Depending upon the principle on which magnification is based, microscopes are of two categories;
- Light or Optical microscope- It includes Bright-field, Dark-field, Fluorescence and Phase-contrast.
- Electron Microscope– It includes Transmission electron and Scanning electron microscope.
In light microscopy, magnification is obtained by system of optical lenses using light waves.
In a light microscope, visible light passes through the specimen (the biological sample) and is bent through the lens system, allowing the user to see a magnified image. An advantage of light microscopy is that it can often be performed on living cells, so it’s possible to watch cells carrying out their normal behaviors (e.g., migrating or dividing) under the microscope.
In bright field light microscope, passes through the specimen and image is formed directly. The microscopic field is brightly lighted and microorganisms appear dark because they absorb some light.
Dark field microscope works under dark-field technique in which background is dark and objects are brilliantly illuminated. It consists of special condenser that transmits hollow cone of light from source of illumination. Microscopic field is dark as light directed towards condenser doesn’t enter objective lens.
Phase contrast microscope is based on principle that light passing through one material and into other material of slightly different refractive index and thickness will undergo change in phase. It can be used to observe living unstained cells.
In Fluorescence microscopes, light passing through lens are absorbed by specimen and later it emits light which forms an image. E.g. Confocal microscopy is type of fluorescence microscope. A confocal microscope uses a laser to excite a thin layer of the sample and collects only the emitted light coming from the target layer, producing a sharp image.
Fluorescent microscope uses ultraviolet light as its light source. When ultraviolet light hits an object, it excites the electrons of the object, and they give off light in various shades of color.