Answer :

Given:


Distance between the objective mirror and the secondary mirror, d = 20 mm


Radius of curvature of the objective mirror, R1 = 220 mm


Therefore, focal length of the objective mirror, f1 = 0.5 × R1 = 110 mm


Radius of curvature of the secondary mirror, R2 = 140 mm


Therefore, focal length of the secondary mirror, f2 = 0.5 × R2 = 70 mm


The image of an object placed at infinity, formed by the objective mirror, acts as a virtual object for the secondary mirror.


Let the virtual object distance for the secondary mirror be u,


u = f1 – d …(1)


u = 110 – 20


u = 90 mm


By applying the mirror formula for the secondary mirror, we can calculate image distance (v) as:


…(2)


By putting the values in equation (2), we get,



v = 315 mm


Hence, the final image will be formed 315 mm away from the secondary mirror.


NOTE: Cassegrainian telescopes are one of the most widely used telescope of current times. It is a combination of a parabolic primary mirror and secondary hyperbolic mirror.


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