Compound Microscope

definition

The compound microscope consists essentially of two or more double convex lenses, fixed in the two extremities of a hollow cylinder.
The lower lens (nearest to the object) is called as the objective; the upper lens (nearest to the eye of the observer), the eyepiece.
The parts of a microscope work together in hospitals and in forensic labs, for scientists and students, bacteriologists and biologists so that they may view bacteria, plant and animal cells and tissues and various microorganisms the world over.

example

Compound microscope

Compound microscope is an optical device used to obtain very large values of magnification. It is used to see microscopic objects like microorganisms. It comprises of two convex lenses and magnification occurs in both of them. Its basic components are:

1. Objective: Convex lens placed near the object. Object is placed just beyond the focal point. Image formed in real, inverted and magnified.
2. Fine adjustment screw: This is used to adjust the position of objective lens. A small movement in this causes a lot of change in magnification.
3. Eyepiece: Convex lens placed near the eye. It forms a virtual object of the real image formed by the objective lens. Hence, its position is designed such that image from the objective falls between its focus and centre.
4. Rough adjustment screw: This is used to form a clear image and focus at the correct part of the object to be seen.
Working Principle:
Light from a light source (mirror or electric lamp) passes through a thin transparent object. The objective lens produces a magnified real image first image) of the object. This image is again magnified by the ocular lens (eyepiece) to obtain a magnified virtual image (final image), which can be seen by eye through the eyepiece. As light passes directly from the source to the eye through the two lenses, the field of vision is brightly illuminated.

example

Angular magnification produced by a compound microscope

Example: The separation

L

between the objective (

f_{o} = 0.5 c m

) and the eyepiece (

f_{o} = 5 c m

) of a compound microscope is

7 c m

. Where should a small object be placed so that the eye is least strained to see the image and what is the angular magnification produced?

Solution:
The eye is least strained if the final image is formed at infinitely. In such a case the image formed by objective should fall at the focus of eyepiece.
As

f_{o} = 5 c m, L = 7 c m

, this first image is formed at a distance =

7 - 5 = 2 c m

from objective.
Thus

v = + 2 c m

. Also

f_{o} = 0.5 c m

\frac{1}{v} - \frac{1}{u} = \frac{1}{f _{o}}

or,

\frac{1}{u} = \frac{1}{v} - \frac{1}{f _{o}} = \frac{1}{2} - \frac{1}{0 . 5}

or,

u = - \frac{2}{3} c m

The object should be placed at

\frac{2}{3} c m

from objective.
Angular magnification =

\frac{v}{u} \times \frac{D}{f _{e}}

= \frac{2}{- \frac{2}{3}} \times \frac{2 5}{5}

= - 15

Compound Microscope

definition