Working Principle and Parts of a Compound Microscope (with Diagrams)

Read this article to learn about the working principle and parts of a compound microscope with diagrams!

Working Principle:

The most commonly used microscope for general purposes is the standard compound microscope. It magnifies the size of the object by a complex system of lens arrangement.


It has a series of two lenses; (i) the objective lens close to the object to be observed and (ii) the ocular lens or eyepiece, through which the image is viewed by eye. Light from a light source (mirror or electric lamp) passes through a thin transparent object (Figure 4.4).

Ray Diagram of a Compound Microscope

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. That is why; it is a bright-field microscope.

Parts of a Compound Microscope:

The parts of a compound microscope are of two categories as given below:

(i) Mechanical Parts:

These are the parts, which support the optical parts and help in their adjustment for focusing the object (Figures 4.5 and 4.6).


Parts of a Compound Microscope

A Compound Microscopre

The components of mechanical parts are as follows:


1. Base or Metal Stand:

The whole microscope rests on this base. Mirror, if present, is fitted to it.

2. Pillars:

It is a pair of elevations on the base, by which the body of the microscope is held to the base


3. Inclination joint:

It is a movable joint, through which the body of the microscope is held to the base by the pillars. The body can be bent at this joint into any inclined position, as desired by the observer, for easier observation. In new models, the body is permanently fixed to the base in an inclined position, thus needing no pillar or joint.

4. Curved Arm:

It is a curved structure held by the pillars. It holds the stage, body tube, fine adjustment and coarse adjustment.

5. Body Tube:

It is usually a vertical tube holding the eyepiece at the top and the revolving nosepiece with the objectives at the bottom. The length of the draw tube is called ‘mechanical tube length’ and is usually 140-180 mm (mostly 160 mm).

6. Draw Tube:

It is the upper part of the body tube, slightly narrower, into which the eyepiece is slipped during observation.

7. Coarse Adjustment:

It is a knob with rack and pinion mechanism to move the body tube up and down for focusing the object in the visible field. As rotation of the knob through a small angle moves the body tube through a long distance relative to the object, it can perform coarse adjustment. In modern microscopes, it moves the stage up and down and the body tube is fixed to the arm.

8. Fine Adjustment:

It is a relatively smaller knob. Its rotation through a large angle can move the body tube only through a small vertical distance. It is used for fine adjustment to get the final clear image. In modern microscopes, fine adjustment is done by moving the stage up and down by the fine adjustment.

9. Stage:

It is a horizontal platform projecting from the curved arm. It has a hole at the center, upon which the object to be viewed is placed on a slide. Light from the light source below the stage passes through the object into the objective.

10. Mechanical Stage (Slide Mover):

Mechanical stage consists of two knobs with rack and pinion mechanism. The slide containing the object is clipped to it and moved on the stage in two dimensions by rotating the knobs, so as to focus the required portion of the object.

11. Revolving Nosepiece:

It is a rotatable disc at the bottom of the body tube with three or four objectives screwed to it. The objectives have different magnifying powers. Based on the required magnification, the nosepiece is rotated, so that only the objective specified for the required magnification remains in line with the light path.

(ii) Optical Parts:

These parts are involved in passing the light through the object and magnifying its size.

The components of optical parts include the following:

1. Light Source:

Modern microscopes have in-built electric light source in the base. The source is connected to the mains through a regulator, which controls the brightness of the field. But in old models, a mirror is used as the light source. It is fixed to the base by a binnacle, through which it can be rotated, so as to converge light on the object. The mirror is plane on one side and concave on the other.

It should be used in the following manner:

(a) Condenser Present:

Only plane side of the mirror should be used, as the condenser converges the light rays.

(b) Condenser Absent:

(i) Daylight:

Plane or concave (plane is easier)

(ii) Small artificial light:

High power objective: Plane side

Low power objective: Concave side

2. Diaphragm:

If light coming from the light source is brilliant and all the light is allowed to pass to the object through the condenser, the object gets brilliantly illuminated and cannot be visualized properly. Therefore, an iris diaphragm is fixed below the condenser to control the amount of light entering into the condenser.

3. Condenser:

The condenser or sub-stage condenser is located between the light source and the stage. It has a series of lenses to converge on the object, light rays coming from the light source. After passing through the object, the light rays enter into the objective.

The ‘light condensing’, ‘light converging’ or ‘light gathering’ capacity of a condenser is called ‘numerical aperture of the condenser’. Similarly, the ‘light gathering’ capacity of an objective is called ‘numerical aperture of the objective’. If the condenser converges light in a wide angle, its numerical aperture is greater and vice versa.

If the condenser has such numerical aperture that it sends light through the object with an angle sufficiently large to fill the aperture back lens of the objective, the objective shows its highest numerical aperture (Figure 4.7). Most common condensers have numerical aperture 1.25.

Numerical Aperture of Condensers

If the numerical aperture of the condenser is smaller than that of the objective, the peripheral portion of the back lens of the objective is not illuminated and the image has poor visibility. On the other hand, if the numerical aperture of condenser is greater than that of the objective, the back lens may receive too much light resulting in a decrease in contrast.

There are three types of condensers as follows:

(a) Abbe condenser (Numerical aperture=1.25): It is extensively used.

(b) Variable focus condenser (Numerical aperture =1.25)

(c) Achromatic condenser (Numerical aperture =1.40): It has been corrected for both spherical and chromatic aberration and is used in research microscopes and photomicrographs.

4. Objective:

It is the most important lens in a microscope. Usually three objectives with different magnifying powers are screwed to the revolving nosepiece.

The objectives are:

(a) Low power objective (X 10):

It produces ten times magnification of the object.

(b) High dry objective (X 40):

It gives a magnification of forty times.

(c) Oil-immersion objective (X100):

It gives a magnification of hundred times, when immersion oil fills the space between the object and the objective

The scanning objective (X4) is optional. The primary magnification (X4, X10, X40 or X100) provided by each objective is engraved on its barrel. The oil-immersion objective has a ring engraved on it towards the tip of the barrel.

Resolving Power of Objective:

It is the ability of the objective to resolve each point on the minute object into widely spaced points, so that the points in the image can be seen as distinct and separate from one another, so as to get a clear un-blurred image.

It may appear that very high magnification can be obtained by using more number of high power lenses. Though possible, the highly magnified image obtained in this way is a blurred, one. That means, each point in the object cannot be found as widely spaced distinct and separate point on the image.

Mere increase in size (greater magnification) without the ability to distinguish structural details (greater resolution) is of little value. Therefore, the basic limitation in light microscopes is one not of magnification, but of resolving power, the ability to distinguish two adjacent points as distinct and separate, i.e. to resolve small components in the object into finer details on the image.

Resolving power is a function of two factors as given below:

(a) Numerical aperture (n.a.)

(b) Wavelength of the light (λ)

(a) Numerical aperture:

Numerical aperture is a numerical value concerned with the diameter of the objective lens in relation to its focal length. Thus, it is related to the size of the lower aperture of the objective, through which light enters into it. In a microscope, light is focused on the object as a narrow pencil of light, from where it enters into the objective as a diverging pencil (Figure 4.8).

Numberical Aperture of Objectives

The angle 9 subtended by the optical axis (the line joining the centers of all the lenses) and the outermost ray still covered by the objective is a measure of the aperture called ‘half aperture angle’.

A wide pencil of light passing through the object ‘resolves’ the points in the object into widely spaced points on the lens, so that the lens can produce these points as distinct and separate on the image. Here, the lens gathers more light.

On the other hand, a narrow pencil of light cannot ‘resolve’ the points in the object into widely spaced points on the lens, so that the lens produces a blurred image. Here, the lens gathers less light. Thus, the greater is the width of the pencil of light entering into the objective (29), the higher is its ‘resolving power’.

The numerical aperture of an objective is its light gathering capacity, which depends on the site of the angle 8 and the refractive index of the medium existing between the object and the objective.

Numerical aperture (n.a.) = n sin θ


n = Refractive index of the medium between the object and the objective and

θ = Half aperture angle

For air, the value of ‘n’ is 1.00. When the space between the lower tip of the objective and the slide carrying the object is air, the rays emerging through the glass slide into this air are bent or refracted, so that some portion of it do not pass into the objective. Thus, loss of some light rays reduces numerical aperture and decreases the resolving power.

However, when this space is filled with an immersion oil, which has greater refractive index (n=1.56) than that of air (n=1.00), light rays are refracted or bent more towards the objective. Thus, more light rays enter into the objective and greater resolution is obtained. In oil immersion objective, which provides the highest magnification, the size of the aperture is very small.

Therefore, it needs bending of more rays into the aperture, so that the object can be distinctly resolved. That is why, immersion oils, such as cedar wood oil and liquid paraffin are used to fill the gap between the object and the objective, while using oil-immersion objective.

(b) Wavelength of light (λ):

The smaller is the wavelength of light (λ), the greater is its ability to resolve the points on the object into distinctly visible finer details in the image. Thus, the smaller is the wavelength of light, the greater is its resolving power.

Limit of resolution of objective (d):

The limit of resolution of an objective (d) is the distance between any two closest points on the microscopic object, which can be resolved into two separate and distinct points on the enlarged image.

Points with their in-between distance less than ‘d’ or objects smaller than ‘d’ cannot be resolved into separate points on the image. If the resolving power is high, points very close to each other can be seen as clear and distinct.

Thus, the limit of resolution (the distance between the two resolvable points) is smaller. Therefore, smaller objects or finer details can be seen, when’d’ is smaller. Smaller ‘d’ is obtained by increasing the resolving power, which in turn is obtained by using shorter wavelength of light (λ) and greater numerical aperture.

Limit of resolution = d = λ/2 n.a.


λ = Wave length of light and

n.a. = Numerical aperture of the objective.

If λ green = 0.55 p and n.a. = 1.30, then d = λ/2 n.a. = 0.55/2 X 1.30 = 0.21 µ. Therefore, the smallest details that can be seen by a typical light microscope is having the dimension of approximately 0.2 µ. Smaller objects or finer details than this cannot be resolved in a compound microscope.

5. Eyepiece:

The eyepiece is a drum, which fits loosely into the draw tube. It magnifies the magnified real image formed by the objective to a still greatly magnified virtual image to be seen by the eye (Figure 4.9).

Eyepiece (Ocular)

Usually, each microscope is provided with two types of eyepieces with different magnifying powers (X10 and X25). Depending upon the required magnification, one of the two eyepieces is inserted into the draw tube before viewing. Three varieties of eyepieces are usually available.

They are the Huygenian, the hyper plane and the compensating. Among them, the Huygenian is very widely used and efficient for low magnification. In this eyepiece, two simple Plano-convex lenses are fixed, one above and the other below the image plane of the real image formed by the objective.

The convex surfaces of both the lenses face downward. The lens towards the objective is called ‘field lens’ and that towards eye, ‘eye lens’. The rays after passing through the eye lens come out through a small circular area known as Rams-den disc or eye point, where the image is viewed by the eye.

Total magnification:

The total magnification obtained in a compound microscope is the product of objective magnification and ocular magnification.

Mt = Mob X Moc


Mt = Total magnification,

Mob = Objective magnification and

Moc = Ocular magnification

If the magnification obtained by the objective (Mob) is 100 and that by the ocular (Moc) is 10, then total magnification (Mt) = Mob X Moc =100 X 10 =1000. Thus, an object of lq will appear as 1000 µ.

Useful magnification:

It is the magnification that makes visible the smallest resolvable particle. The useful magnification in a light microscope is between X1000 and X2000. Any magnification beyond X2000 makes the image blurred.

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