A Microscope, Defined
A microscope is an instrument for viewing objects that are too small to see clearly with the naked eye. It produces an enlarged image that reveals detail you couldn’t otherwise make out. Most microscopes use light and glass lenses, but some use beams of electrons, and others trace a surface with an ultra-fine probe.
Where the Word Comes From
“Microscope” comes from two Greek words: mikros, meaning “small”, and skopein, meaning “to look at” or “to examine”. The name was coined in 1625, for one of the early compound instruments. That was a few decades after the first microscopes appeared in Europe. “Microscopy” is the practice of using microscopes.
What a Microscope Actually Does
It’s easy to think a microscope just makes things bigger. A useful microscope actually does three jobs:
- Magnifies: It makes the image larger than the object.
- Resolves: It separates fine details that would otherwise blur together. This is what truly reveals structure.
- Provides contrast: It makes the specimen stand out from its background, through lighting, stains or special optics.
Magnification without resolution gives a bigger blur, not more information. This idea, and the vocabulary that goes with it, is covered in our introduction to microscopy.
How a Basic Microscope Works
The simplest microscope is a single convex lens, such as a magnifying glass. When you hold a small object just inside the lens’s focal length, the lens bends the light so that your eye sees a larger, upright image. Our article on the principle of the simple microscope explains this.
A compound microscope, the classic laboratory instrument, uses two lens stages:
- The objective lens, close to the specimen, forms a magnified image inside the microscope tube.
- The eyepiece magnifies that image again for your eye.
To find the total magnification, multiply the two values. For example, a 10x eyepiece with a 40x objective gives 400x. The way lenses bend light to make this possible is explained in how lenses magnify.
The Main Kinds of Microscope
| Kind | How it forms an image | Typical use |
|---|---|---|
| Simple microscope (magnifier, loupe) | One convex lens | Close inspection of small objects |
| Compound light microscope | Objective plus eyepiece, light through the specimen | Cells, tissues, microorganisms on slides |
| Stereo (dissecting) microscope | Two optical paths for a 3D view | Insects, plants, rocks, electronics |
| Electron microscope (TEM, SEM) | Beam of electrons focused by magnets | Viruses, cell ultrastructure, materials |
| Scanning probe microscope (e.g. AFM) | A fine tip traced over a surface | Surfaces and molecules at nanometer scale |
For more on each family, see types of microscopes used in biology and what a light microscope is.
How Small Can a Microscope See?
The human eye can typically make out objects down to roughly a tenth of a millimeter (about 100 micrometers) under good conditions. Different microscopes take over from there:
| Object | Approximate size | What you need |
|---|---|---|
| Human hair (width) | Roughly 50–100 µm | Naked eye (just) or a hand lens |
| Paramecium | Often 50–300 µm | Low-power compound or stereo microscope |
| Human red blood cell | About 7–8 µm | Compound microscope, 400x |
| Typical bacterium | About 1–5 µm long | Compound microscope, 1000x |
| Most viruses | Mostly below 0.2 µm | Electron microscope |
| Ribosome | About 20–30 nm | Electron microscope |
| Atoms | Fractions of a nanometer | Advanced electron or scanning probe microscopes |
Sizes are approximate and vary within each group.
Who Invented the Microscope?
No single person can be credited with certainty. Simple magnifying lenses were used for centuries. The compound microscope is traditionally credited to Dutch spectacle makers Hans and Zacharias Janssen, around the 1590s, though historians debate the evidence. Galileo, Cornelis Drebbel, Robert Hooke and Antonie van Leeuwenhoek all played major roles in the 1600s. Our history of the microscope tells the full story.
Microscope vs Telescope
Both instruments use lenses, and both can have an objective and an eyepiece. A microscope is designed to examine tiny objects close at hand, so its objective has a very short focal length. A telescope is designed to view large objects far away, so it uses a long focal length and a large lens or mirror to gather faint light. The details are in our article on how a simple microscope works and how it differs from a telescope.