# NCERT Physics Class 12 Exemplar Ch 9 Ray Optics and Optical Instruments Part 1 (For CBSE, ICSE, IAS, NET, NRA 2023)

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

Ans:

• 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?

Ans:

• 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?

Ans:

• 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.

Ans:

• 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.

Ans:

At minimum deviation

Given