12th physics. NCERT notes

Astronomical Telescopes

It is used to observe distinct images of heavenly bodies.

It consists of 2 lenses, the objective lens O of large focal length and large aperture and the eyepiece E which has a small focal length and small aperture.

In the normal adjustment of the telescope, the final image is formed at infinity.


The objective forms the real and inverted image of the distant object at its focal plane. The distance of the eyepiece from the objective is adjusted the final image is formed at the least distance vision. In case the position of the eye is so adjusted that the final image is formed at infinity.

To increase magnifying power of an astronomical telescope in normal adjustment, the focal length of objective lens should be the large and focal length of eye lens should be small.

Note: In normal adjustment, distance between the lenses = f0 + fe.

When final image is formed at the least distance of distinct vision from the eye

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