Introduction to Microscopy: What Microscopes Do and Key Terms

What Is Microscopy?

Microscopy is the use of instruments to look at things too small to see clearly with the naked eye. The word “microscope” comes from the Greek mikros (small) and skopein (to look at), and the name was first used in the 1620s for an early compound instrument.

Microscopy is both a set of tools and a set of skills. The tools range from a simple hand lens to electron microscopes that fill a room. The skills include preparing a specimen, lighting it well, focusing carefully and interpreting what you see. A good microscope used badly gives a worse image than a modest one used well. That is why beginners benefit from learning the basic ideas before buying anything expensive.

If you just want a short definition, our article on what a microscope is gives the quick answer. This guide goes further and introduces the ideas and vocabulary that the rest of this section builds on.

The Three Jobs of a Microscope

A microscope is often described as a device that “makes things bigger”. That’s only part of it. A useful microscope has to do three things at once.

Magnification

Magnification is how much larger the image appears compared with the object. On a typical compound microscope, you work it out by multiplying the eyepiece magnification by the objective magnification. A 10x eyepiece with a 40x objective gives 400x total.

Magnification on its own isn’t enough, though. Enlarging a blurry image just gives you a bigger blur.

Resolution

Resolution (also called resolving power) is the ability to show two close points as separate rather than merged into one. It is the property that actually reveals detail. For ordinary light microscopes, resolution is limited by the wavelength of visible light. The best instruments can separate details roughly 0.2 micrometers (200 nanometers) apart.

Magnifying beyond what the resolution supports is called empty magnification. That is why toy microscopes advertising 2000x or more usually disappoint: the image is larger, but it holds no more detail.

Contrast

Contrast is the difference between the specimen and its background. Many biological specimens, such as living cells or bacteria, are almost transparent, so they’re hard to see even when perfectly focused. Microscopists add contrast with stains, or with optical techniques such as darkfield and phase contrast. These are described in our guide to the 7 types of light microscopes.

A Short History in Four Steps

The history of the microscope can be summarized in four broad stages:

1. Simple magnifiers. Curved glass and crystal were known to magnify in ancient times, and spectacles appeared in Europe in the late 13th century.
2. Early compound and simple microscopes (late 1500s to 1600s). Dutch spectacle makers are traditionally credited with the first compound microscope, though the evidence is debated. Robert Hooke published Micrographia in 1665. Antonie van Leeuwenhoek used tiny single-lens microscopes to become one of the first people to describe bacteria and other microorganisms.
3. Scientific optics (1800s). Better achromatic lenses, and the work of Ernst Abbe and Carl Zeiss, turned the microscope into a precision instrument.
4. Beyond visible light (1900s onward). The electron microscope was first built in the early 1930s. Scanning probe and super-resolution techniques followed later in the century.

Our history of the microscope covers this story in more detail, including the debate over who made the first one.

The Main Types of Microscopes

Microscopes fall into three broad families, depending on what they use to form the image.

Light microscopes are the ones most people will ever use, and they split into two everyday types:

• Compound microscopes use two lens stages (objective and eyepiece). They usually magnify from about 40x to 1000x and are used for thin, translucent specimens on slides.
• Stereo microscopes (dissecting microscopes) give a lower-magnification, three-dimensional view of solid objects such as insects, rocks, coins or circuit boards.

There is also the simple microscope, a single lens such as a magnifying glass or loupe. Our explanation of how a simple microscope works shows why even one lens can magnify. For a wider survey, see types of microscopes used in biology and our explainer on what a light microscope is.

How Powerful Is an Electron Microscope?

Electrons have a far shorter wavelength than visible light, so electron microscopes can resolve much finer detail. Transmission electron microscopes can resolve structures at the nanometer scale and below, and the most advanced instruments can image arrangements of atoms. Magnifications in the hundreds of thousands, or even more than a million times, are possible.

The trade-offs are cost, size, complex sample preparation, and the fact that specimens normally have to be dead and held in a vacuum. Our comparison of light vs electron microscopes goes through the pros and cons.

Parts of a Compound Microscope

Students are often asked to name “the 12 main parts” of a microscope. Exact lists vary between textbooks, but these are the parts you’ll find on most compound microscopes:

1. Eyepiece (ocular). The lens you look through, usually 10x.
2. Head or body tube. Holds the eyepiece(s) at the correct distance from the objectives. Heads can be monocular (one eyepiece), binocular (two) or trinocular (two plus a camera port).
3. Revolving nosepiece (turret). Holds several objectives and rotates to change magnification.
4. Objective lenses. The main magnifying lenses near the specimen, commonly 4x, 10x, 40x and 100x.
5. Stage. The flat platform that holds the slide.
6. Stage clips or mechanical stage. Hold the slide in place. A mechanical stage moves it smoothly with two knobs.
7. Coarse focus knob. Moves the stage (or the objectives) in large steps for initial focusing.
8. Fine focus knob. Makes small adjustments for sharp focus, especially at high power.
9. Condenser. A lens system under the stage that concentrates light onto the specimen.
10. Iris (aperture) diaphragm. An adjustable opening, usually on the condenser, that controls the cone of light and affects contrast and resolution.
11. Illuminator. The light source, usually an LED or halogen lamp. Older microscopes used a mirror instead.
12. Arm. The curved part connecting the head to the base, and the place to hold when carrying the microscope.
13. Base. The stable bottom that supports everything.

To see how light travels through these parts, read how a compound microscope works.

Key Microscope Terms: A Beginner’s Glossary

These are the words you’ll see most often in manuals, reviews and forums.

• Brightfield: The standard viewing mode, in which the specimen appears dark or colored against a bright background.
• Darkfield: A lighting method that makes the specimen glow against a black background. Useful for unstained, transparent subjects.
• Coverslip (cover glass): A thin square of glass placed over a specimen on a slide. Many objectives are designed for coverslips about 0.17 mm thick.
• Depth of field: How much of the specimen’s thickness is in focus at once. It becomes very shallow at high magnification.
• Diopter adjustment: A focusing ring on one eyepiece that compensates for differences between your two eyes.
• Empty magnification: Magnification beyond what the optics can resolve. The image gets bigger but shows no more detail.
• Field of view: The diameter of the area you can see. It shrinks as magnification increases.
• Immersion oil: A special oil placed between a 100x oil-immersion objective and the slide to improve resolution.
• Interpupillary distance: The spacing between the two eyepieces on a binocular head, adjusted to match your eyes.
• Köhler illumination: A standard method of aligning the light source and condenser for even, bright lighting.
• Micrometer (µm): One-thousandth of a millimeter, the usual unit for cell sizes. A human red blood cell is roughly 7–8 µm across.
• Numerical aperture (NA): A number engraved on each objective that indicates its light-gathering ability and resolving power. Higher is better.
• Parfocal: Objectives that stay nearly in focus when you switch between them.
• Plan objective: An objective corrected to give a flat field, so the edges and center are in focus together.
• Achromat / apochromat: Grades of lens correction for color fringing. Achromats are standard; apochromats are more highly corrected and more expensive.
• Reticle (graticule): A scale or crosshair placed in the eyepiece for measuring specimens.
• Specimen: Whatever you are looking at.
• Stain: A dye used to add color and contrast to parts of a specimen.
• Wet mount: A slide made by placing a specimen in a drop of liquid (often water) under a coverslip.
• Working distance: The gap between the front of the objective and the top of the coverslip when the specimen is in focus. It gets smaller at higher magnification.

What Microscopes Are Used For

Microscopes are used in far more fields than biology class:

• Education: Teaching cell biology, microbiology and scientific method in schools and universities.
• Medicine and pathology: Examining blood smears, tissue biopsies and samples that may contain bacteria, fungi or parasites.
• Microbiology and research: Studying living cells, microorganisms and cell processes.
• Forensics: Comparing hairs, fibers, glass fragments, tool marks and other trace evidence.
• Materials science and industry: Inspecting metals, coatings, semiconductors and manufactured parts for defects.
• Geology: Identifying minerals in thin rock sections, often with polarized light.
• Electronics: Soldering and inspecting small components, usually under a stereo microscope.
• Surgery: Operating microscopes are used in delicate procedures such as eye surgery and neurosurgery.
• Hobbies: Exploring pond life, pollen, diatoms, crystals, insects and minerals at home.

Where to Go Next

If you’re new to microscopy, a sensible reading order is:

1. How lenses magnify, the physics behind every optical microscope.
2. Real vs virtual images in a microscope, which explains what you are actually seeing when you look into the eyepiece.
3. How a compound microscope works, following the light from lamp to eye.
4. Getting started with amateur microscopy, if you want to set up at home.

Microscopy rewards patience. Learn the basic ideas, practice focusing and lighting, and an ordinary drop of pond water can show you a surprising amount.