You've probably used a microscope in school -- maybe to observe the wings of an insect or to get a closer look at a leaf. If so, then you know microscopes are used in the classroom to illuminate the surface of your subject of study. These microscopes use transparent glass lenses to magnify the image of whatever you are observing.
However, the size of the smallest features that we can distinguish under the microscope is on the order of the wavelength of the light used. Visible light, which is the one our eyes are sensitive to, ranges between 390 and 700 nanometers (one nanometer is one billionth of a meter). This means that we cannot observe things that are smaller than a few hundred nanometers using our eyes and visible light.
With the advancement of science and technology, we realized there is a whole world of things that we can observe and study at small scales. For example, the size of atoms and molecules is just fractions of a nanometer. An electron microscope allows us to see at these small scales.
Electron microscopes work by using an electron beam instead of visible light and an electron detector instead of our eyes. An electron beam allows us to see at very small scales because electrons can also behave as light. It has the properties of a wave with a wavelength that is much smaller than visible light (a few trillionths of a meter!). With this wavelength we can distinguish features down to a fraction of a nanometer. Let's explore the different types of electron microscopes, how they work and some of their applications.