CBSE Class 12th Physics Notes for Chapter 9 Ray Optic and Optical Instruments

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  • Reflection is governed by the equation ∠i = ∠r’ and refraction by the Snell’s Law, sin i/sin r = n, where the incident ray , reflected ray, refracted ray and normal lie in the same plane. i, r and r’ are the angle of incidence, reflection and refraction respectively.


  • The critical angle of incidence ic for a ray incident from a denser to rarer medium, is that angle for which the angle for refraction is 90o. For i > ic , total internal reflection occurs.


  • Optical fibres: Optical fibres consist of glass fibres coated with a thin layer of material of lower refractive index. Light incident at one angle at one end comes out at the other, after multiple internal reflections, even if thee fibre is bent.


  • Cartesian sign convention: Distances measured in the same direction as the incident light are positive while those measured in the opposite direction are negative. The heights measured upwards above x-axis are taken as positive and the heights measured downwards are taken as negative.


  • Mirror equation: 1/v + 1/u = 1/f , where u and v are object and image distances respectively and f is the focal length of the mirror.


  • Dispersion: Dispersion is the splitting of light into its constituent colours.


  • The eye: The eye has a convex lens of focal length about 2.5 cm. This focal length can be varied somewhat so that the image is always formed on the retina. This ability of the eye is called accommodation. In a defective eye, if the image is focussed before the retina (myopia), a diverging corrective lens is needed to correct the defect. If the image is focussed beyond the retina (hypermetropia), a converging corrective lens is needed. Astigmatism is corrected by using cylindrical lenses.


  • Magnifying power (m) of a simple microscope is given by m= 1 + (D + f) where D=25 cm is the least distance of distinct vision and f is the focal length of the convex lens.
  • Magnifying power (m) of a telescope is the ratio of the angle subtended at the eye by the image to the angle subtended at the eye by the object.

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