Solved: PHYSICS Question for NEET

NEET PHYSICSMedium

An observer is using an astronomical refracting telescope to observe planets in normal adjustment. The focal lengths of the objective and eyepiece used in the telescope construction are $20 \text{ m}$ and $2 \text{ cm}$ respectively. Consider the following statements about the telescope: (A) The distance between the objective and the eyepiece is $20.02 \text{ m}$ . (B) The magnification of the telescope is $-1000$ . (C) The image of the planet is erect and diminished. (D) The aperture of the eyepiece is smaller than that of the objective.

The correct statements are:

A, B, and C

B, C, and D

C, D, and A

A, B, and D

Step-by-Step Solution

Analyze Given Data:

Focal length of objective ( $f_o$ ) = $20 \text{ m}$ .
Focal length of eyepiece ( $f_e$ ) = $2 \text{ cm} = 0.02 \text{ m}$ .
Normal adjustment implies the final image is formed at infinity.

Evaluate Statement A (Length):

In normal adjustment, the distance between the lenses (length of the telescope, $L$ ) is the sum of their focal lengths.
$L = f_o + f_e = 20 \text{ m} + 0.02 \text{ m} = 20.02 \text{ m}$ .
Statement A is Correct.

Evaluate Statement B (Magnification):

The magnifying power ( $m$ ) in normal adjustment is given by $m = -f_o / f_e$ .
$m = -20 / 0.02 = -1000$ .
The negative sign indicates the image is inverted.
Statement B is Correct.

Evaluate Statement C (Image Nature):

An astronomical refracting telescope produces an inverted image. It is not erect.
Statement C is Incorrect.

Evaluate Statement D (Aperture):

The objective lens must have a large aperture to gather sufficient light from distant objects and to provide high resolution ( $RP \propto a$ ). The eyepiece has a smaller aperture.
Statement D is Correct.

Conclusion: Statements A, B, and D are correct.

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