NCERT Physics Class 12 Exemplar Ch 9 Ray Optics and Optical Instruments Part 1

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Very Short Answer

Q 17. Will the focal length of a lens for red light be more, same or less than that for blue light?


  • As the refractive index for red is less than that for blue, parallel beams of light incident on a lens will be bent more towards the axis for blue light compared to red.

  • Thus the focal length for blue light will be smaller than that for red.

Q 18. The near vision of an average person is 25cm. To view an object with an angular magnification of 10, what should be the power of the microscope?


  • The near vision of an average person is 25cm. To view an object with magnification 10,

Q 19.An unsymmetrical double convex thin lens forms the image of a point object on its axis. Will the position of the image change if the lens is reversed?


  • No. The reversibility of the lens makes equation.

Q 20 Three immiscible liquids of densities and refractive indices are put in a beaker. The height of each liquid column is . A dot is made at the bottom of the beaker. For near normal vision, find the apparent depth of the dot.


  • Let the apparent depth be for the object seen from then

  • If seen from the apparent depth is.

  • Seen from outside, the apparent height is

Q 21. For a glass prism the angle of minimum deviation is equal to the angle of the prism. Find the angle of the prism.


At minimum deviation


Minimum deviation equal to the angle of the prism

Minimum Deviation Equal to the Angle of the Prism

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