Optical microscope
An optical microscope is a microscope based on optical lenses. The scientist most closely associated with its development is Anton van Leeuwenhoek.
Simple microscope
Leeuwenhoek's microscopes consisted of a single, small, convex lens mounted on a plate with a mechanism to hold the material to be examined (the sample or specimen). This use of a single convex lens is called the simple microscope, which includes the magnifying glass, the hand lens, and the jeweller's loup.
Compound microscope
The diagram below shows a compound microscope. In its simplest form as used by Robert Hooke it had a single glass lens of short focal length for the objective, and another single glass lens as the eyepiece or ocular.
Microscopes of this kind are usually more complex with multiple lens components in both objective and eyepiece. These multi-component lenses are designed to reduce aberrations, particularly chromatic aberration and spherical aberration. In modern microscopes the mirror is replaced by a lamp unit providing stable, controllable illumination.
Compound microscopes are used to study thin specimens as they have a very limited depth of field. Typically they would be used to examine a smear, a squash preparation, or a thinly sectioned slice of some material. Commonly they depend on light passing through the sample from below, and special techniques are usually necessary to increase the contrast in the image to useful levels (see Contrast methods below).
To study the thin structure of metals (see metallography) and minerals, another type of microscope is used, where the light is reflected from the examined surface. The light is fed through the same objective using a semi-transparent mirror.
Due to diffraction, even the best optical microscope is limited to a resolution of 0.2 micrometers.
Binocular microscope
This instrument is designed differently from the diagram above, and serves a slightly different purpose. It uses two eyepieces (or sometimes two complete microscopes) to provide slightly different viewing angles to the left and right eyes. In this way it produces a three-dimensional (3-D) visualisation of the sample being examined.
The binocular microscope is often used to study the surfaces of solid specimens.
Specialist designs
Other types of optical microscope include
- The inverted microscope for studying samples from below
- The student microscope designed for low cost, durability, and ease of use
- The research microscope which is usually an expensive tool with many specialist enhancements.
Contrast methods
To see any detail at all in a thin specimen it is usually essential to increase the contrast. Methods often used include
- Staining the specimen with a dye
- Phase contrast microscopy
- Interference contrast microscopy
- Differential interference contrast microscopy
- Fluorescence microscopy
- Dark field microscopy
Sometimes a combination of several of these are used at the same time as the various methods may bring out different features in the sample.
