Question

In an external focusing telescope, the focusing of the objective is achieved by:

• A. movement of a negative concave lens
• B. movement of objective
• C. movement of cross-staff
• D. double convex lens

Hint:

External focusing = move objective lens for clarity.

Answer 1

The Correct Option is B

In an external focusing telescope, focusing is critical for achieving a clear image. To adjust the focus of the telescope, the optical path must be altered such that the light rays from the object converge correctly at the focal point of the eyepiece. This is achieved by moving the objective lens. Here's why moving the objective lens is the correct approach:

1. The objective lens is the primary lens that collects light from the distant object and creates an image at its focal point.
2. Adjusting the position of the objective lens alters the distance between the objective and the eyepiece. This distance is crucial for focusing the image properly.
3. By moving the objective, you ensure that the converging light rays form a sharp image that aligns with the focal plane of the eyepiece, making it clear to view.
4. This technique is widely used because it allows for precise control over the focusing process, accommodating different object distances and ensuring optimal image quality.

Therefore, the correct method for focusing the objective in an external focusing telescope is through the movement of the objective lens.