Microscopes

Last updated: October 30, 2008.

The plant on your windowsill is buzzing with life, turning sunlight into sugar all day long. Mold is slowly gobbling up the apples in your fruit bowl. Your bed is creeping with dust mites. The air is packed with pollen...

It's a truly amazing thought: there are zillions of things happening all around us, all the time, that are far too tiny for our eyes to see! But never fear, because we have an equally amazing way to get around it. Powerful microscopes shed new light on the teeny tiny and make the invisible, visible. They've played an enormous part in science by taking us deep into worlds we've come to think of as "microscopic". Just as telescopes scale us up to meet the planets and stars, so microscopes scale us down into the tiny world of atoms and cells. Let's take a closer look at how they work!

Photo: A typical optical microsope. Photo by Stephen Ausmus courtesy of US Department of Agriculture: Agricultural Research Service (USDA-ARS).

What does invisible really mean?

Lots of things are invisible, but that doesn't mean they're not there. Radio and TV broadcasts are constantly whistling through your head from powerful transmitters, but unless you happen to have a cunning piece of electronic equipment at your disposal—namely a radio or TV set—you won't be able to understand them. We're used to the world being the totality of things we can see; that there are worlds out there our eyes aren't tuned into is both a physical problem and a philosophical conundrum.

Imagine if your eyes were as powerful as microscopes and you could see all the germs crawling about on your hands. Your brain would be so busy boggling that you wouldn't be able to concentrate on bigger things at a more meaningful scale. Through millions of years of evolution, our eyes and brains are programmed to worry about the things that matter most—things on a similar scale to our bodies. We simply don't have the time or the brain capacity to worry about absolutely everything that's going on. If you were constantly staring at the bugs on your fingers, you could easily get so distracted that you'd walk straight under a bus! Don't understand? Let's put it this way. The smaller the things you look at, the more there is to see, the more information there is to process, and the longer it takes. If you could see microsopically all day long, you'd have to react much more slowly to the world around you—and that extra reaction time would threaten your life.

This, then, is what invisible means: our bodies are finely tuned to the business of day-to-day living on a human scale and efficiently designed to ignore everything else.

Photo: A scientist studies leaves for traces of ticks. Photo by Scott Bauer courtesy of US Department of Agriculture: Agricultural Research Service (USDA-ARS).

Why do we need microscopes?

Once upon a time, we used to ignore things we couldn't see. But thanks to modern science, we know there's a whole lot happening on the microscopic scale that can help us to live our lives more effectively. Scientists have known since the 17th century that the insides of living things are made up of tiny functioning factories called cells; understanding how they work helps us to tackle sickness and disease. More recently, during the 20th century, scientists figured out how materials are made of atoms and how atoms themselves are built from smaller "subatomic" particles; understanding atomic structure paved the way for all kinds of amazing inventions, from electronic transistors to nuclear power.

How microscopes work

Microscopes are effectively just tubes packed with lenses, curved pieces of glass that bend light rays passing through them. The simplest microscope of all is a magnifying glass made from a single convex lens, which typically magnifies by about 5-10 times. Microscopes used in homes, schools, and professional laboratories are actually compound microscopes and use at least two lenses to produce a magnified image. There's a lens above the object (called the objective lens) and another lens near your eye (called the eyepiece or ocular lens). Each of these may, in fact, be made up of a series of different lenses. Most compound microscopes can magnify by 10, 20, 40, or 100 times, though professional ones can magnify by 1000 times or more. For greater magnification than this, scientists generally use electron microscopes.

So what does a microscope actually do? Imagine a fly sitting on the table in front of you. The big, fat, compound eye on the front of its head is just a few millimeters across, but it's made up of around 6000 tiny segments, each one a tiny, functioning eye in miniature. To see a fly's eye in detail, our own eyes would need to be able to process details that are millimeters divided into thousands—millionths of a meter (or microns, as they're usually called). Your eyes may be good, but they're not that good. To study a fly's eye really well, you'd need it to be maybe 10-100 cm (4-40 in) across: the sort of size it would be in a nice big photo. That's the job a microscope does. Using very precisely made glass lenses, it takes the minutely separated light rays coming from something tiny (like a fly's eye) and spreads them apart so they appear to be coming from a much bigger object.

Photo: Most microscopes have several different objective lenses that turn around on a thumb-wheel to give different levels of magnification. Going from right to left, the lenses you can see here magnify by twenty times (20x), forty times (40x), and a hundred times (100x). Photo by Stephen Ausmus courtesy of US Department of Agriculture: Agricultural Research Service (USDA-ARS).

What are electron microscopes?

Even the best microscopes have their limits: they can't magnify more than a few thousand times. Why? One way of understanding light is to say that it's made up of energetic particles called photons, effectively about 200 times thinner than a human hair. But what if we want to look at things smaller than this? In that case, we can't use light: we have to use smaller particles instead—namely electrons, the tiny negatively charged particles that whizz around inside atoms. Microscopes that work in this way are called electron microscopes.

Photo: Ordinary microscopes are "powered" by light. When light shines on the specimen at the bottom, it travels straight through or reflects off the surface, passing up through the lenses into the eyepiece. Microscopes that use light are called optical microscopes to distinguish them from electron microscopes. Photo by Peggy Greb courtesy of US Department of Agriculture: Agricultural Research Service (USDA-ARS).