The Compound Microscope Help
Optical microscopes are designed to greatly magnify the images of objects too small to resolve with the unaided eye. Microscopes, in contrast to telescopes, work at close range. The design is in some ways similar to that of the telescope, but in other ways it differs. The simplest microscopes consist of single convex lenses. These can provide magnification factors of up to 10× or 20×. In the laboratory, an instrument called the compound microscope is preferred because it allows for much greater magnification.
A compound microscope employs two lenses. The objective has a short focal length, in some cases 1 mm or less, and is placed near the specimen or sample to be observed. This produces an image at some distance above the objective, where the light rays come to a focus. The distance (let’s call it s) between the objective and this image is always greater than the focal length of the objective.
The eyepiece has a longer focal length than the objective. It magnifies the real image produced by the objective. In a typical microscope, illumination can be provided by shining a light upward through the sample if the sample is translucent. Some microscopes allow for light to be shone downward on opaque specimens. Figure 19-13 is a simplified diagram of a compound microscope showing how the light rays are focused and how the specimen can be illuminated.
Fig. 19-13. Illumination and focusing in a compound optical microscope.
Laboratory-grade compound microscopes have two or more objectives, which can be selected by rotating a wheel to which each objective is attached. This provides several different levels of magnification for a given eyepiece. In general, as the focal length of the objective becomes shorter, the magnification of the microscope increases. Some compound microscopes can magnify images up to about 2,500 times. A hobby-grade compound microscope can provide decent image quality at magnifications of up to about 1,000 times.
A compound microscope is focused by moving the entire assembly, including both the eyepiece and the objective, up and down. This must be done with a precision mechanism because the depth of field (the difference between the shortest and the greatest distance from the objective at which an object is in good focus) is exceedingly small. In general, the shorter the focal length of the objective, the smaller is the depth of field, and the more critical is the focusing. High-magnification objectives have depths of field on the order of 2 μm (2 × 10 −6m) or even less.
If the eyepiece is moved up and down in the microscope tube assembly while the objective remains in a fixed position, the magnification varies. However, microscopes usually are designed to provide the best image quality for a specific eyepiece-to-objective separation, such as 16 cm (approximately 6.3 in).
If a bright enough lamp is used to illuminate the specimen under examination, and especially if the specimen is transparent or translucent so that it can be lit from behind, the eyepiece can be removed from the microscope and a decent image can be projected onto a screen on the ceiling of the room. A diagonal mirror can reflect this image to a screen mounted on a wall. This technique works best for objectives having long focal lengths, and hence low magnification factors.