How microscopes magnify

A microscope is something that uses a lens or lenses to make small objects look bigger and to show more detail. This means that a magnifying glass can count as a microscope! It also means that making your own microscope is straightforward.

You’ll meet some highly complex microscopes in our collection of microscope resources, but most of them are just ways of viewing objects in more detail with lenses. A lot of technological innovations have been added to microscopes over time (mostly to make them simpler to use and improve the quality of the images seen). These changes can make microscopes appear more complicated, but they don’t alter the basic science of the microscope lenses.

The first microscopes

Very early microscopes were extremely simple but were still remarkably good at magnifying small objects. For instance, microscopes made by the Dutchman Antonie van Leeuwenhoek in the 1600s were merely a small glass ball (which acted as a lens) set into a metal frame. Using this simple apparatus, van Leeuwenhoek discovered the world of what he called ‘animalcules’ (microorganisms) – tiny single-celled creatures that are far too small to be seen with the naked eye.

How lenses magnify

When you look through a simple light microscope or a magnifying glass, you are looking through a biconvex lens (one that’s bent like the back of a spoon on both sides) made of glass. The object being viewed is on the far side of the lens. Light from the object passes through the lens and is bent (refracted) towards your eye, so it seems as though it comes from a much bigger object.

In practice, modern microscopes contain a series of lenses rather than just one. They have an objective lens (which sits close to the object) and an eyepiece lens (which sits closer to your eye). Both of these contribute to the magnification of the object. The eyepiece lens usually magnifies 10x, and a typical objective lens magnifies 40x. (Microscopes usually come with a set of objective lenses that can be interchanged to vary the magnification.) You can calculate the total magnifying power of the microscope by multiplying the magnifying powers of the objective lens and the eyepiece (so 10 x 40 = total magnification of 400x).

Including more lenses doesn’t change the basic principle of how a microscope magnifies but it does enable higher magnifications and gives a better quality image.

Lenses and image quality

Microscope lenses differ widely in quality, and this can affect how clearly you can see an image. The quality of the glass used and the shape of the lens both affect its overall quality. Misalignment of lenses within the microscope can also limit resolution. In practice, this means that students using classroom microscopes may not be able to view samples that are close to the theoretical limits of resolution of a light microscope.

Lenses in electron microscopes

Electron microscopes use a beam of electrons instead of visible light to illuminate the object being viewed. However, electrons can’t pass through glass, so the lenses that are used in light microscopes can’t be used to bend the electron beam.

To get around this issue, scientists designed an alternative lens – a coil of wire surrounding the electron beam. When electricity runs through the wire, it generates a magnetic field within the coil. Because electrons are charged, the microscope’s electron beam bends in response to the magnetic field as it passes through the coil. In this way, the coils act as lenses – they bend the electron beam, just as glass lenses bend light in an optical microscope.

Microscopes without lenses

Some modern instruments that don’t contain lenses are still known as microscopes because they magnify objects. For example, the scanning tunnelling microscope and the atomic force microscope measure the shape of a surface by measuring the distance between the microscope’s probe and the surface. These microscopes generate images at very high resolution. You can learn more about them in the article Nanoscience that introduces our wide range of nanoscience resources.