Question

Explain the working of an astronomical telescope using refraction of light.

Answer 1

Construction of a refracting telescope:

The astronomical telescope consists of two lenses: the objective and the eyepiece. The objective has a larger focal length and diameter to accommodate the maximum amount of light coming from distant (astronomical) objects. A parallel beam of rays from an astronomical object is made to fall on the objective lens of the telescope. It forms a real, inverted and diminished image A'B' of the object. The eyepiece is so adjusted that A'B' lies just at the focus of the eyepiece. Therefore, a highly magnified image of the object is formed at infinity. The same has been shown in the figure below.

Working: When the objective lens is directed towards a distant object, the light rays coming from it are nearly parallel and pass through the objective lens. Since the objective lens has a large aperture, it gathers a maximum amount of light. It produces a real, inverted, and diminished image at the focal plane of the objective lens. The eyepiece is then adjusted so that this image lies just within its focal length and acts as the object for the eyepiece, which functions like a simple microscope. The final image formed is highly magnified, virtual, located on the same side as the object, and inverted with respect to the original object. This image can be viewed by placing the eye close to the eyepiece. If the image formed by the objective lens is exactly at the focal plane of the eyepiece, the final image is formed at infinity.