question_answer 34)

A cassegrain telescope uses two mirrors asShown in figure. Such a telescope is built withmirrors 20 mm apart. If the radius of curvatureof large mirror is 220 mm and the small mirroris 140 mm, where will be the final image of anobject at infinity be?  

Answer:

Object at infinity will form an image at 110 mm. This image will act as virtual object for thesmaller mirror. Distance of virtual object for small mirror, u=110-20 =90 mmUsing mirror formula, we get Distance of final image is 315 mm fromthe small mirror on the right side.