# (a) Give two reasons to explain why reflecting telescopes are preferred over refracting type. (b) Use mirror equation to show that convex mirror always produces a virtual image independent of the location of the object.

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(i) There is no chromatic aberration in case of reflecting telescopes as the objective is a mirror.

(ii) Spherical aberration is reduced in case of reflecting telescopes by using mirror objective in the form of a paraboloid.

(b) For convex mirror, focal length is always positive, f = +ve An object is placed on the left side of the mirror. So, the object distance, u = −ve or u < 0. Using the mirror formula we have, 1f=1v+1u⇒1v=1f-1u Since f > 0 and u < 0, then from the above equations, we get that 1v>0⇒v<0 Hence, a virtual image is always formed at the back side of the mirror. Therefore, the image formed by the convex mirror is always virtual in nature, independent of the location of the object.