Science: Light Energy: Lens Formula, Magnification and Dispersion of Light through a Prism (For CBSE, ICSE, IAS, NET, NRA 2022)

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Lens Formula

the distance of object from the optical center of the lens to be u distance of image from the optical center to be v and focal length of the lens is f then the relationship between u, v and f for lens can be shown as:

This is called lens formula. Focal length for convex lens is positive, for concave lens it is taken negative.


The ratio of the size of the image to that of the object is called as the magnification of the lens.

Dispersion of Light through a Prism

  • A prism is a transparent medium bounded by any number of surfaces in such a way that the surface on which light is incident and the surface from which light emerges are plane and non-parallel. Generally equilateral, right angled isosceles or right-angled prisms are used.
  • When white light or sun light passes through a prism it splits up into constituent colors. This phenomenon is called dispersion and arises due to the fact that refractive index of prism is different for different colors of light. So, different colors in passing through a prism are deviated through different angles. Rainbow, the most colorful phenomenon in nature, is primarily due to the dispersion of sunlight by raindrops suspended in air.
  • The incident beam of white light needs to be resolved into its constituent wavelengths before it can provide relevant chemical information on molecular structure and composition of materials.
Dispersion of Light through Prism

Eyes and Its Defects


In this defect the distant objects are not clearly visible i.e.. , far point is at a distance lesser than infinity and hence image of distant object is formed before the retina . This defect is removed by using diverging (concave) lens. Myopia is also called short sightedness or near sightedness.

Eyes and Defects: Myopia

Hyper Metropia

It is also called long sightedness or far sightedness. In it the near objects are not clearly visible i.e.. near point is at a distance greater than 25 cm. So, the image of near object is formed behind the retina. This defect is removed by using converging lens.

Hyper Metropia


In this defect both near and far object are not clearly visible i.e.. , far point is lesser than infinity and near point greater than 25 cm. This can be removed either by using two separate spectacles one for myopia and other for hypermetropia or by using bifocal lens. It is an old age disease. At old age ciliary muscles lose their elasticity so they cannot change the focal length of eye lens effectively and eye losses its power of accommodation.


It is due to imperfect spherical nature of eye lens. The focal length of eye lens is in two orthogonal directions become different so they cannot see objects in two orthogonal directions simultaneously. This defect in direction can be removed by using cylindrical lens in a particular direction.

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