Optical microscope

The optical microscope, also referred to as a light microscope, is a type of microscope that commonly uses visible light and a system of lenses to generate magnified images of small objects. Optical microscopes are the oldest type of microscope, with the present compound form first appearing in the 17th century. Basic optical microscopes can be very simple, although many complex designs aim to improve resolution and sample contrast. Objects are placed on a stage and may be directly viewed through one or two eyepieces on the microscope. A range of objective lenses with different magnifications are usually mounted on a rotating turret between the stage and eyepiece(s), allowing magnification to be adjusted as needed. In high-power microscopes, both eyepieces typically show the same image, but with a stereo microscope, slightly different images are used to create a 3-D effect. A camera may be used to capture the magnified image (micrograph). Objects can be lit in a variety of ways. Transparent objects can be lit from below and solid objects can be lit with light coming through (bright field) or around (dark field) the objective lens. Polarised light may be used to determine crystal orientation of metallic objects. Phase-contrast imaging can be used to increase image contrast by highlighting small details of differing refractive index. The maximum resolving power of optical microscopes is typically limited to around 200 nanometers because of the diffraction limit of visible light. While larger magnifications are possible no additional details of the object are resolved. Alternatives to optical microscopy which do not use visible light include scanning electron microscopy, transmission electron microscopy and scanning probe microscopy, which can achieve much greater magnifications than optical microscopy.

Types

There are two basic types of optical microscopes: simple microscopes and compound microscopes. A simple microscope uses the optical power of a single lens or group of lenses for magnification. A compound microscope uses a system of lenses (one set enlarging the image produced by another) to achieve a much higher magnification of an object. The vast majority of modern research microscopes are compound microscopes, while some cheaper commercial digital microscopes are simple single-lens microscopes. Compound microscopes can be further divided into a variety of other types of microscopes, which differ in their optical configurations, cost, and intended purposes.

Simple microscope A simple microscope uses a lens or set of lenses to enlarge an object through angular magnification alone, giving the viewer an erect enlarged virtual image. The use of a single convex lens or groups of lenses are found in simple magnification devices such as the magnifying glass, loupes, and eyepieces for telescopes and microscopes.

Compound microscope

A compound microscope uses a lens close to the object being viewed to collect light (called the objective lens), which focuses a real image of the object inside the microscope. That image is then magnified by a second lens or group of lenses (called the eyepiece) that gives the viewer an enlarged inverted virtual image of the object. The use of a compound objective-eyepiece combination allows for much higher angular magnification: For an object of height h {\displaystyle h} , it can at most occupy an unmagnified angular size θ 0 = h / d 0 {\displaystyle \theta _{0}=h/d_{0}} while remaining in focus, achieved when it is placed at the near point distance d 0 {\displaystyle d_{0}} of the eye (about 11 cm). The virtual image created by the compound microscope achieves an angular size of

θ = − h s / f objective f eyepiece {\displaystyle \theta =-hs/f_{\text{objective}}f_{\text{eyepiece}}} , where s {\displaystyle s} is the distance between the neighboring objective and eyepiece focal points. This is an angular magnification of θ / θ 0 = − s d 0 / f objective f eyepiece {\displaystyle \theta /\theta _{0}=-sd_{0}/f_{\text{objective}}f_{\text{eyepiece}}} . Common compound microscopes often feature exchangeable objective lenses, allowing the user to quickly adjust the magnification. A compound microscope also enables more advanced illumination setups, such as phase contrast.

Other microscope variants There are many variants of the compound optical microscope design for specialized purposes. Some of these are physical design differences allowing specialization for certain purposes:

Stereo microscope, a low-powered microscope which provides a stereoscopic view of the sample, commonly used for dissection. Comparison microscope has two separate light paths allowing direct comparison of two samples via one image in each eye. Inverted microscope, for studying samples from below; useful for cell cultures in liquid or for metallography. Fiber optic connector inspection microscope, designed for connector end-face inspection Traveling microscope, for studying samples of high optical resolution. Other microscope variants are designed for different illumination techniques:

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