Refraction

When the light (a type of electromagnetic wave) travels through from one medium to another medium other than the vacuum, it deflects from its path. This deflection of light is Refraction. Refraction of the light is the most common phenomenon as because of this property of light one can see the objects. Lenses of human eyes are like a convex lens and when the light hits on the lens it converges on the retina and image of the object forms on the retina. This converging of light is an example of refraction. Making of optical lenses, binoculars, microscopes and other optical devices use this property of light. Natural phenomenon like mirages and rainbows appear due to the refraction of light.

Refraction and its Uses

Refraction is the bending of light and this can be visualised when it passes from one transparent medium to another transparent medium. The banding capacity of any medium is termed as refractive index. Use of this property of light is very common in nature as well as artificial. Therefore, there are two types of refraction, namely, natural and artificial refraction. A common example of natural refraction is atmospheric refraction. Due to this atmospheric refraction, the light from the sun reaches on the earth and spreads all over. Refraction depends on the density of the medium. The density of the medium slows down the speed of light and therefore light bends.

In the atmosphere, the density of air is low at high altitudes and it becomes denser at low altitudes. The density of air also depends on temperature and pressure. At high altitudes, the density of air is low hence it does not spread the light much. But when the light emerges deep down the density of air increases which refracts the light more and spreading of light occurs more. This is phenomenon is more predominant at the time of sunrise and sunset. Before the time of sunrise, though the sun does not appear the light emerges from the horizon. Similarly, even after the sunset, light appears at the horizon.

Temperature variation also affects the refraction of light. When we see the objects over the fire or heated surface, we find that objects are shimmering. This shimmering of objects is due to the mixing of hot air (near the fire of heated surface) with the cold air. The same phenomenon occurs in hot summer days on the roads. In summer day, another phenomenon occurs which is a mirage. In this, if we see the road at a certain angle, it appears that water has covered the surface. This illusion is only due to refraction.

When we see the objects placed inside water, they appear closer actually they are. Similarly, we immerse a stick partially in a glass of water in a tilted way we see that the stick which merged in the water is slightly bent. So the bending of light depends on the angle at which it is falling. If the angle of light is perpendicular to the surface, it does not bend but it looks thick. In this way, we can say that when the light enters in a medium its speed becomes slower and the path deflects.

Artificial refraction has medical uses also. With the help of this property of light, refractive errors of human eyes are corrected and corrective lenses are made.

Laws of Refraction

The refraction of light is governed by three laws.

First law: The first law of refraction states that the incident ray, the refracted ray and the normal to the interface of two transparent media at the point of incidence, all lie in the same plane.

Second law: The second law states that when a ray of light passes through a rarer medium to a denser medium, the refracted ray comes closer to the normal while the ray of light passes through a denser medium to a rarer medium, the refracted ray moves away from the normal. This law of refraction is termed as Snell’s law.  According to Snell’s law, the ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant. This constant is the refractive index.

\(\frac{sin i}{sin r}\) = constant

This constant or refractive index is denoted by ‘n’.

Another way of representing ‘n’ is as the ratio of the velocity of light in one medium to the velocity of light in another medium. When light falls on the water surface, the medium above the water surface is air and velocity of light in the air is assumed to be equal to ‘c’, equivalent to velocity of light in vacuum. Assuming that the velocity of light in the water is ‘u’ then

Refractive Index = n = \(\frac{v}{c}\)

So we can say that

\(\frac{sin i}{sin r}\) = n = \(\frac{v}{c}\)

When the light passes through one medium (1) to the second medium (2), then the refractive index is:

\(n_{21}\) = \(\frac{v_{1}}{v_{2}}\),

Here, \(v_{1}\) is the velocity of light in the first medium, \(v_{2}\) is the velocity of light in the second medium and \(n_{21}\) is the refractive index of one medium with respect to the second medium.

Third law: According to the third law, the incident ray passing through the normal always goes straight in the media after refraction.

 Application of Refraction of Light

It has many applications in nature and human life. In nature, the formation of rainbows and mirages are because of the refraction of light. Rainbows form when the light passes through the droplets presented in the moist air. The light gets refracted and forms a rainbow.

In human life, refraction of light plays an important role. It has many applications in optics and imaging technology. Some of the applications are:

• Refraction concave and convex glasses are used to correct the refractive errors of human eyes
• Peepholes in the doors, magnifying glasses, binoculars, cameras, lenses in the projectors, etc. use the principle of refraction of light.

FAQs on Refraction

Q.1: Define refraction of light?

Answer: when the light passes through any medium, it deflects from its path. This deflection of light is Refraction.

Q.2: What are the three laws of refraction?

Answer: Following the three laws of refraction:

1. The incident ray, the normal and the refracted ray at a point of incidence all lie in the same plane.
2. When a ray of light passes through a rarer medium to a denser medium, the refracted ray is closed to the normal while the ray of light passes through a denser medium to a rarer medium, the refracted ray is away from the normal.
3. An incident ray passing through the normal always goes straight.

Q.3: What are the factors affecting the refraction of light?

Answer: There are following factors affecting refraction of light,

1. Density on the medium: The density of the medium affects the refraction of the light. When the density of the medium is low, it does not refract the light much. The denser medium refracts light as it slows down the velocity of light which causes refraction.
2. Temperature and pressure: The effect of temperature and pressure can be understood by the differences in the refractive index of air at higher and lower altitudes. At high altitudes, where the temperature and pressure are low the density of air is low and therefore it refracts the light much less than at low altitudes where the pressure is high and the temperature is moderate.
3. The wavelength of the light: The light comprises of 7 major colours. These colours are having different wavelengths. When light passes through the medium, the colour which has low wavelength refracts less and the colour which has a higher wavelength refracts more.