Modern Physics

Light refraction: elements, laws and experiments

The refraction of light is the optical phenomenon that occurs when light obliquely strikes the separation surface of two media with different refractive indices. When this happens, light changes direction and speed.

Refraction occurs, for example, when light passes from air to water, as it has a lower refractive index. It is a phenomenon that can be seen perfectly in the pool, observing how the body forms under the water it seems to deviate from the direction it should have had.

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

The explanation for the refraction of light was offered by Dutch physicist Willebrord Snell van Royen, who established a law to explain what became 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 different colors. So, through the phenomenon of light refraction, he proved that white light is made up of all the colors in the rainbow.

refraction elements

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

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

n = c / v

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

Laws of refraction

Snell’s law is often called the law of refraction, but the truth is, the laws of refraction can be said to be two.

first law of refraction

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

second law of refraction

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

1 sin i = n 2 sin r

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

Principle of Fermat

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

This principle states that the actual path followed by a ray of light traveling between two points in space requires the least time to travel.

Consequences of Snell’s Law

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

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

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

b) If n 2 <n 1 ; sen r> sen io sea r> i

Therefore, when a ray of light passes from a medium with a higher refractive index to one with a lower index, the refracted ray deviates from the normal.

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

Boundary angle and total internal reflection

Another important consequence of Snell’s law is what is known as the boundary angle. This is called the angle of incidence which corresponds to an angle of refraction of 90°.

When this happens, the refracted ray moves close to the surface separating the two media. This angle is also called the critical angle.

For angles greater than the limit, the phenomenon called total internal reflection occurs. When this occurs, refraction does not occur, as the entire light beam is internally reflected. 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 optical fiber without energy loss. Thanks to this, we can enjoy the high data transfer speeds offered by fiber optic networks.


A very basic experiment to observe the phenomenon of refraction is to introduce a pencil or pen into a glass filled with 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 you would expect it to have.

You can also try a similar experiment with a laser pointer. Obviously, 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 test with different angles of incidence and see how the angle of refraction varies as they change.


The causes of this optical effect must be looked for in the refraction of light that causes the pencil (or laser beam) image to be deflected underwater in relation 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 in our daily lives. Some of them we’ve already named, others we’ll count below.

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

Another effect of refraction is the rainbow, which occurs because light is refracted as it passes through water droplets 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 watch the sun set when several minutes have passed since it actually happened.

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