Light Refraction

What is the refraction of light?

The refraction of light is when the light wave passes from one material medium to another when propagating, after which there is an immediate change in its direction and speed. It is a process related to the reflection of light and can occur at the same time.

Light can propagate in material media such as vacuum, water, air, diamond, glass, quartz, glycerin, and all kinds of transparent or translucent materials. In each medium, light travels at a different speed.

There is refraction of light when, for example, it passes from air to water, in which its angle and speed of movement vary.

In all light refraction phenomenon, the following elements participate:

  • incident ray: ray of light that reaches the surface between both media;
  • refracted ray: ray that is deflected when the light wave passes through the surface;
  • regular line: imaginary line perpendicular to the surface, established from the point where both rays coincide;
  • angle of incidence: angle between the incident ray and the line normal. It is expressed with the symbol θ1;
  • refraction angle: is the angle between the refracted ray and the normal line. It is expressed with the symbol θtwo.

light refraction

Light refraction

The speed of light in each medium is given by a quantity called refractive index. The refractive index of these material media is determined by calculating the relationship between the speed of light in a vacuum and the speed of the medium in question. The formula to calculate the refractive index is:

n equals ac over v


  • n is the refractive index of the medium;
  • c is the speed of light in a vacuum;
  • v is the speed of light in the other medium.
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Today the refractive indices of many material media are known. Some examples are:

material medium Refractive index
Empty 1
Air 1,0002926
Water 1.3330
Quartz 1,544
common glass 1.45
Diamond 2.43

Laws of light refraction

Two laws of light refraction are known that explain the behavior of this phenomenon.

First law of refraction of light

According to the first law of refraction of light, the ray of incidence, the ray of reflection, and the line normal lie in the same plane. Consequently, when the phenomenon is observed from above we can capture continuity between both rays.

Second law of refraction of light or Snell’s Law

Snell’s law or second law of refraction of light determines that this occurs when the refractive index of the two media is different and the light ray falls obliquely on the surface that separates them.

With this in mind, Snell’s law establishes the formula for calculating the angle of refraction of light. It applies to any electromagnetic wave. It gets its name from the Dutch mathematician Willebrord Snell van Royen, who discovered it in the 16th century.

The formula for Snell’s law is as follows:

n com 1 subscribed espaço sen teta com 1 subscribed espaço equal to espaço n com 2 subscribed espaço sen teta com 2 subscribed


  • n1 is the refractive index of the medium in which the incident ray is found;
  • θ1 is the angle of incidence of said ray;
  • ntwo is the refractive index of the medium in which the refracted ray is manifested;
  • θtwo is the angle of refraction of the refracted ray.

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Examples of light refraction

Some common examples of light refraction can be found in the following phenomena:

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Teaspoon in a cup of tea

light refraction

When we introduce a teaspoon into a cup of tea, we can see as if it were divided. It is an effect of the refraction of light that produces that optical illusion.

The same phenomenon occurs when we put a pencil or a straw in water. The illusion is produced that these are bent due to the refraction of light.


light refraction

A rainbow is produced by the refraction of light when it passes through small droplets of water suspended in the atmosphere. The light, when entering this area, breaks down and generates the colorful effect.

See also: Rainbow

Halos of sunlight

light refraction

It is a phenomenon similar to the rainbow and that occurs at some points on the globe or in very specific atmospheric conditions. This occurs when ice particles accumulate in the troposphere, which refract light and decompose it, allowing a colored ring to be distinguished around the light source.

Refracted light in a diamond

light refraction

Diamonds are also capable of refracting light, breaking it down into multiple colors.

Lenses and magnifying glasses

light refraction

Both the magnifying glasses and the lenses that we use are based on the principles of light refraction, since they must capture the light and deform the image so that it can be interpreted by the eye.

Rays of sunlight on the sea

light refraction

We can see sunlight changing angle and speed, and it scatters as it enters the sea through the surface.

light through a stained glass window

light refraction

The refraction of light also occurs through glasses or crystals, which filter the light and diffuse it in the environment.

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Reflection and refraction of light

light refraction

Reflection of light

The reflection and refraction of light are related, although different processes. The reflection of light consists of its bouncing when it strikes a surface or interface that separates two different media. For example, the reflection of a landscape in a lake or of a face in a mirror are light reflection phenomena.

Instead, the refraction of light occurs when light passes through the surface that separates two media to continue its propagation. By penetrating the second material medium, the light beam will change its angle and speed.

See also Reflection.