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.