REFRACTION OF LIGHT: ELEMENTS, LAWS AND EXPERIMENT - PHYSICAL - 2025

The refraction of light is the optical phenomenon that occurs when light is obliquely incident on the separation surface of two media with different refractive index. When this happens the light changes its direction and speed.

Refraction occurs, for example, when light passes from air to water, since this has a lower refractive index. It is a phenomenon that can be perfectly appreciated in the pool, when observing how the shapes of the body under water seem to deviate from the direction they should have.

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It is a phenomenon that affects different types of waves, although the case of light is the most representative and the one with the most presence in our day to day life.

The explanation for the refraction of light was offered by the Dutch physicist Willebrord Snell van Royen, who established a law to explain it that has become known as Snell's Law.

Another scientist who paid special attention to the refraction of light was Isaac Newton. To study it, he created the famous glass prism. In the prism, light penetrates it through one of its faces, refracting and decomposing into the different colors. In this way, through the phenomenon of refraction of light, he proved that white light is composed of all the colors of the rainbow.

Elements of refraction

The main elements that must be considered in the study of the refraction of light are the following: -The incident ray, which is the ray that strikes obliquely on the separation surface of the two physical media. -The refracted ray, which is the ray that passes through the medium, modifying its direction and speed. -The normal line, which is the imaginary line perpendicular to the separation surface of the two media. -The angle of incidence (i), which is defined as the angle formed by the incident ray with the normal. -The angle of refraction (r), which is defined as the angle formed by the normal with the refracted ray.

-In addition, the refractive index (n) of a medium must also be considered, which is the quotient of the speed of light in vacuum and the speed of light in the medium.

n = c / v

In this regard, it should be remembered that the speed of light in vacuum takes the value of 300,000,000 m / s.

Refractive index of light in different media

The refractive indices of light in some of the most common media are:

Laws of refraction

Snell's Law is often referred to as the law of refraction, but the truth is that it can be said that there are two laws of refraction.

First law of refraction

The incident ray, the refracted ray and the normal are in the same plane of space. In this law, also deduced by Snell, reflection also applies.

Second law of refraction

The second, the law of refraction or Snell's law, is determined by the following expression:

n ₁ sin i = n ₂ sin r

Where n _{1 is} the index of refraction of the medium from which the light comes; i the angle of incidence; n ₂ the index of refraction of the medium in which the light is refracted; r is the angle of refraction.

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Fermat's principle

From the principle of minimum time or Fermat's principle, both the laws of reflection and the laws of refraction, which we have just seen, can be deduced.

This principle states that the real path followed by a ray of light that moves between two points in space is the one that requires the least time to travel.

Consequences of Snell's law

Some of the direct consequences that are deduced from the previous expression are:

a) If n ₂ > n ₁; sin r <sin io let r <i

So when a ray of light passes from a medium with a lower refractive index to another with a higher refractive index, the refracted ray approaches normal.

b) If n2 <n ₁; sin r> sin io let r> i

So when a light ray passes from a medium with a higher refractive index to another with a lower index, the refracted ray moves away from the normal.

c) If the angle of incidence is zero, then the angle of the refraction ray is zero.

Limit angle and total internal reflection

Another important consequence of Snell's law is what is known as the limit angle. This is the name given to the angle of incidence that corresponds to an angle of refraction of 90º.

When this happens, the refracted ray moves flush with the separation surface of the two media. This angle is also called the critical angle.

For angles greater than the limit angle, the phenomenon known as total internal reflection occurs. When this happens, no refraction occurs, since the entire beam of light is reflected internally. Total internal reflection only occurs when moving from a medium with a higher refractive index to a medium with a lower refractive index.

One application of total internal reflection is the conduction of light through the optical fiber without loss of energy. Thanks to it, we can enjoy the high data transfer speeds offered by fiber optic networks.

Experiments

A very basic experiment to be able to observe the phenomenon of refraction consists of introducing a pencil or a pen in a glass full of water. As a result of the refraction of light, the submerged part of the pencil or pen appears slightly broken or deviated from the path that one would expect it to have.

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You can also try a similar experiment with a laser pointer. Of course, it is necessary to pour a few drops of milk into the glass of water to improve the visibility of the laser light. In this case, it is recommended that the experiment be carried out in low light conditions to better appreciate the path of the light beam.

In both cases, it is interesting to try different angles of incidence and observe how the angle of refraction varies as they change.

Causes

The causes of this optical effect must be found in the refraction of light that causes the image of the pencil (or the beam of light from the laser) to appear deviated underwater with respect to the image we see in the air.

The refraction of light in everyday life

The refraction of light can be observed in many situations of our day to day. Some we have already named, others we will comment on below.

One consequence of refraction is that pools appear to be shallower than they actually are.

Another effect of refraction is the rainbow that occurs because light is refracted by passing through the water droplets present in the atmosphere. It is the same phenomenon that occurs when a beam of light passes through a prism.

Another consequence of the refraction of light is that we observe the sunset of the Sun when several minutes have passed since it actually happened.

References

1. Light (nd). In Wikipedia. Retrieved on March 14, 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 March 12, 2019, from en.wikipedia.org.
4. Light (nd). On Wikipedia. Retrieved on March 13, 2019, from en.wikipedia.org.
5. Lekner, John (1987). Theory of Reflection, of Electromagnetic and Particle Waves. Springer.
6. Refraction (nd). On Wikipedia. Retrieved on March 14, 2019, from en.wikipedia.org.
7. Crawford jr., Frank S. (1968). Waves (Berkeley Physics Course, Vol. 3), McGraw-Hill.