Light microscope

A light microscope illuminates the sample, or specimen, by focusing light through a lens onto the microscope slide. The light passes through the specimen and into the objective lenses. Typically, a light microscope contains several objective lenses for different magnifications. The objectives are fixed on a revolving nosepiece that sits above the sample on the stage, this nosepiece can be turned to move a different objective into the light path. Each objective is marked with a colored ring indicating the magnification. The size of the objective is also indicative of the magnification; the 5x objective is much shorter than the 100x objective. The total magnification of the image results from multiplying the objective magnification by the eyepiece magnification, which normally is 10x. Thus, if you observe a slide using the 5x objective, you are magnifying the sample 50 times.

Figure 1: Parts of the light microscope. The specimen is placed in the specimen holder on the stage. Light from the light source passes through the sample and reaches the objective, which magnifies the sample. The light then reaches the eyepieces and/or the camera, which can both be used to observe the magnified image.

To examine a specimen, the revolving nosepiece is turned to the lowest magnification before the microscope slide is placed onto the microscope stage, as it is much easier to focus on the specimen at low magnification. Focusing is done by moving the stage up or down until the specimen is in the right position. The objectives are designed to be parfocal, which means they stay in focus upon switching to the next magnification. The stage of the microscope can also be moved horizontally to explore different areas of the slide.