Question

Describe a Terrestrial telescope. Calculate its magnifying power.

Answer 1

Terrestrial telescope

It is used to see erect image of distant earthly

objects. For obtaining erect image an extra convex lens is placed between objective and eye piece.
Principle. Same as in astronomical telescope.

Construction: It consists of an objective and an eye piece Objective is of large aperture and focal length and eye piece is of small focal length and aperture. An erecting lens placed at a distance 2f from the image to form erect image. The distance between the objective and eye piece can be adjusted by using a rack and pinion arrangement.

The course of rays is as shown in Fig. The objective produces real and inverted image A₁B₁ on its focal plane of distant object. In order to form erect image A₁₁B₁₁, a convex lens called erecting lens is placed at a distance. 2f from image A₁B₁ and a real image A₁₁B₁₁ is formed at 2f on other side of the lens. The eye piece is placed at such a distance so that the image is either formed at infinity (for normal adjustment) or at least distance of distinct vision.

For normal adjustment

Total length of the telescope = f₀ + 4f + f_e

And magnification (similar as in case of astronomical telescope)

M = \(\frac{f_0}{f_e}\)

When image is formed at least distance of distinct vision.
For normal adjustment, eye lens is placed at a distance exactly equal to its focal length from the image PQ which forms the virtual object for the concave lens. So the diverged rays become parallel to each other and the final image is formed at infinity. In this position, the length of telescope is f₀ - f_e, where f₀ is focal length of objective and fe is focal length of eye lens.

Magnifying Power Magnifying Power,

M = \(\frac{\text{Angle subtended by the image }}{\text{Angle subtended by the object }}\)

Magnification, M = \(\frac{f_0}{f_e}[1 + \frac{f_e}{D}]\)