- Depth of field - vertical distance, from above to below the focal plane, that yields an acceptable image
- Field of view - area of the specimen that can be seen through the microscope with a given objective lens
- Focal length - distance required for a lens to bring the light to a focus (usually measured in microns)
- Focal point/focus - point at which the light from a lens comes together
- Magnification - product of the magnifying powers of the objective and eyepiece lenses
- Numerical aperture - measure of the light-collecting ability of the lens
- Resolution - the closest two objects can be before they're no longer detected as separate objects (usually measured in nanometers)
The Parts of a Light Microscope
A light microscope, whether a simple student microscope or a complex research microscope, has the following basic systems:
- Specimen control - hold and manipulate the specimen stage - where the specimen rests clips - used to hold the specimen still on the stage (Because you are looking at a magnified image, even the smallest movements of the specimen can move parts of the image out of your field of view.) micromanipulator - device that allows you to move the specimen in controlled, small increments along the x and y axes (useful for scanning a slide)
- Illumination - shed light on the specimen (The simplest illumination system is a mirror that reflects room light up through the specimen.) lamp - produces the light (Typically, lamps are tungsten-filament light bulbs. For specialized applications, mercury or xenon lamps may be used to produce ultraviolet light. Some microscopes even use lasers to scan the specimen.) rheostat - alters the current applied to the lamp to control the intensity of the light produced condenser - lens system that aligns and focuses the light from the lamp onto the specimen diaphragms or pinhole apertures - placed in the light path to alter the amount of light that reaches the condenser (for enhancing contrast in the image) Diagram of a typical student light microscope, showing the parts and the light path
- Lenses - form the image objective lens - gathers light from the specimen eyepiece - transmits and magnifies the image from the objective lens to your eye nosepiece - rotating mount that holds many objective lenses tube - holds the eyepiece at the proper distance from the objective lens and blocks out stray light
- Focus - position the objective lens at the proper distance from the specimen coarse-focus knob - used to bring the object into the focal plane of the objective lens fine-focus knob - used to make fine adjustments to focus the image
- Support and alignment arm - curved portion that holds all of the optical parts at a fixed distance and aligns them base - supports the weight of all of the microscope parts The tube is connected to the arm of the microscope by way of a rack and pinion gear. This system allows you to focus the image when changing lenses or observers and to move the lenses away from the stage when changing specimens.
Some of the parts mentioned above are not shown in the diagram and vary between microscopes. Microscopes come in two basic configurations: upright and inverted. The microscope shown in the diagram is an upright microscope, which has the illumination system below the stage and the lens system above the stage. An inverted microscope has the illumination system above the stage and the lens system below the stage. Inverted microscopes are better for looking through thick specimens, such as dishes of cultured cells, because the lenses can get closer to the bottom of the dish, where the cells grow.
Light microscopes can reveal the structures of living cells and tissues, as well as of non-living samples such as rocks and semiconductors. Microscopes can be simple or complex in design, and some can do more than one type of microscopy, each of which reveals slightly different information. The light microscope has greatly advanced our biomedical knowledge and continues to be a powerful tool for scientists.