Optical configurations of microscope

There are two basic configurations of the conventional optical microscope: the simple (single lens) and the compound (many lenses). The vast majority of modern research microscopes are compound microscopes while some cheaper commercial digital microscopes are simple single lens microscopes. A magnifying glass is, in essence, a basic single lens microscope. In general, microscope optics are static; to focus at different focal depths the lens to sample distance is adjusted, and to get a wider or narrower field of view a different magnification objective lens must be used. Most modern research microscopes also have a separate set of optics for illuminating the sample.

A simple microscope is a microscope that uses only one lens for magnification, and is the original design of light microscope. Van Leeuwenhoek's microscopes consisted of a small, single converging lens mounted on a brass plate, with a screw mechanism to hold the sample or specimen to be examined. Demonstrations by British microscopist have images from such basic instruments. Though now considered primitive, the use of a single, convex lens for viewing is still found in simple magnification devices, such as the magnifying glass and the loupe.

A compound microscope is a microscope which uses multiple lenses to collect light from the sample and then a separate set of lenses to focus the light into the eye or camera. Compound microscopes are heavier, larger and more expensive than simple microscopes due to the increased number of lenses used in construction. The main advantages of multiple lenses are improved numerical aperture (see resolution limit below), reduced chromatic aberration and exchangeable objective lenses to adjust the magnification. A compound microscope also makes more advanced illumination setups, such as phase contrast.

Components of microscope

Figure 1. Basic optical transmission microscope elements

All modern optical microscopes (figure 1) designed for viewing samples by transmitted light share the same basic components of the light path, listed here in the order the light travels through them: In addition the vast majority of microscopes have the same 'structural' components:

Ocular lens (eyepiece) (1)

Objective turret or Revolver or Revolving nose piece (to hold multiple objective lenses) (2)

Objective (3)

Focus wheel to move the stage (4 – coarse adjustment, 5 – fine adjustment)

Frame (6)

Light source, a light or a mirror (7)

Diaphragm and condenser lens (8)

Stage (to hold the sample) (9)

The eyepiece, or ocular, is a cylinder containing two or more lenses; its function is to bring the image into focus for the eye.

Objective turret or Revolver is the part that holds the set of objective lenses. It allows the user to switch between objectives.

Some microscopes make use of oil-immersion objectives or water-immersion objectives for greater resolution at high magnification. These are used with index-matching material such as immersion oil or water and a matched cover slip between the objective lens and the sample.

Adjustment wheels move the stage up and down with separate adjustment for coarse and fine focussing.

The whole of the optical assembly is traditionally attached to a rigid arm, which in turn is attached to a robust U-shaped foot to provide the necessary rigidity. The arm angle may be adjustable to allow the viewing angle to be adjusted.

Many sources of light can be used. At its simplest, daylight is directed via a mirror. Most microscopes, however, have their own adjustable and controllable light source – often a halogen lamp, although illumination using LEDs and lasers are becoming a more common provision.

The stage is a platform below the objective which supports the specimen being viewed. In the center of the stage is a hole through which light passes to illuminate the specimen.

Materials and equipment:

Ready preparations of microorganisms, microscope, immersion oil.

Progress of work:

Examine under immersion objective a preparation of the fixed and painted bacteria in such sequence:

1. Put on a table a microscope and find the best illumination;

2. Place on a sample stage the fixed preparation and fix its by clips;

3. Examine a preparation at dry objectives and find the best place for detailed research;

4. Lift a microscope tube, put immersion lens and put a drop of immersion oils on a preparation;

5. Ship in immersion oil in the bottom frontal lens of immersion objective, looking at a preparation sideways;

6. Looking in an eyepiece, slowly lift a tube of a microscope before occurrence in sight of studied object;

7. Specify focus by the microscopic screw;

8. Study and sketch a preparation;

9. Lift a tube and remove immersion oil.

Control questions:

1. Describe methods of microscopic research of microorganisms:

2. Optical configurations of microscope.

3. Lighting techniques.

4. Composition of microscope.