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# Refraction of Light

## Definition

Light is electromagnetic radiation that the human eye can detect. Light can undergo various phenomena like reflection, refraction, diffraction, polarization and interference. Refraction is the phenomenon by which light rays bend whenever it passes from one medium to another. The bending of light takes place due to the different densities of substances. In short, we can say, light undergoes refraction when it changes its medium.

Other forms of energy, like sound waves, hydrological waves, etc., undergo refraction. Because of the refraction phenomenon, we have different instruments, like lenses, prisms, and magnifying glasses. Moreover, we can focus the light on the retina due to refraction.

## Causes of refraction

Different objects have different refractive indices. Whenever a light source passes through objects of different refractive indices, they experience refraction. In addition, refraction occurs if the speed of light changes while travelling from a rarer to a denser medium and vice versa. The straight incident light bends as soon as the medium changes.

Consider an example of light striking a glass of water. When the light strikes the surface of the water, it bends. This is because the refractive of water is different from that of the air. Since the refractive index of water is more than that of air, the light ray bends towards the normal. We conclude that if the light travels from a rarer medium to a denser medium, the ray of light will bend towards the normal.

Let us take another example. Take a thick glass slab with water on its top. When we incident a ray of light above the water, the light bends towards the normal. The same incident light bends more to the normal while passing through the glass slab. This is because the refractive index of the glass slab is more than that of the water.

Moreover, the light ray moves away from the normal and comes back to the same line of sight as the source while coming out of the glass slab. This is because the ray of light travels at a specific angle with the normal in a specific medium.

We can make two conclusions from the second experiment:
1. The angle of refraction is the same for substances with a specific refractive index.
2. The ray of light moves away from the normal if the medium changes from denser to rarer.

## Laws of refraction

According to the laws of refraction, two things can be drawn:

1. The incident light ray, the refracted light ray, and the normal all lie on the same plane.

2. The sine angle of incidence is equal to the sine angle of refraction. This is known as Snell’s law. Mathematically, Snell’s law is given by: sin i / sin r = constant.

## Refractive index

A refractive index is a quantity to denote how fast the light travels in a given media. Mathematically, it is the ratio of the speed of light in a vacuum to the speed of light in the medium. A refractive index is a dimensionless number. It is represented by η.
Therefore, refractive index, η = c / v

Based on the refractive index, we can determine whether the light will travel towards the normal or away from the normal. The light rays travelling in higher refractive index media will always bend towards the normal. This is because the higher the refractive index of the media, the greater is the density.

## Refraction in nature

1. We can see the twinkling of stars due to the refraction of light at night. Star is a celestial body, like planets, having its light. When the rays of the starlight reach the Earth’s atmosphere, it undergoes refraction. This is because the different zones on Earth have different densities due to which the starlight bends. As a result, we see the twinkling of stars.

2. Sunrise and sunset seem earlier due to the refraction of light near the horizon.

3. We might have mirages in a hot desert or summers. This is due to the refraction of light that the sunlight bends while travelling through hot air.

4. A deep swimming pool will look shallower due to the bending of light.

5. Rainbow forming after rain is also due to the refraction of white light.

6. Prism undergoes refraction and splits the white light into its seven constituent colours – violet, indigo, blue, green, yellow, orange and red.

## Applications of refraction

1. Magnification uses the phenomenon of refraction to form an image of an object.

2. The principle of refraction is used in spectacles for people with defective eye sights.

3. Refraction is widely used in telescopes, movie projectors, household cameras, doors and peepholes.

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