Q. 19

Read the follo

Answer :

(i)-(b) Convex


The lenses and must be convex. The focal length of the lens is large with a large aperture in order to accommodate maximum light rays coming from the outer-space with a minimum drop in its intensity.


The lens, , is of smaller focal length. It produces an enlarged but sharp virtual image of the distant object i.e. m (magnification is inversely correlated to the focal length; in other words, smaller the focal length, greater will be the size of the image thus obtained)


(ii)-(b) 1:4


Power of a spherical lens is given as



The power of each lens is;


and


It is given, that the ratio of the power of and is 4:1. The focal length of the lenses can be written as



Hence f1/f2=1/4


(iii)-(a) Ratio of the height of the image to the height of the object


The linear magnification of a spherical lens is given as



(iv)-(c) 8 cm


The lateral magnification of a spherical lens is given as



Hence, u = 8 cm


(v)-(c) Not-so-thick lenses would not make the telescope very heavy and they will also allow a considerable amount of light to pass through them.


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