Explain the Bright Field Microscopes?

Figure illustrates the compound bright field microscope. This is the most commonly used microscope in biology and microbiology courses. It is called so because it forms a dark image against a brighter background. It contains 2 lens systems for magnification: ocular lens in eyepiece and the objective lens located in nosepiece.

The specimen is illuminated by a light focused on it by a sub-stage lens called a condenser. With this microscope, specimens are made visible because of the differences in contrast that exist between them and the surrounding medium. Contrast differences arise because cells absorb or scatter light in varying degrees. Live cells, however, are difficult to observe through this microscope due to absence of contrast between specimen and the surrounding medium. It is therefore, used to observe nonviable and stained preparations where contrast is increased and variations in colour between cell structure is evident.