjoying it is due to the total reflection of light.

Retroreflection

Retroreflection is an application of light reflection, consisting of reflecting light back towards the origin or source, whatever its angle of incidence. To achieve this, flat reflective surfaces are used.

A specific application of retroreflection is in traffic signs. This allows the light from the headlights to be reflected directly in the direction of origin. Thus, the signal appears enhanced so that the driver receives the warning of the danger.

Experiment

The phenomenon of light reflection can be verified with some simple home experiments like the one we propose below. You can do it at home without any risk and see how physics is at your fingertips.

Total internal reflection of light experiment

To carry out this experiment you only need a glass, water, milk and a laser pointer.

First, fill the glass with water, then add a few drops of milk. Once this is done, the laser pointer is pointed at the glass below the water level. In this way, the light beam from the pointer will pass through the water and will be reflected on the surface of the water with the air.

Furthermore, thanks to the milk drops dissolved in the water, it is possible to follow the complete path of the light wave and to observe perfectly both the incident ray and the reflected ray. In any case, the ideal is to carry out the experiment in a room with little light to better appreciate the pattern of the reflected wave.

Causes

As we have explained previously, this phenomenon only takes place when the light passes from a medium with a higher refractive index to one with a lower index. In the same way, for this to happen, the light has to affect the separation of the media with an angle greater than the so-called critical angle.

conclusion

The reflection of light is a natural phenomenon that accompanies us daily in our lives. This is so, to such an extent, that we perceive colors thanks to it. There is evidence of its study already in classical Greece, although it was not until the seventeenth century with Snell that the rules that govern it began to be defined.

At present, its applications are many and varied. Some, surely you would not have imagined them, and it is that they intervene in such unexpected processes as the transmission of information through fiber optic cables.

Not only physics is in everything when it surrounds us, light also inseparably accompanies us in our discovery of reality. Not in vain, it is thanks to her that we perceive the world around us.

References

1. Light (nd). In Wikipedia. Retrieved on February 27, 2019, from en.wikipedia.org.
2. Burke, John Robert (1999). Physics: the nature of things. Mexico DF: International Thomson Editores.
3. Total internal reflection (nd). On Wikipedia. Retrieved on February 28, 2019, from en.wikipedia.org.
4. Light (nd). On Wikipedia. Retrieved on March 1, 2019, from en.wikipedia.org.
5. Lekner, John (1987). Theory of Reflection, of Electromagnetic and Particle Waves. Springer.