A small telescope has an objective lens of focal length 140 cm and an eyepiece of focal length 5.0 cm. What is the magnifying power of the telescope for viewing distant objects when - Physics

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Numerical

A small telescope has an objective lens of focal length 140 cm and an eyepiece of focal length 5.0 cm. What is the magnifying power of the telescope for viewing distant objects when

(a) the telescope is in normal adjustment (i.e., when the final image is at infinity)?

(b) the final image is formed at the least distance of distinct vision (25 cm)?

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Solution

Focal length of the objective lens, fo = 140 cm

Focal length of the eyepiece, fe = 5 cm

Least distance of distinct vision, d = 25 cm

(a) When the telescope is in normal adjustment, its magnifying power is given as:

`"m" = "f"_"o"/"f"_"e"`

= `140/5`

= 28

(b) When the final image is formed at d,the magnifying power of the telescope is given as:

`"f"_"o"/"f"_"e"  [1 + "f"_"e"/"d"]`

= `140/5 [1 + 5/25]`

= 28[1 + 0.2]

= 28 × 1.2

= 33.6

Concept: Optical Instruments: Telescope
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Chapter 9: Ray Optics and Optical Instruments - Exercise [Page 347]

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NCERT Physics Class 12
Chapter 9 Ray Optics and Optical Instruments
Exercise | Q 9.28 | Page 347
NCERT Physics Class 12
Chapter 9 Ray Optics and Optical Instruments
Exercise | Q 34 | Page 349

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  1. the telescope is in normal adjustment,
  2. the final image is formed at the least distance of distinct vision.

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