Telescope

Description:

Telescope is an optical instrument used to view distant objects using lenses. It is made-up of two convex lens named as objective lens (lens which is towards the object) and eye-piece (lens which is nearer to eye).

As telescope is used to view objects that are placed at a very large distance, the incident rays from the object are parallel to each other. These parallel rays after refraction through the lens converge to a single point on the focal plane.

The purpose of the objective lens is to form an image of the object, once the image is formed, the eye-piece is used to magnify the image. The eye-piece works as a simple microscope.

Case 1: Setting for relaxed eyes, for parallel rays

Magnification can be also given as −

Magnification = Angle subtended by Image/Angle subtended by Object

∴ m = β/γ

For small angle, we assume that −

m = tan β/tan γ

m = A′B"/B′C₂/A′B"/B′C₁

m = B′C₁/B′C₂

∴ m = f₀/-f_e

Where,

f₀ = Focal length of objective lens.

f_e = Focal length of eye-piece.

As the magnification increases the intensity/brightness of the image will decrease. To reduce this effect we need to increase the aperture of the lens i.e. objective lens of the telescope should have aperture as large as possible.