## Refraction at a Rarer Medium – Class 12 | Chapter – 10 | Physics Short Notes Series PDF for NEET & JEE

Refraction at a Rarer Medium: Refraction at a rarer medium is used in optical devices, such as lenses and prisms, to manipulate the path of light and produce images. The ability to control the path of light through refraction is an important aspect of many technologies, including cameras, telescopes, and microscopes.

## Refraction at a Rarer Medium

When a wave passes from a denser medium to a rarer medium, the speed of the wave increases, and the wave is refracted away from the normal to the surface. The angle of incidence is greater than the angle of refraction, and the refracted wave bends away from the normal.

In this case, the ratio of the sine of the angle of incidence to the sine of the angle of refraction is greater than one, and is given by Snell’s law:

sin θ1 / sin θ2 = v1 / v2 = n2 / n1

Where,

• θ1 is the angle of incidence,
• θ2 is the angle of refraction,
• v1 and v2 are the velocities of the wave in the two media, and
• n1 and n2 are the refractive indices of the two media.

When light passes from air to water, for example, the refractive index of water is greater than that of air, and the speed of light is slower in water. As a result, the angle of incidence is greater than the angle of refraction, and the light is refracted away from the normal.

This phenomenon is commonly observed in everyday life, such as when a straw in a glass of water appears to be bent at the surface of the water. The light from the straw is refracted as it passes from the water to the air, causing the straw to appear to be bent.

## Applications of Refraction at a Rarer Medium

The refraction of waves when they pass from a denser medium to a rarer medium, or from a medium of higher refractive index to one of lower refractive index, has a wide range of practical applications. Some of the most common applications are:

• Lenses: Refraction at a rarer medium is the basis for the operation of lenses, which are used in eyeglasses, telescopes, microscopes, and cameras. The curved surface of the lens is designed to bend light so that it converges or diverges to form an image.
• Magnifying glass: The magnifying glass works on the same principle as the lens, where the curved surface of the glass is used to focus the light and produce a magnified image.
• Fiber optics: Refraction at a rarer medium is used in the design of fiber optic cables, which are used to transmit information over long distances. Light is transmitted through the cable by repeatedly undergoing total internal reflection at the boundary between the fiber and the surrounding medium, which allows the light to travel without significant attenuation.
• Prism: A prism is an optical device that is used to separate light into its constituent colors. This is achieved by the different amounts of refraction of different colors of light as they pass through the prism.
• Mirage: Refraction at a rarer medium causes the mirage, where the image of an object appears displaced from its actual position due to the bending of light through the atmosphere.
• Camera: In a camera, the image is formed by the refraction of light through the lens onto a photosensitive surface.

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