NEETPhysicsRay Optics
Given below are two statements: one is labelled as Assertion (A) and the other is labelled as Reason (R). Assertion (A): In an astronomical telescope, the length of the telescope tube is maximum when the final image is formed at the least distance of distinct vision. Reason (R): For the final image to form at the least distance of distinct vision, the intermediate image formed by the objective must lie between the ey
Options
- ABoth Assertion (A) and Reason (R) are true and (R) is the correct explanation of (A).
- BBoth Assertion (A) and Reason (R) are true but (R) is not the correct explanation of (A).
- C(A) is true but (R) is false.
- D(A) is false but (R) is true.
Correct answer
D. (A) is false but (R) is true.
Step-by-step solution
The length of an astronomical telescope tube is given by L = f_o + u_e , where u_e is the distance of the intermediate image from the eyepiece. In normal adjustment (final image at infinity), the intermediate image is formed at the principal focus of the eyepiece, so u_e = f_e . When the final image is formed at the least distance of distinct vision ( D ), the eyepiece acts as a simple magnifier forming a virtual, erect (with respect to the intermediate image) and magnified image. For a convex lens to form a virtua