When a telescope is in normal use, the final image is at

Analyze the question: It asks where the final image is formed when an astronomical telescope is in normal adjustment (normal use).

Explain the concept: An astronomical telescope has two lenses:

• Objective lens: long focal length. It collects light from a distant object and forms a real image at its focal point.
• Eyepiece lens: short focal length. It acts as a magnifier for the image formed by the objective.

In normal adjustment, the two lenses are separated by a distance equal to the sum of their focal lengths:

L = f_o + f_e

Step-by-step solution:

1. Light from a distant object (effectively at infinity) enters as parallel rays.
2. The objective focuses these rays to form a real image at its focal point, F_o.
3. In normal adjustment, this image lies exactly at the focal point of the eyepiece, F_e.
4. An object placed at the focus of a convex lens produces emerging parallel rays, so the final image is formed at infinity.
5. Viewing at infinity is the most relaxed way to use the eye, since the eye's ciliary muscles are not strained. This is why it is called "normal" use.

The magnification in this setting is:

M = f_o / f_e

Why not the others?

• B (The focus): the intermediate image is at the focus, not the final one.
• C (Radius of curvature): this is not relevant to the final image of a telescope.
• D (The near point): this applies to a microscope when it is adjusted for the final image at the least distance of distinct vision (25 cm), not to normal use of a telescope.

Answer: The final image is at infinity, which is option A.

Exam tip: Remember "Normal adjustment = final image at infinity = lens separation f_o + f_e." If you see "normal adjustment" for a telescope, think infinity.