A microscope is an instrument designed to magnify small objects, revealing details not visible to the naked eye. Its primary function involves increasing both the apparent size of a specimen and its resolution, which is the ability to distinguish between two closely spaced points. This capability allows scientists and researchers to explore the intricate structures of cells, microorganisms, and materials. Microscopes have been fundamental to breakthroughs across numerous scientific disciplines, including biology, medicine, and materials science.
Microscopes Using Light
Microscopes that utilize light operate by directing a beam of visible light through or onto a specimen, which then passes through a system of lenses to create a magnified image. This approach allows for the observation of living cells and biological processes in real-time.
Compound Light Microscope
A compound light microscope uses multiple lenses to achieve high magnification, often ranging from 40x to 1000x. Light from a built-in source passes through the specimen, then through objective lenses that collect the light and form an initial magnified image. This image is further magnified by the eyepiece lens, producing a two-dimensional view suitable for observing individual cells, bacteria, and thin tissue sections. These microscopes are commonly used in educational settings and for routine laboratory analysis.
Stereo Microscope
In contrast, a stereo microscope, also known as a dissecting microscope, offers lower magnification, usually between 5x and 80x. It provides a three-dimensional view of the specimen, achieved by using two separate optical paths, one for each eye. This design allows for depth perception and is useful for observing larger, opaque objects that do not require high magnification or transparency. Researchers use stereo microscopes for tasks such as dissecting biological specimens, examining circuit boards, or inspecting insects.
Microscopes Using Electrons
Microscopes employing electrons instead of light waves achieve significantly higher magnification and resolution, enabling the visualization of structures at a much finer scale. These instruments utilize a beam of electrons, which have much shorter wavelengths than visible light, to overcome the diffraction limits inherent in light microscopy. Electron microscopes operate in a vacuum to prevent electron scattering by air molecules, requiring specialized sample preparation.
Transmission Electron Microscope (TEM)
The Transmission Electron Microscope (TEM) functions by transmitting a high-energy beam of electrons through an extremely thin specimen. As electrons pass through the sample, some are scattered while others are transmitted, creating a two-dimensional image based on the electron density of different areas within the specimen. TEMs can achieve magnifications up to 2 million times, revealing the internal structures of cells, viruses, and atomic arrangements within materials. They are used in fields like virology, cell biology, and materials science.
Scanning Electron Microscope (SEM)
The Scanning Electron Microscope (SEM) operates by scanning a focused beam of electrons across the surface of a specimen. When the electron beam interacts with the sample’s surface, it produces various signals, including secondary electrons, backscattered electrons, and X-rays, which are then detected to create an image. This process generates a highly detailed, three-dimensional-appearing image of the specimen’s surface topography. SEMs are widely used for examining the surfaces of insects, microchips, and the intricate surface features of cells, with magnifications typically ranging from 10x to 500,000x.
Microscopes Using Probes
Scanning probe microscopes represent a distinct class of imaging tools that do not use lenses or electron beams to form an image. Instead, they rely on a physical probe that interacts directly with the surface of a sample to generate an image, often at atomic or molecular resolution. This direct interaction allows for unique insights into surface properties beyond simple topography. These microscopes are valuable for studying materials at the nanoscale.
Scanning Tunneling Microscope (STM)
The Scanning Tunneling Microscope (STM) operates by bringing an extremely sharp, electrically conductive tip very close to a conductive sample surface, typically within a few angstroms. When a small voltage is applied between the tip and the sample, electrons can “tunnel” across the tiny gap, creating a tunneling current. As the tip scans across the surface, a feedback loop adjusts its height to maintain a constant tunneling current, and these height changes are recorded to generate an atomic-resolution topographical map. STM is widely used in nanotechnology and surface science to image individual atoms and explore electronic properties.
Atomic Force Microscope (AFM)
The Atomic Force Microscope (AFM) uses a sharp tip attached to a flexible cantilever to scan a sample surface. Unlike STM, AFM measures the interatomic forces, such as van der Waals forces, between the tip and the sample, rather than an electrical current. As the tip scans, the cantilever bends in response to these forces, and this deflection is detected by a laser beam reflected off the cantilever onto a photodetector. A significant advantage of AFM is its ability to image both conductive and non-conductive surfaces in various environments, including air and liquid. AFM is applied to image biological molecules like DNA and proteins, polymers, and a wide range of material surfaces.