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### Compound Microscope

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Compound microscope: It is an optical device used to see highly magnified image of tiny objects.

Construction: It consist of two convex lenses of short focal length arranged co-axial at the free end of tube, at a suitable fixed distance.

Objective: It is a convex lens of very short focal length and small aperture, kept near the object.

Eye piece: It is also a convex lens of comparatively larger focal length and larger aperture kept near the eye.

Working:

Case I

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

The object AB is placed at slightly larger than the focal length of objective lens.

The objective lens forms a real, inverted and magnified image A’B’ of the object AB on the other side of lens.

The distance between the objective and eyepiece is so adjusted that the image A’B’ lie within the focus of eyepiece.

This image A’B’ acts as an object for eyepiece.

The eyepiece E form a virtual and magnified final image A”B” which is inverted to object.

Magnifying power: The ratio of the angle subtended by the eye by the final virtual image, to the angle subtended at the eye by the object

Magnifying power

Where angle subtended by the eye by final image

angle subtended by the eye by object.

h’ =height of image, h = height of object

The eye piece acts as a simple microscope.

where D = distance between eye piece to final image

and = focal length of eye piece.

As Length of microscope tube.

Where distance between objective lens to image A’B’

distance between objective lens to object AB

focal length of objective lens.

Case II: When the final image is formed at infinity

When A’B’ lie on focus of eye piece.

so final image A”B” is formed at infinity.

Magnifying power =

Note: Magnifying power of the compound microscope is large when both and are small.