Question

An astronomical telescope in normal adjustment receives light from a distant source S the tube length is now decreased slightly, then

• A. no image will be formed
• B. a virtual image of S will be formed at a finite distance
• C. a large, real image of S will be formed behind the eye-piece, far away from it
• D. a small, real image of S will be formed behind the eye-piece close to it

Answer 1

Answer: (B)

In an astronomical telescope, $L=f_0+f_e$ When tube length is decreased, the image formed by the objective lens will lie between principal focus and optical centre of eye lens instead of lying at the focus of eye lens. Therefore, a virtual image will be formed at a finite distance from the eye lens.