Page 25 - 10-phy-12 Geometrical Optics

Experiment Set up a Telescope.

12.6

(b)

Date ________________

Background A telescope is a device
Information used to see the objects which are
located at very large distances
and cannot be seen clearly with
naked eye. Atelescope consists of
two lenses.

(1) Objective:

It is the convex lens

directed towards the distant

object. It has large focal length 'fo'

DO YOU KNOW? and a large aperture. It produces
an inverted, diminished and real image of the object.

Galileo in 1609 constructed (2) Eyepiece: Fig. 12.6b.1
a telescope which provides

an erect image of an object The eyepiece of the telescope is also a convex lens close to the eye. It has

with the help of only two short focal length 'fe'. It magnifies the diminished image formed by the objective.
lenses. The eyepiece is a
concave lens. Galilean's Construction

telescope is used to see the The objective is mounted on one end of a wide tube while the eyepiece is

object on earth. mounted on one end of a narrow tube which can slide into the wide tube. Thus the

distance between the eyepiece and the objective can be changed.

Length of telescope

Distance between objective lens and eyepiece is called length of
telescope. Length of telescope is L=fo +fe

Rays coming fO fe
from far object

A I¢ I

Objective lens B
I1 (L1)
Eye piece
(L2)

B¢ Fig. 12.6b.2

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