The universe around us exhibit objects that span a large range in size.  Within this range is the  radius of our gigantic visible cosmos represented by the farthest visible galaxy at 10²⁸ meters away to the tiniest atomic nucleus with a diameter near 10^-15 meters.  The unaided human eye can only perceive a narrow range of distance and sizes.  In order to perceive the very far, telescopes have been used.  Various microscopes have been invented to aid us in seeing objects in the microscopic and nanoscale realm.

The size/distance regime where the unaid eye, telescopes, and microscopes operate best at is shown in the graphics above.
 
 

Exploring with Microscopes The development of the three major types of microscopes over time is outlined in the graph below.  It took humans about 400 years to develop optical microscopes to the current level of sophistication using visible light, starting with Robert Hooke.  No radical developments occur until about 1933 when Ernst Ruska invented the electron microscope.  He used short-wavelength electrons to image the microscopic objects with high magnification and resolution.  Another radical step was taken by Gerd Binnig and Heinrich Rohrer in 1981 when they developed the scanning tunneling microscope that can visualize a surface with high magnification and resolution together in three-dimensions.  Their revolutionary idea was to use a sharp mechanical tip instead of employing electromagnetic radiations of ever shorter wavelengths.

A brief overview of history of microscopes. Roll your mouse over various areas of the graph to read more about it.

The optical microscope is aptly named since it is often used to resolve objects in the micrometer (10-6m) range. Meanwhile, electron microscopes (EM) and scanning probe microscopes (SPM) are often used to resolve objects a thousand times smaller than the micrometer. These tools can resolve objects in the nanometer (10-9m) range. Although we still call EM and SPM "microscopes," it might be more appropriate to label them "nanoscopes." As we continue to use and develop these micro and nanoscopes we continue to learn more about the new micro/nano frontier, the home of the building blocks of life and matter.

Optical Microscopes   A modern Optical Microscope     The first microscopes were optical microscopes, enabling the human eye to see much smaller objects. These early visualization tools used glass lenses, using the phenomenon of refraction, to focus light to magnify the image of an object. Major scientific discoveries began about 60 years after the invention of the microscope. As optical microscopes improved, smaller and smaller objects could be seen and more and more discoveries were made. Images taken with an Optical microscope  Optical microscopes are by far the most common types of microscopes and are still important in the exploration of the micro/nano universe today. A good optical microscope can generally distinguish objects as small as 200nm (2 X 10-7 m). 200nm are about the diameter of a large virus or very small bacterium, such as mycoplasm.

Electron Microscopes   A Transmission Electron Microscope The limit of resolution for optical microscopes, about 200nm, is a physical limit imposed by the wavelength of light. While this is very small, it is still more than 1000 times larger than the diameter of a carbon atom. Since optical resolution is limited by light, visualization of objects smaller than 200nm had to come from a microscope that does not use light. Electron microscopes use accelerated electrons instead of light, and magnetic coils instead of glass lenses to make an image.   This technique offers great depth of field for imaging objects and high magnification of up to 50,000X.  The sample surface must be electrically conducting to achieve high resolutions.   This technique requires the laborious thinning of a sample to allow the electrons to go through the sample.  Atomic-scale resolutions have been achieved using these instruments.  To get useful results from either of these microscopes, the sample must be able to survive conditions of high vacuum and heating from the microscope's electron beam.    Images taken with a Scanning Electron Microscope The electrons have an associated wavelength that is 10 to 100 thousand times smaller than the wavelength of light. Because of this, electron microscopes are able to resolve objects 1000 times smaller than the smallest resolvable object in a light microscope. The electron microscope has extended our vision into the realm of sub-cellular organelles, viruses, proteins, DNA, molecules and even atoms. It has radically changed all science and engineering.

Scanning Probe Microscopes   An Atomic Force Microscope Electron microscopes are invaluable tools for the exploration of the micro/nano universe, but have many limitations. In general, samples must be examined under a vacuum and require careful preparation. However, the newest family of microscopes, called scanning probe microscopes (SPM) can achieve electron microscope resolution in plain air or even liquid. Also, SPM samples require much less preparation than electron microscope samples. This has enabled imaging of many more types of samples, including living cells.      An ant's eye image taken with an AFM.    The head of an ant is too large for an    AFM. Scanning probe microscopes can visualize a sample surface, under high magnifications and resolution,  in three dimensions and make micro/nano images of sample properties that other microscopes cannot see. These include thermal properties, friction, hardness, magnetic properties and chemical binding. Instead of using light or focussed electrons, scanning probe microscopes use a tiny needle like probe attached to a cantilever that is scanned back and forth across a surface. The interactions between the probe and the atoms composing the sample can be recorded and processed to form an image. These images are changing the way we see the micro/nano universe.