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Refraction

Liquid crystal that changed the Display world Liquid crystal that changed the display world The breakthrough has come with the development of liquid crystals to over power the traditional monitors, that consume lot of power. The technology of liquid crystal display (LCD) uses less power. LCD works using the phenomenon of refraction, where the light passes through many components like filters, glass plates, liquid crystal, etc.,. To give a clear picture, lets discuss in detail about the amazing applications of refraction.

Learning objectives

After completing the topic, the student will be able to:

  • Distinguish and differentiate between reflection and refraction and explore their properties.
  • Discuss and explore about the applications of refraction as observed in daily life.
  • Explore the concept of refractive index and laws of refraction and its applications.
  • Examine, explore and probe about the types and magnification of convex/concave lenses and their combination.
  • Discuss and explain the aberration/defects involved in different types of lenses and measures to minimize the same.
  • Outline and formulate the concept of total internal reflection based on the critical angle. Also explore its vast applications in the optical fiber communications.
Glass block
A light ray incident on a glass block A light ray incident on a glass block bends towards the normal and while it leaves it bends back again away from the normal.
Refraction

Light travels at different speeds in different materials. Light travels at 3 × 108 m/sec in vacuum, at a slightly lower speed in air and at 2 × 108 m/sec in glass. In diamond, light travels at about 40% of its speed in vacuum. When light travels from one medium to another, its speed changes. This change causes light rays to bend. Thus when light bends in passing from one medium to another, we call the process as refraction.

We know that water or glass is transparent. But when water is running down a faucet, we can see the water stream. Isn’t it? The reason is: Even though water or glass are transparent to visible light, we can “see” them only at their edges, where the refraction conditions are pronounced. If there is a uniform sheet of glass or water in front of us, it would be difficult to see them. Don′t we sometimes wonder if the car windscreen is wound up or down? In quite a lot of public places, such as hospitals, plain glass doors are marked with a horizontal white bar – so that persons do not try to walk through the door!

A transparent substance in which light travels is known as a medium. Air, glass, water, ice, diamond etc., are all examples of medium. Different media are said to have different optical densities. A medium in which speed of light is more when compared with other medium is known as optically rarer medium. Air is an optically rarer medium as compared to water, glass and alcohol. A medium in which speed of light is less when compared with other medium is known as optically denser medium. Glass is optically denser medium than air and water. When a ray of light goes from a rarer medium to a denser medium, it bends towards the normal (at the point of incidence). When a ray of light goes from a denser medium to a rarer medium, it bends away from the normal (at the point of incidence).

Light on a glass window Light ray incident on a Window glass at different angles The light rays entering from a one medium and emerging out back in to the same medium will be parallel.
Refraction through a rectangular glass slab

Consider the pane of thick window glass as shown in the figure. When light falls normally to the surface of a glass slab (from point A through the glass to point B), it will go in a straight–line path. In this case, light encounters the glass perpendicularly and hence there is no bending of the ray of light.

But when it goes from A to C, the emerging light is displaced but parallel to the incident light. A ray of light Aa travelling in air is incident on the glass block. On entering the glass block, it gets refracted and bends towards the normal. Again a change of direction takes place when the ray of light ac, travelling in glass emerges into air at point c. Since the ray of light ac goes from a denser medium glass into the rarer medium air, it bends away from the normal and goes in the direction cC. The incident ray Aa and the emergent ray cC are parallel to each other.

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