Optical Instrument (NEET LEVEL)

Q.1

A compound microscope consists of an objective of focal length f_o and an eyepiece with magnification 25. The microscope is designed so that the object is focused in a plane 16 cm away from the focal point of the objective lens. When properly adjusted, the eyepiece and the objective are 17.4 cm apart. What is f_o? (Assume that the eyepiece magnification is based on an image at infinity and a near point at 25 cm).

(a)    0.4 cm         (b)   0.2 cm        (c) 0.3 cm           (d) 0.4 cm           

Ans.  (a)

Q.2

The objective and the eyepiece of a refracting telescope have focal lengths of 320 cm and 4 cm respectively. The telescope is used to view Neptune and the final image is set at infinity. The diameter of Neptune is 4.96 × 10⁷ m and the distance from Earth at the time of observation is 4.4 × 10¹² m. Find the angle subtended by the final telescopic image of Neptune.

(a)    0.8 m rad        (b)   0.9 m rad        (c) 0.7 m rad          (d) 0.6 m rad

Ans.  (b)

Q.3

An astronomial telescope has an angular magnification of magnitude 5 for distant objects. The separation between the objective and eyepiece is 36 cm and the final image is formed at infinity. Determine the focal length of objective and eyepiece.

(a)    f_o = 20  and f_e = 5 cm                                                      

(b)    f_o = 10  and f_e = 8 cm                

(c)    f_o = 30  and f_e = 6 cm                                                      

(d)   f_o = 10  and f_e = 5 cm

Ans.  (c)

Q.4

The magnifying power of a telescope is found to be 9 and the separation between the lenses is 20 cm for relaxed eye. What are the focal lengths of the component lenses?

(a)      f_o = 16 cm and f_e = 1 cm                            

(b)     f_o = 17 cm and f_e = 3 cm

(c)     f_o = 16 cm and f_e = 4 cm                            

(d)    f_o = 18 cm and f_e = 2 cm

Ans.  (d)

Q.5

The total magnification produced by a compound microscope is 20. The magnification produced by the eyepiece is 5.  The microscope is focused on a certain object. The distance between the objective and eyepiece is observed to be 14 cm.  If least distance of distinct vision is 20 cm. Calculate focal length of the objective and the eyepiece.

(a)    \displaystyle {{f}_{e}}=4cm\,\,and\,\,\,{{f}_{o}}=\,3\,cm                                                   

(b)    \displaystyle {{f}_{e}}=5cm\,\,and\,\,\,{{f}_{o}}=\,2\,cm

(c)    \displaystyle {{f}_{e}}=6cm\,\,and\,\,\,{{f}_{o}}=\,1\,cm                                                         

(d)    \displaystyle {{f}_{e}}=8cm\,\,and\,\,\,{{f}_{o}}=\,4\,cm

Ans.  (b)

Q.6

A convex lens of focal length 25 cm is placed coaxially in contact with a concave lens of focal length 20 cm. Determine the power of the combination.  Will the system be converging or diverging in nature?

(a)    –1 D ; Divergent                                                          

(b)    1 D ; Convergent                                     

(c)    –0.5 D ; Divergent                                                       

(d)    0.5 D ; Convergent

Ans.  (a)

Q.7

A compound microscope uses an objective lens of focal length 4 cm and eyepiece lens of focal length 10 cm. An object is placed at 6 cm from the objective lens.  Calculate the magnifying power of the compound microscope. Also, calculate the length of the microscope.

(a) \displaystyle M=-6,\,\,Length\,\,=\,\,18.14\,cm                                            

(b) \displaystyle M=-7,\,\,Length\,\,=\,\,17.14\,cm

(c) \displaystyle M=-5,\,\,Length\,\,=\,\,16.14\,cm                                            

(d) \displaystyle M=-7,\,\,Length\,\,=\,\,19.14\,cm

Ans.  (d)

Q.8

Two convex lenses of focal length 20 cm and 1 cm constitute a telescope. The telescope is focused on a point which is 1m away from the objective.  Calculate the magnification produced and the length of the tube if the final image is formed at a distance 25 cm from the eyepiece.

(a) \displaystyle M\,=-5.5 and \displaystyle L\,=26.96\,\,cm                                            

(b) \displaystyle M\,=-8.5 and \displaystyle L\,=22.96\,\,cm            

(c) \displaystyle M\,=-6.5 and \displaystyle L\,=25.96\,\,cm                                            

(d) \displaystyle M\,=-4.5and \displaystyle L\,=23.96\,\,cm

Ans.  (c)

Q.9

The objective of an astronomical telescope has a diameter of 150 mm and a focal length of 4.00 m. The eyepiece has a focal length of 25.00 mm.  Calculate the resolving power of telescope.
(X = 6000A for yellow colour)

(a)    \displaystyle 0.34\,\times \,{{10}^{7}}                           

(b)    \displaystyle 0.34\,\times \,{{10}^{6}}                           

(c)    \displaystyle 0.34\,\times \,{{10}^{4}}               

(d)    \displaystyle 0.24\,\times \,{{10}^{6}}

Ans.  (b)

Q.10

An astronomical telescope uses two lenses of powers 10 D and 1 D. What is its magnifying power in normal adjustment?

(a)    –10          (b)   10           (c) –9            (d)–11

Ans.  (a)