Parts and Use of the Light Microscope

A light microscope uses visible light and glass lenses to magnify small objects, and it is used by following a set order: low power first, then higher power with fine focusing. In brief: total magnification equals the eyepiece magnification multiplied by the objective magnification, so a ×10 eyepiece with a ×40 objective gives ×400. A typical school light microscope can show cells clearly, but it cannot show structures smaller than about 0.2 micrometers.

The light microscope is the main instrument for studying cells in school and many laboratories. This guide names the parts and their functions, explains correct use step by step, defines magnification and resolution, works through a magnification calculation and warns about the mistakes that damage slides and lenses.

What are the parts of a microscope?

A compound light microscope has two sets of lenses, the eyepiece and the objectives, which together form the magnified image. Other parts support the slide, control the light and move the lenses.

• Eyepiece (ocular lens): The lens that you look through, commonly ×10.
• Objective lenses: Usually three or four lenses on a rotating nosepiece, for example ×4, ×10 and ×40. The shortest is the lowest power.
• Stage and stage clips: The platform that holds the slide in place.
• Light source and diaphragm: A lamp or mirror below the stage supplies light, and the diaphragm adjusts the amount of light and the contrast.
• Condenser: A lens under the stage that focuses light onto the specimen.
• Coarse and fine focus knobs: The large knob moves the stage or the body quickly. The small knob gives the final sharp image.
• Arm and base: The frame used for carrying the instrument. Always hold it by the arm with one hand and support the base with the other.

How do you use a light microscope?

1. Place the microscope on a stable surface and switch on the light.
2. Rotate the nosepiece to the lowest-power objective until it clicks into position.
3. Place the slide on the stage with the specimen over the opening, and secure it with the clips.
4. Looking from the side, use the coarse focus to bring the stage and objective close together, but do not let them touch.
5. Look through the eyepiece and turn the coarse focus slowly so that the objective moves away from the slide until the image appears.
6. Sharpen the image with the fine focus and adjust the diaphragm for good contrast.
7. Center the part you want to study. Then switch to the next higher objective and focus using only the fine focus.
8. When you have finished, return to the lowest power, lower the stage, remove the slide and clean up.

Starting at low power gives a wide field of view that makes it easy to find the specimen. Using only fine focus at high power prevents the lens from hitting the slide, which could crack the slide and scratch the lens.

What is magnification?

Magnification is how many times larger an image appears compared with the real object. For a compound microscope, total magnification is the eyepiece magnification multiplied by the objective magnification.

The related formula is: magnification = size of image ÷ actual size of object. The formula can be rearranged to find any of the three values, and the units of image size and actual size must be the same. A common unit is the micrometer (µm); 1 millimeter equals 1000 micrometers.

What is resolution?

Resolution is the ability to show two nearby points as separate. It is the smallest distance between two points that can be seen as two. High magnification without good resolution only gives a bigger blurred image.

Resolution is limited by the wavelength of light. Visible light has wavelengths of roughly 400 to 700 nanometers, and a light microscope can resolve details of about 200 nanometers (0.2 µm) at best. Electron microscopes use beams of electrons, whose wavelength is far shorter, and they can show much finer detail, down to a fraction of a nanometer in the best instruments.

Microscope parts and functions

Worked example: magnification and actual size

A student uses a ×10 eyepiece and a ×40 objective to look at a cheek cell. The image of the cell measures 6 mm across in a drawing made from the view.

1. Total magnification: 10 × 40 = ×400.
2. Actual size: actual size = image size ÷ magnification = 6 mm ÷ 400 = 0.015 mm.
3. Convert to micrometers: 0.015 mm × 1000 = 15 µm.

The cell is 15 µm across, a reasonable size for a human cheek cell. Check the result with a rough rule: if the magnification is several hundred times and the drawing is millimeters across, the real size should be tens of micrometers.

The field of view also changes. At ×40 total magnification (a ×4 objective), the field might be about 4.5 mm wide. At ×400, it is ten times narrower, about 0.45 mm. A rule of thumb is that the field of view becomes smaller by the same factor that the magnification increases.

How did the microscope develop?

Compound microscopes were made in the Netherlands around 1600, and the names of Hans and Zacharias Janssen and Hans Lippershey are linked to early designs, although the exact inventor is uncertain. Robert Hooke and Antonie van Leeuwenhoek used microscopes in the seventeenth century to make the first observations of cells and microorganisms. Better lenses and illumination in the nineteenth century, including improvements by Ernst Abbe and Carl Zeiss, raised resolution to the theoretical limit of light.

Where are light microscopes used today?

Light microscopes are used in schools, hospitals and research. Pathologists examine stained tissue samples to diagnose diseases such as cancer, and technicians count blood cells and look for parasites such as the malaria organism. Water-quality and food laboratories identify microorganisms. Modern variants include fluorescence microscopes, which label specific molecules with glowing dyes, and phase-contrast microscopes that make living, transparent cells visible.

Which mistakes are common?

Mistake 1: Starting at high power

You will probably not find the specimen, and you risk hitting the slide. Always begin with the lowest objective.

Mistake 2: Using the coarse focus at high power

Even a small movement can push the lens into the slide. Use only the fine focus after changing to a higher objective.

Mistake 3: Confusing magnification with resolution

Magnification makes the image bigger. Resolution decides whether it shows more detail. Beyond a certain point, extra magnification adds no detail.

Mistake 4: Mixing units in size calculations

Convert all values to the same unit before dividing, and remember that 1 mm = 1000 µm.