 # Refraction of Light

Definition of Refraction

When a light ray is incident on the surface separating two media, the direction of the ray changes. This phenomenon is known as the refraction of light. The speed of light is maximum in the vacuum. In any medium, light travels with less speed. Due to this, the direction of light changes at the interface of two different media. The frequency of the incident light remains constant but the speed and wavelength change. When a light ray enters a denser medium, it bends closer to the normal whereas for a lighter medium, the ray shifts away from the normal.

Laws of Refraction

Refraction is governed by the two laws as follows,

1. The incident ray, refracted ray and the normal to the refracting surface at the point of incidence, belong to the same plane.

2. Snell’s Law: The ratio of the sine of the angle of incidence (angle between the incident ray and the normal) to the sine of the angle of refraction (angle between the refracted ray and the normal) is constant. The constant depends on the two media and the wavelength of the incident light.

Similar laws are followed in the refraction of other waves (e.g. sound wave) as well.

Why Does Refraction Occur?

The frequency (v) of light remains constant when the light ray is incident on the interface of two media. But light moves with different speeds in different media.

c = vλ

As a result, the wavelength (μ) of light also changes at the interface. This, in turn, causes a change in direction, which is refraction. Clearly during refraction,

• The frequency of light remains constant.

• Speed of light and wavelength changes causing the incident light to deviate.

Refractive Index

If  the angle of incidence is θᵢ and the angle of refraction is θᵣ on the interface of two media, according to Snell’s law of refraction,

$\frac{sinθᵢ}{sinθᵣ}$ = μ = constant

This constant μ is called the relative refractive index of the second medium (refracted ray propagates here) with respect to the first medium (incident ray propagates here). If the first medium is taken to be vacuum, μ is called the absolute refractive index.

If the speed of light in a medium is μ , its absolute refractive index is given by,

μ = $\frac{c}{v}$

c is the speed of light in vacuum. is always greater than 1 (e.g. refractive index of glass is 1.51) and equal to 1 for vacuum only. μ is greater for a dense medium. In terms of absolute refractive indices μ₁ and μ₂ of the two media, Snell’s law can be expressed as,

μ₁sini = μ₂sinr

Here, i and r are the incident angle and refracted angle respectively.

Types of Refraction

• Refraction from rarer to denser medium: In such cases, the relative refractive index is greater than 1. The angle of incidence is greater than the angle of refraction i.e. the refracted ray shifts towards the normal. Refraction of light in water from air is an example of this category.

• Refraction from denser to rarer medium: The relative refractive index is less than 1. The angle of refraction is less than the angle of refraction i.e. the refracted ray shifts away from the normal. Refraction from glass to air falls in this category.

Effects of Refraction in Nature

• Rainbow is formed after a shower due to the refraction of sunlight by surfaces of water droplets.

• When the sun rises, the light coming from the sun refracts through the Earth’s atmosphere and reaches our eye before the sun actually crosses the horizon. Similarly, after the sunset, light rays from the sun get refracted and reach us. Due to this, the sunrise is a bit advanced and the sunset is delayed.

• Because of the heterogeneous atmosphere of the Earth, the apparent position of distant stars varies slightly, causing twinkling of stars.

Application of Refraction of Light

• Lenses are transparent objects which can focus light using refraction. Lenses are widely used in cameras and binoculars.

• In cosmology, different telescopes are used to study the faraway objects of the universe. Microscopes are widely used in science to study very small objects, which are otherwise invisible to the naked eye. These optical instruments use refraction to form magnified images of distant or small objects.

• Prisms are wedge-shaped transparent objects, often used as optical filters to extract out a particular wavelength. Light rays of different wavelengths propagate with different speeds in a medium such that they refract at different angles. This phenomenon is called dispersion (observed in prisms).

• An important application of refraction is optical fibers which are used for communication with minimum energy dissipation.

Did you know?

• A light ray shifts away from the normal after getting refracted from a denser to a rarer medium. The angle of refraction increases with the incident angle. For a particular incident angle namely the critical angle, the refracted ray passes along the interface i.e. the angle of refraction is 90. For incident angles greater than the critical angle, the angle of refraction exceeds 90and the refracted ray comes back in the denser medium itself. This phenomenon is called total internal reflection. Mirages in deserts or cold oceans are optical illusions formed by this process.

• Due to total internal reflection, an observer inside water sees the objects above the water surface to lie inside a cone of half-angle 49, which is the critical angle of water and air.

• A convex lens of varying focal length, in the human eye, forms the image of an object on the retina. It is an effect of the refraction of light.