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FSc ICS Notes Physics XI Chapter 10 Optical Instruments Exercise Short Questions

FSc ICS Notes Physics XI Chapter 10 Optical Instruments Exercise Short Questions

FSc ICS Notes Physics XI Chapter 10 Optical Instruments Exercise Short Questions

Question 10.1 What do you understand by linear magnification and angular magnification? Explain how a convex lens is used as a magnifier?
Answer 10.1 Linear Magnification: It is defined as the ratio of the size of image to the size of object.
                                             M = I / O = q / p
Angular Magnification: It is the ratio of the angle subtended by the image as seen through the optical instrument to the angle subtended by the object i.e.
                                             M = β / α
A Convex lens as a Magnifier: A convex lens of short focal length can be used as magnifying glass because when an object is placed close to the lens then the image formed is erect, virtual and very much magnified.

Question 10.2 Explain the difference between angular magnification and resolving power of an optical instrument. What limits the magnification of an optical instrument?
Answer 10.2 Angular magnification means how large or magnified image is formed by the instrument but resolving power is its ability to provide the details of an object under examination. The magnification of an optical instrument is limited due to defects in the lenses. Such as chromatic and spherical aberrations.

Question 10.3 Why would it be advantageous to use blue light with a compound microscope?
Answer 10.3 In case of grating spectrometer, the resolving power 'R' of the grating is defined as:
                           R = λ / (λ2 - λ1)
Thus we see that a grating with high resolving power can distinguish small distance in wavelength. If blue light is used in compound microscope, it increase the resolving power and more details on an object can be studied.
Question 10.4 One can buy a cheap microscope for use by the children. The image seen in such a
microscope have colored edges. Why is this so?
Answer 10.4 It is due to the defects of lenses known as chromatic aberrations. This is because of the prism like formation of the lens which cause dispersion of white light.

Question 10.5 Describe with the help of diagrams, how (a) a single biconvex lens can be used as a
magnifying glass. (b) biconvex lenses can be arranged to form a microscope.
Answer 10.5 (a)  Ray diagram of biconvex lens used as magnifying glass.

           (b)   Ray diagram of two biconvex lens arranged to make microscope.

Question 10.6 If a person were looking through a telescope at the full moon, how would the appearance of the moon be changed by covering half of the objective lens.
Answer 10.6  The person will see the full image of the moon if half of the objective lens of a telescope is covered but its brightness is reduced because less light is transmitted through the lens.

Question 10.7 A magnifying glass gives a five times enlarged image at a distance of 25 cm from the
lens. Find, by ray diagram, the focal length of the lens.
Answer 10.7  Magnification = m = 5       Image distance = 25 cm         Focal length of lens = ?
                            M = 1 + d/f
                             f = d / M – 1
                             f = 25/5-1
                             f = 6.2 cm

Question 10.8 Identify the correct answer.
1) The resolving power of a compound microscope depends on:
  1. The refractive index of the medium in which the object is placed.
  2. The diameter of the objective lens.
  3. The angle subtended by the objective lens at the object.
  4. The position of an observer’s eye with regard to the eye lens.
2) The resolving power of an astronomical telescope depends on:
  1. The focal length of the objective lens.
  2. The least distance of distinct vision of the observer.
  3. The focal length of the eye lens.
  4.  The diameter of the objective lens.
Answer 10.8  (1) The formula for resolving power of a lens of diameter D is given by:
                      R = D / 1.22 λ
 Hence the resolving power of a lens depends upon the diameter of the objective so (b) is correct.
(2) As we know that the resolving power of an astronomical telescope depends upon the diameter of the objective lens so (d) is correct.

Question 10.9 Draw sketches showing the different light paths through a single-mode and multi-mode fibre. Why is the single-mode fibre preferred in telecommunications?
Answer 10.9  It has a very thin core of about 5 * 10(-6 )m and has a relatively larger cladding. It can carry more than 14 TV channels or 14000 phone calls so it is preferred in telecommunication.

Question 10.10 How the light signal is transmitted through the optical fibre?
A10.10  The light signals are transmitted through the optical fibre on the principle of
  1. Total Internal Reflection.
  2. Continuous Refraction.

Question 10.11 How the power is lost in optical fibre through dispersion? Explain.
Answer 10.11 If the source of light signals is not monochromatic then, the light will disperse while propagating through the core of the optical fibre into different wavelength so the light of different wavelengths reaches the other end of the fibre at different times and the signal received is distorted. So the power is lost in optical fibre through dispersion.

Written By: Asad Hussain.


  1. Very helpful ☺ thanks a lot

  2. too good thanks a lot
    can you write a blog for the rearrangement of all formulas for lenses?


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