The University of Victoria in British Columbia has the world's most powerful microscope. It is a Scanning Transmission Electron Holography Microscope (STEHM) that weighs seven tons and has a height of 4.5 meters. This microscope can magnify up to 30 million times.
The history of microscopes can be traced back to the sixteenth century, when its crude form was invented. This instrument got more and more sophisticated with the advent in science and technology; and is now available in various types, which are used for different purposes. The most common among them is the oldest and simplest type, called the optical or light microscope, which comes in different types that include simple, compound, and dissecting microscopes. Electron microscopes are available as transmission electron microscopes (TEM), scanning electron microscopes (SEM), reflection electron microscopes (REM), scanning transmission electron microscopes (STEM), etc. Another type is scanning acoustic microscope, which makes use of sound waves to detect and measure variations in acoustic impedance.
What is a Microscope?
It is an instrument that is used to see those substances and organisms, which cannot be seen with the naked eye. The basic model of a microscope contains one or more lenses that make small objects look bigger, so that the details can be viewed. Nowadays, various advanced models of microscopes are being used for different purposes. Even though there are different types of microscopes, the three basic types are optical (light), electron, and scanning probe microscopes. Though microscopes are classified on the basis of different factors, the main difference between them lies in the mode of illumination or the means of image generation. In optical microscopes, light interacts with the sample and creates the magnified image. In case of electron microscopes, a beam of electrons is used. Given below is a brief overview about optical and electron microscopes.
As mentioned earlier, these are the oldest and simplest types of microscopes, which are otherwise known as light microscopes. They use visible light and a combination of lenses to enlarge the images of the samples.
A simple microscope bears a single lens or a combination of lenses, but magnifies objects to a certain limit only. In other words, high magnification is not possible with simple microscopes.
Compound Optical Microscope
In compound microscopes, the objective lens captures the light from the sample. It focuses this light to form an image of the sample inside the microscope. The user views the image through another set of magnifying lens called eyepieces. Compound microscopes provide high magnification and their maximum limit is 1000x. These microscopes are mainly seen in biology classrooms, and are used for studying plant and animal cells, pathogens, etc. They are also used in forensic labs. Though high magnification is possible with compound microscopes, they have a low resolution.
Stereo or Dissecting Microscope
This device provides a three-dimensional image of the sample. Though useful for observing large, opaque specimens; this microscope is not capable of high magnification. It can magnify up to 100x only. Unlike a compound microscope, you may not be able to view individual cells using a stereo microscope. However, it is better than the former for viewing large, thick objects. This microscope is used for science dissection projects, for inspection of circuit boards, and for watch repair.
Mainly used by research organizations, confocal microscopes use laser light and glass lenses with dichromatic mirrors to create three-dimensional images of the sample. When compared to a conventional microscope, a confocal one has confocal pinholes. These pinholes are positioned in such a way that only the light from the plane of focus falls on the detector. So out-of-focus lights are removed, and this enhances the clarity of the images to a great extent. The laser light scans the sample in depth and the images are displayed on a digital computer screen. The advantage of this device is that it produces thin optical sections of thick specimens. Confocal microscopes are mainly used for biomedical research.
An electron microscope uses a beam of highly energetic electrons to illuminate the sample. As compared to the optical ones, electron microscopes produce highly magnified images with finer details. Electrostatic and electromagnetic lenses are used in electron microscopes. These lenses control the electron stream and focus it to a specific plane of the sample. This method is similar to that of an optical microscope, which uses light to focus on the specimen.
Transmission Electron Microscope
The traditional version of the electron microscope, called transmission electron microscope, makes use of high voltage electron beam for forming two dimensional images. Usually, thin slices of the specimen are used for best results. Some parts of the sample may allow the beam to pass through, while others scatter the same. The beam that emerges from the sample, is magnified by the lens system. Sometimes, a monitor is also attached to this device. This microscope is capable of high magnification. It offers high resolution too. So this device is used in nanotechnology.
Scanning Electron Microscope
This device does not allow the electron beam to pass through the specimen, but the beam scans the surface of the sample and measures the rate of reflection to form the image. This microscope produces three-dimensional images. They are also capable of high magnification and resolution.
There are two other types called the reflection electron microscope and the scanning transmission electron microscope. The reflection electron microscope uses the reflected electrons from the specimen to form the image. Usually, techniques like reflection high energy electron diffraction (RHEED) and reflection high-energy loss spectroscopy (RHELS) are used in these microscopes. In case of a scanning transmission electron microscope, a highly focused electron probe raster-scans a thin specimen, and images are formed on the basis of the scattering pattern of electrons.
Scanning Probe Microscopes
As the name rightly suggests, a scanning probe microscope has a physical probe that scans the surfaces of the specimen and sends information to a computer that collects the data and generates images on the basis of that data. This microscope is so powerful that it can create images with a resolution of less than one nanometer. Unlike an optical microscope, the surface of the specimen is not seen by the user. The probe moves over the sample several times and collects information. There are different types of scanning probe microscopes. They include atomic force microscopes, magnetic force microscopes, scanning tunneling microscopes, scanning thermal microscopes, scanning voltage microscopes, etc.
This is only a brief overview about the common types of microscopes. Advanced versions like digital microscopes, USB computer microscopes and pocket microscopes are also getting popular. Acoustic microscopes used sound waves for imaging. They are mainly used for finding faults in samples during the manufacturing process. A microscope is deemed fit for a particular task, based on its illumination method, the type of image it produces, magnification, depth of field, and resolution.