An astronomical telescope is said to be in normal adjustment when the?

Analyze the question: It asks for the condition that defines "normal adjustment" for an astronomical telescope.

Concept: An astronomical telescope has two converging lenses:

• the objective lens (long focal length, f₀)
• the eyepiece (short focal length, fₑ)

In normal adjustment, the telescope is set so that the final image is formed at infinity. This lets the eye view the image in a relaxed state, with no strain from accommodation.

Step-by-step reasoning:

1. Light from a distant object (effectively at infinity) enters the objective. The objective forms a real image at its focal point F₀.
2. For the final image to be at infinity, the eyepiece must receive light from an object at its own focal point Fₑ.
3. So the objective's image must fall at the eyepiece's focal point. The two focal points coincide.
4. The distance between the lenses is then: L = f₀ + fₑ
5. The angular magnification is: M = f₀ / fₑ

Checking the options:

• A. Eye is accommodated: Wrong. In normal adjustment the eye is relaxed, not accommodated.
• B. Focal length of objective is longer than the eyepiece: This is true of a telescope in general, but it is a design feature and does not define normal adjustment.
• C. Final image at the near point: Wrong. That is a different setting, which gives the maximum angular magnification but causes eye strain.
• D. Final image is at infinity: Correct. This is the definition of normal adjustment.

Exam tip: Remember "Normal = Naturally relaxed = image at infinity." For a telescope in normal adjustment, the lens separation is f₀ + fₑ. For a microscope or telescope with the image at the near point, the eye is strained.