Structure, Principle, Operation and Maintenance of Ordinary Optical Microscope
1. Structure
Magnification of the specimen is mainly achieved by the objective lens. The higher the magnification of an objective lens, the shorter its focal length. A shorter focal length results in a smaller distance between the objective lens and the slide (working distance). The oil immersion objective has an extremely short working distance and requires extra caution during use. The eyepiece only provides magnification and cannot improve resolution; the standard eyepiece magnification is 10×. The condenser focuses light onto the specimen before it enters the objective lens, forming a wide-angle conical light cone, which is critical for improving the resolution of the objective lens. The condenser can be moved up and down to adjust brightness, and the iris diaphragm controls the size of the incident light beam.
For the microscope light source, both natural light and artificial lighting are usable, though artificial lighting is preferred because its color temperature and intensity are easy to control. General microscopes can use ordinary lamps, while high‑grade microscopes require dedicated microscope lamps for optimal performance. For applications requiring intense illumination, such as darkfield observation and photomicrography, halogen lamps are commonly adopted as the light source. An optical microscope consists of two major parts: an optical magnification system and a mechanical assembly. The optical system generally includes the eyepiece, objective lens, condenser and light source; the mechanical system usually comprises the tube, nosepiece, stage, arm and base.
2. Principle
The resolving power (resolution) of a microscope is determined by the wavelength of the light used and the numerical aperture of the objective lens. Resolution can be improved by shortening the light wavelength or increasing the numerical aperture. Visible light covers a narrow wavelength range. Ultraviolet light has a shorter wavelength for higher resolution but cannot be observed directly by the naked eye. Therefore, there is a limit to improving the resolution of an optical microscope by reducing light wavelength. Increasing the numerical aperture is the preferred method for better resolution. Numerical aperture can be raised by increasing the refractive index of the medium. Air has a refractive index of 1, while cedar oil has a refractive index of 1.51, close to that of the cover glass (1.52). This allows light to travel through the slide and cedar oil into the objective without refraction, thus enhancing resolution. The total magnification of a microscope equals the product of the eyepiece magnification and objective magnification, and higher objective magnification corresponds to higher resolution.
