# Conceptual Questions Based on Class 12 Physics Optical Instruments

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## Conceptual Questions Based on Class 12 Physics Optical Instruments

Here we are providing conceptual questions based on class 12 physics chapter 9 Optical Instruments. These questions are prepared by experts and are very useful from exam point of view. Students can practice these questions for scoring better marks in the exam.

Q.1. How will the magnifying power of a refracting type astronomical telescope be affected on increasing for its eyepiece (i) the focal length and (ii) the aperture? Justify your answer.

Solution, (i) As m = fo/fe, so that magnifying decreases when focal length of eyepiece increases.

(ii) Magnifying power is not affected by the increase in. aperture of the eyepiece.

Q.2. What are the advantages of binocular vision?

Solution. The advantages of seeing an object with two eyes are:

1.   The distance of the object can be correctly estimated.

2.   The complete three-dimensional effect of the object can be obtained.

3.   The two eyes give relief to one another after every fraction of second.

Q.3. How is the working of a telescope different from a microscope?

Solution. Working differences between a telescope and a microscope:

(ii) A telescope produces high resolution while a microscope produces high magnification.

(ii) The objective of a telescope forms the image of a very far off object at, or within, the focus of its eyepiece. The microscope does the same for a small object kept just beyond the focus of its objective.

(iii) The final image formed by a telescope is magnified relative to its size as seen by the unaided eye while the final image formed by a microscope is magnified relative to its absolute size.

(iv) The objective of a telescope has large focal length and large aperture while the corresponding quantities for a microscope have very small values.

(v) In a telescope, the distance between the objective and eyepiece is adjusted to focus the object at infinity. In a microscope the distance between the objective and eyepiece is fixed, the distance of the objective is changed to focus the object.

Q.4. Which two of the following lenses L1, L2 and L3 will you select as objective and eyepiece for constructing best possible (i) telescope (ii) microscope? Give reason to support your answer.

Solution. (i) Telescope. L1 as objective and L3 as eyepiece.

Reason. The objective should have large aperture and large focal length while the eyepiece should have small aperture and small focal length. Then the light gathering power and magnifying power will be larger.

(ii) Microscope, L3 as objective and L2 as eyepiece.

Reason. Both the lenses of the microscope should have short focal lengths and the focal length of the objective should be smaller than that of the eyepiece. Magnifying power will be larger for short focal lengths of objective and eyepiece.

Q.5. In a telescope, the objective has a large aperture while the eyepiece has a small aperture. Why?

Solution. The objective of large aperture has a large light gathering capacity and it forms bright images of even distant faint stars. Moreover, the large aperture of the objective increases the resolving power of the telescope.

The eyepiece of small aperture is taken so that entire light forming the final image may enter the small pupil of the eye and a brighter image is seen.

Q.6. Explain the basic differences between the construction and working of a telescope and a microscope.

Solution.

Construction differences:

Working Differences:

Q.7. Which two main considerations are kept in mind while designing the ‘objective’ of an astronomical telescope?

Solution. Two main considerations with an astronomical telescope are: (i) its light gathering power and (ii) its resolving power.

The light gathering power depends on the area of the objective. With larger diameters, fainter objects can be observed. The resolving power, or the ability to observe two objects distinctly, which are very nearly in the same direction, also depends on the diameter of the objective.

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