Optical system
The main task of the optical system is to form an enlarged image of the studied specimen on the observer's retina. This system consists of two main elements fixed in the body tube.
The objective is located in the lower part of the body tube and faces directly toward the specimen. It is a complex system of lenses divided into two types:
- Dry objectives: operate in a normal air environment. They have standard magnifications: $\times4$, $\times8$, $\times10$, $\times20$, $\times40$.
- Immersion objectives: designed for high magnification ($\times90$). For them to work, the lens must be immersed in a special medium — a drop of cedar oil applied to the coverslip.
The eyepiece is located in the upper part of the body tube and faces the researcher's eye. Eyepieces are interchangeable elements with magnifications of $\times7$, $\times10$, or $\times15$.
To find the resulting magnification, multiply the objective and eyepiece values. For example, using a $\times20$ objective and a $\times10$ eyepiece, the total magnification will be 200 times.
Illumination system
The illumination system is responsible for passing light through the studied specimen. Its components are located strictly along the path of the light beam.
- Light source: can be built-in or external (desk lamp, natural light from a window).
- Mirror: directs light rays from below onto the specimen. It has two working surfaces. The flat surface is used in daylight because the sun provides parallel rays. The concave surface is needed with artificial light (from a lamp) to gather diverging rays. In modern models with built-in illumination, the mirror may be absent.
- Diaphragm: regulates the light flux. It consists of opaque blades forming an aperture of variable diameter. When working at low magnification, the aperture is narrowed to eliminate spherical aberration, and at high magnification, it is opened.
- Condenser: a system of lenses that collects and focuses light rays precisely onto the specimen. The focus can be adjusted by raising or lowering the condenser using a special screw. Often, this element is structurally integrated with the diaphragm.
Mechanical system
The mechanical system functions as a framework. It is necessary for fixing the optics and manipulating the studied object.
The main load-bearing units include the limb (stand) and the arm (column). The optical elements are located in a tube called the body tube, and the specimen itself is placed on the stage.
Special focusing mechanisms—screws—are used for focusing. They are connected to the arm and allow raising or lowering it along with the body tube:
- Coarse adjustment knob (coarse screw) is used for rough adjustment and only when working at low magnification.
- Fine adjustment knob (fine screw) is necessary for precise focusing when working at high magnification.
The mechanical system also includes mechanisms for moving the stage in the horizontal plane, which allows precise positioning of the object under the objective lens.
Principles of microscopy and light path
Since the study is conducted in transmitted light, the histological specimen must be extremely thin and transparent. Another feature is that the optics form an inverted image.
Light follows a strictly defined path from the source to the researcher's eye:
- Light source.
- Mirror (directs the beam upward).
- Diaphragm and condenser (restrict and focus rays on the object).
- Specimen.
- Objective (forms the primary magnified image).
- Body tube (transmits the image).
- Eyepiece (transmits the image for perception by the eye).
Briefly: answers to topical questions
What is the physical meaning of using cedar oil?
The physical purpose of using cedar oil in immersion microscopy is to eliminate the sharp change in the refractive index of light rays in an air medium. A drop of oil is applied to the coverslip, placing it between the objective and the specimen; this eliminates the air gap. When using an immersion objective, the magnification increases up to 1000x.
What is the resolving power of the light microscope?
Resolving power is the minimum distance between two points visible separately. For a light microscope, it is about 0.25 µm.
The following values are also given for light microscopy:
| Parameter | Significance |
|---|---|
| Theoretical resolution | 0.2 µm |
| Practical resolution | 0.4 µm |
What types of objective aberrations exist, besides spherical?
In addition to spherical aberration, chromatic distortions and field curvature occur in the microscope's optical system. Special types of objectives are used to compensate for these flaws:
- Achromats — correct chromatic aberration for two colors.
- Apochromats — correct chromatic aberration for three colors.
- Plan objectives — eliminate field curvature.
The light path is easy to remember from bottom to top: Source → Mirror → Condenser/Diaphragm → Specimen → Objective → Body tube → Eyepiece.
Class and exam questions
How to calculate the total magnification of a microscope?
You must multiply the objective magnification value by the eyepiece magnification value. For example, a 40x objective and a 10x eyepiece give a total magnification of 400x.
What is cedar oil used for?
It is used when working with immersion objectives (x90 magnification). Oil is applied to the coverslip and serves as the medium into which the objective lens is immersed.
Which side of the mirror should be chosen when working with a desk lamp?
The concave surface of the mirror should be used. It helps gather diverging rays from an artificial light source.
Why is it necessary to close the aperture at low magnification?
Reducing the diaphragm aperture at low magnification is necessary to limit the light flux and eliminate spherical aberration.
What else to study on this topic
- Operating principle of dry and immersion objectives
- Spherical aberration and methods for its correction
- Rules for positioning an object on the microscope stage
- Features of inverted image formation
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