Principle And Structure Of Optical Microscope

- Apr 15, 2020-

Microscope is one of the important tools to study microbiology. According to different research purposes and requirements, ordinary optical microscope, dark field microscope, phase contrast microscope, fluorescence microscope and electron microscope can be selected respectively. In the detection of food microorganisms, ordinary optical microscope (referred to as microscope) is commonly used.

Optical microscope is a kind of optical instrument which uses optical principle to magnify and image the tiny objects which can not be distinguished by human eyes, so that people can extract the information of microstructure. Optical microscope is to use the principle that the optical lens with curved surface makes the object magnify and image, to enlarge the small object to the size that the human eye can observe.

Optical microscopes are usually magnified by two stages, one by the objective lens and the other by the eyepiece. The object to be observed is located in front of the objective lens, which is magnified by the objective lens to form an inverted real image, and then the real image is magnified by the eyepiece to form a virtual image by the second stage. What the human eye sees is a virtual image, and the total magnification of the microscope is the product of the magnification of the objective lens and the magnification of the eyepiece.

Magnification refers to the magnification of the linear dimension, not the area ratio. The limit of magnification of microscope is the effective magnification. The resolution of microscope refers to the small space between two objects which can be clearly distinguished by microscope. Resolution and magnification are two different but interrelated concepts.

When the resolution of the selected objective lens is not high enough (the numerical aperture is not large enough), the microscope can not distinguish the fine structure of the object. At this time, even if the magnification is increased excessively, only an image with large outline but unclear details is obtained, which is called invalid magnification. On the contrary, if the resolution has met the requirements and the magnification is insufficient, the microscope has the resolution ability, but the image is too small to be seen clearly by the human eye. Therefore, in order to give full play to the resolution of the microscope, it is necessary to match the numerical aperture with the total magnification of the microscope

Structure of microscope: optical part

1. objective lens

The lower end of the lens is close to the object to be tested, which is the main optical device on the microscope. It is composed of multiple lenses. Generally, the microscope has 3-4 objective lenses, which are divided into two groups: drying system and oil mirror. The drying system is divided into low power and high power mirrors according to the magnification. The magnification of each objective is identified by its shape. The larger the lens length, the smaller the lens aperture, and the larger the magnification; otherwise, the smaller the magnification. The lower edge of the oil lens is generally engraved with a circle of black lines. In addition, each objective lens is engraved with marks such as magnification, aperture number, etc

2. eyepiece

One of the important optical parts of the eyepiece, which is close to the eye of the examiner. The eyepiece is composed of two lenses with multiple engraved on it. In order to point out the image, a black filament is often installed in the mirror as a pointer.

3. concentrator

The concentrator is located on the secondary stage below the loading stage and consists of several lenses. It can move up and down, adjust and gather the light provided by the light source, so as to concentrate on the slide of the carrier.

4. light source

The light source provides the light needed for observation and is installed under the microscope.

Structure of microscope: mechanical part

1. mirror arm

The mirror arm is at the back of the mirror barrel, which is convenient for the microscope to move, and serves as the holding part when the mirror barrel is tilted.

2. base

The base of the microscope is used to support the whole mirror, in the shape of horseshoe.

3. loading stage

Under the objective lens, it is square or round, which is used to carry the inspected object. There are holes in the center, which can transmit light. A spring clip is installed on the table, and the specimen piece can be fixed during the microscopic inspection. Some tables are equipped with adjustment knobs and twirling knobs, which can move the specimen back and forth

4. aperture

Under the concentrator, it can be opened or closed at will to adjust the amount of incoming light.

5. Power supply and brightness adjustment knob

It is used to turn on the light source and adjust the brightness of the light source.

6. Coarse quasi focus screw and fine quasi focus screw

The coarse quasi focus screw can make the stage rise and fall at a large distance. The fine focus screw can adjust the distance.