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Optics MCQs for NEET — Physics Questions with Answers

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The magnifying power of a refracting telescope is given by the ratio of the focal length of the objective lens ($f_o$) to the focal length of the eyepiece ($f_e$). If a telescope has an objective of focal length 100 cm and an eyepiece of focal length 1 cm, what is its magnifying power?

A. 1 B. 10 C. 100 D. 0.01

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Explanation

The NCERT text provides an example: 'For example, consider a telescope whose objective has a focal length of 100 cm and the eyepiece a focal length of 1 cm. The magnifying power of this telescope is m = 100/1 = 100.' This directly applies the formula $m = f_o / f_e$.

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What are the two main considerations for an astronomical telescope, as mentioned in the NCERT text?

A. Field of view and chromatic aberration B. Light gathering power and resolving power C. Focal length and aperture D. Cost and portability

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Explanation

The NCERT excerpt states, 'The main considerations with an astronomical telescope are its light gathering power and its resolution or resolving power.'

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The resolving power of an optical telescope primarily depends on:

A. The focal length of the eyepiece B. The focal length of the objective C. The diameter of the objective D. The material of the lenses

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Explanation

The NCERT text mentions, 'The resolving power, or the ability to observe two objects distinctly... also depends on the diameter of the objective. So, the desirable aim in optical telescopes is to make them with objective of large diameter.'

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Why do modern telescopes often use a concave mirror instead of a lens for the objective?

A. Mirrors are easier to clean. B. Mirrors have no chromatic aberration. C. Mirrors produce a magnified, erect image directly. D. Lenses are fundamentally more complex to grind.

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Explanation

The NCERT states, 'For these reasons, modern telescopes use a concave mirror rather than a lens for the objective. Telescopes with mirror objectives are called reflecting telescopes. There is no chromatic aberration in a mirror.'

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Which of the following is an advantage of a reflecting telescope's mirror objective over a lens objective, concerning mechanical support?

A. Mirrors can be made lighter than lenses of equivalent optical quality. B. Mirrors can be supported over their entire back surface. C. Lenses require more complex mounting mechanisms. D. Mirrors are inherently more rigid.

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Explanation

The NCERT text highlights, 'Mechanical support is much less of a problem since a mirror weighs much less than a lens of equivalent optical quality, and can be supported over its entire back surface, not just over its rim.'

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What is a Cassegrain telescope?

A. type of refracting telescope using a prism for inversion. B. reflecting telescope that uses a convex secondary mirror to focus light through a hole in the primary mirror. C. telescope designed for terrestrial observations only. D. telescope that combines lenses and mirrors to eliminate all aberrations.

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Explanation

The NCERT diagram (Fig. 9.26) and description explain: 'One such arrangement using a convex secondary mirror to focus the incident light, which now passes through a hole in the objective primary mirror, is shown in Fig. 9.26. This is known as a Cassegrain telescope, after its inventor.'

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The largest reflecting telescopes in the world, as mentioned in the context, are known as:

A. Yerkes Telescope B. Mt. Palomar telescope C. Cassegrain telescope in Kavalur D. Keck telescopes

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Explanation

The NCERT states, 'The largest reflecting telescopes in the world are the pair of Keck telescopes in Hawaii, USA, with a reflector of 10 metre in diameter.'

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A refracting telescope is used for both terrestrial and astronomical observations. What additional component do terrestrial telescopes have compared to astronomical ones?

A. larger objective lens B. shorter eyepiece focal length C. pair of inverting lenses D. concave secondary mirror

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Explanation

The NCERT text specifies, 'Terrestrial telescopes have, in addition, a pair of inverting lenses to make the final image erect.'

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If an astronomical telescope has an objective lens of focal length 1.5 m and an eyepiece of focal length 1.0 cm, what is its angular magnification?

A. 1.5 B. 15 C. 150 D. 1500

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Explanation

The angular magnification (m) for a telescope is given by $m = f_o / f_e$. Given $f_o = 1.5$ m $= 150$ cm and $f_e = 1.0$ cm. So, $m = 150 ext{ cm} / 1.0 ext{ cm} = 150$. This is directly from practice problem 9.14 (a) in the NCERT.

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What is an advantage of a Cassegrain telescope's design?

A. It eliminates spherical aberration completely. B. It achieves a large focal length in a short telescope tube. C. It is less expensive to produce than a simple refracting telescope. D. It provides only virtual images.

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Explanation

The NCERT text states regarding the Cassegrain telescope, 'It has the advantages of a large focal length in a short telescope.'

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