# Physics Class 12 NCERT Solutions: Chapter 9 Ray Optics and Optical Instruments Part 6 (For CBSE, ICSE, IAS, NET, NRA 2022)

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Q: 12. A person with a normal near point using a compound microscope with objective of focal length and an eyepiece of focal length can bring an object placed at from the objective in sharp focus. What is the separation between the two lenses? Calculate the magnifying power of the microscope,

Answer:

Focal length of the objective lens,

Focal length of the eyepiece,

Object distance for the objective lens,

Least distance of distant vision,

Image distance for the eyepiece,

Object distance for the eyepiece

Using the lens formula, we can obtain the value of as:

We can also obtain the value of the image distance for the objective lens using the lens formula.

The distance between the objective lens and the eyepiece

The magnifying power of the microscope is calculated as:

Hence, the magnifying power of the microscope is .

Q: 13. A small telescope has an objective lens of focal length 144 cm and an eyepiece of focal length 6.0 cm. What is the magnifying power of the telescope? What is the separation between the objective and the eyepiece?

Answer:

Focal length of the objective lens

Focal length of the eyepiece,

The magnifying power of the telescope is given as:

The separation between the objective lens and the eyepiece is calculated as:

Hence, the magnifying power of the telescope is and the separation between the objective lens and the eyepiece is .

Q: 14. (A) A giant refracting telescope at an observatory has an objective lens of focal length . If an eyepiece of focal length is used, what is the angular magnification of the telescope?

(B) If this telescope is used to view the moon, what is the diameter of the image of the moon formed by the objective lens? The diameter of the moon is , and the radius of lunar orbit is

Answer:

Focal length of the objective lens,

Focal length of the eyepiece,

(A) The angular magnification of a telescope is given as:

Hence, the angular magnification of the given refracting telescope is .

(B) Diameter of the moon,

Radius of the lunar orbit,

Let be the diameter of the image of the moon formed by the objective lens.

The angle subtended by the diameter of the moon is equal to the angle subtended by the image.

Hence, the diameter of the moon՚s image formed by the objective lens is