Bright-Field Microscopy

Bright-field microscopes are the most common type of microscope. They use glass lenses to help focus and magnify a specimen.

Basic setup
Ocular: The eyepiece lens that one looks into. Can have a magnification of 5x, 10x, 15x, or 20x. The most commonly used one, and the one that we use in lab is the 10x ocular.

Objectives: The lenses closest to the sample. They revolve around on a turret, and are built to be parfocal, which means that the sample should stay in focus when we switch from one objective to the other. Available magnifications are 4x (scanning), 10x (low power), 40x (high power), and 100x (oil immersion).

Specimen

Condenser: Glass lens just below the specimen

Light source: Visible light, which consists of electromagnetic radiation of wavelengths ~400-800nm

Appearance of field and object
When no specimen is present, the appearance of the field is a white circle. The specimen, in order to be visible, must be of a darker color, because it provides a greater contrast. While the specimen is usually colorless, it is possible to stain a specimen to make it more visible under the light microscope.

The magnification multiplier is determined by the magnification of the ocular multiplied by the magnification of the objective.

Uses
In general laboratories, used to see organisms that are no smaller than 200nm. These include:

Relatively large, eukaryotic cells: yeasts, molds, fungi, algae, protozoa, helminths, etc.

Prokaryotes and most bacteria except for mycoplasma, the smallest bacteria

Viruses cannot be seen