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Refraction Basics

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Refraction Basics - Lesson Summary


Light bends while travelling from one medium to another as its velocity differs from one medium to another. The change in the directionof the path of light, when it passes from one transparent medium to the another transparent medium is called refraction of light.The refraction is a surface phenomenon.

The speed of light in optically rarer medium is larger compared to that in optically denser medium. Hence, while travelling from one medium to another, light bends. Light ray passing from rarer to denser medium bends towards the normal. This makes the angle of incidence (angle between the incident ray and the normal at the point of incidence) larger than that of the angle of refraction (angle between the normal and the refracted ray). The incident ray, the normal and the refracted ray, all lie in a plane. If the light ray retraces its path while travelling from denser to rarer, the angle of incidence is lesser than that of the refraction. This is the principle of reversibility.

The extent to which a light ray bends depends on the refrangibility of the ray with respect to the medium. The ratio of velocity of light in vacuum to that in a medium, which is the absolute refractive index (m) of the medium, is the measure of the ability of light to get bend in the given medium. Measuring speed of light is difficult. Hence, Snell’s law helps to determine the refractive index. According to Snell’s law,
µ =  Sin i Sin r                     

When a light ray, incident at an angle, passes through a glass slab, the emergent ray is shifted laterally. The lateral shift depends on the thickness and refractive index of the glass slab.

When a light ray bends from denser to rarer medium, it bends away from the normal. If the angle of incidence gradually increases, the angle of refraction too increases. At a particular angle of incidence in the denser medium, the refracted ray emerges along the surface. That particular angle is the critical angle. If the angle of incidence is greater than the critical angle, the ray undergoes total internal reflection. It is due to this phenomenon we observe mirages in deserts.

The bottom of a water glass appears to rise upwards when viewed normally. This is due to the vertical shift of the bottom of the glass, which takes place because of refraction.

Terms Used for Lens
Centre of Curvature: The centre of the imaginary glass sphere of which the lens is a part, is called centre of curvature.

Principal Axis: An imaginary line joining the centres of curvature of the two spheres, of which lens is a part, is called Principal Axis.

Optical Centre: A point within the lens, where a line drawn through the diameter of lens meets principal axis, is called optical centre.

Principal Focus for Convex Lens: It is a point on the principal axis of a convex lens,  where parallel beam of light rays, travelling parallel to principal axis, after passing through the lens actually meet.

Principal Focus for Concave Lens: It is a point on the principal axis of a concave lens, from where parallel beam of light rays, travelling parallel to principal axis, after passing through the lens, appears to come.

Focal Length: The distance between principal focus and optical centre is called focal length.
Aperture: The effective diameter of the lens through which refration takes place is called aperture of lens.

Optic centre is a point on the axis of a lens such that any light ray passing through this point emerges without refraction.
      •  Principal focus is a point on the axis of a lens.
      •  Principal focus is also known as the focal point.

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