Microscope

A microscope is an instrument used to examine objects and structures too small to be distinguished clearly by the unaided eye. Different microscopes form images using light, beams of electrons, or interactions between a fine probe and a specimen. They reveal features ranging from cells and microorganisms to nanoscale surfaces. Microscopy encompasses both the instruments and the methods used to prepare specimens, acquire images, and interpret their information. (microscopyu.com)

Historical development

Microscopes became important scientific instruments during the seventeenth century. In 1665, Robert Hooke published Micrographia, illustrating observations made with a compound microscope. His description of the compartments in cork introduced the term “cell.” Antonie van Leeuwenhoek used small, single-lens instruments to investigate previously unseen organisms, helping establish the observational foundations of microbiology. Improvements in lens construction during the nineteenth century made microscopic observations more detailed and reliable. (blog.sciencemuseum.org.uk)

A major change followed the development of electron optics. Ernst Ruska’s early instruments demonstrated electron-optical imaging in 1931, opening a route beyond the resolution of conventional light microscopy. In 1981, IBM researchers introduced the scanning tunneling microscope, which examined surfaces through a probe–specimen interaction rather than a conventional image-forming lens. These developments expanded microscopy from visible-light observation into atomic-scale investigation. (nobelprize.org)

Magnification and resolution

Magnification describes how much larger an image appears than its object. Resolution describes the ability to distinguish neighboring details. Enlarging an unresolved image does not reveal additional structure; this is called empty magnification. Image usefulness therefore depends on resolution, contrast, and the amount of detectable signal, not simply on the stated magnification. (microscopyu.com)

In conventional optical microscopy, diffraction limits the separation at which nearby features can be distinguished. A commonly used estimate of lateral resolution is

where $d$ is the resolvable distance, $\lambda$ is the wavelength, and numerical aperture is $\mathrm{NA}=n\sin\theta$. Here, $n$ is the refractive index of the medium and $\theta$ is the collection cone’s half-angle. Shorter wavelengths and higher numerical apertures generally improve resolution. Ordinary high-performance visible-light microscopy resolves lateral details on the order of 200 nanometres; the exact limit depends on the imaging conditions and resolution criterion. (microscopyu.com)

Optical microscopes

A simple microscope uses a single magnifying lens. A compound optical microscope uses an objective to form an enlarged intermediate image and an eyepiece to magnify it for observation. Many modern systems place a tube lens between the objective and intermediate image. A camera can record the image instead of, or alongside, visual observation. Other essential components include illumination, a specimen stage, focusing mechanisms, and, in transmitted-light instruments, a condenser. (microscopyu.com)

Different arrangements emphasize different specimen properties. Bright-field microscopy records variations in transmitted light, often aided by staining. Phase-contrast microscopy converts phase differences caused by transparent specimens into intensity differences, making unstained living cells easier to distinguish. Differential interference contrast uses optical interference to emphasize variations in optical path length. These techniques create contrast from different physical properties rather than merely enlarging the same view. (microscopyu.com)

Fluorescence microscopy detects light emitted after excitation. Filters separate the excitation light from the weaker emitted signal, allowing selected structures to stand out against a dark background. Fluorescent labels can identify particular proteins or cellular components. Confocal microscopy rejects much of the out-of-focus light with a spatial aperture, producing optical sections that can be assembled into three-dimensional representations. (microscopyu.com)

Electron microscopy

An electron microscope uses accelerated electrons and electromagnetic lenses. Electrons have wavelengths much shorter than visible light, making substantially finer resolution possible, although lens aberrations, specimen properties, and operating conditions also constrain performance. (jeol.com)

A transmission electron microscope (TEM) forms images from electrons passing through a sufficiently thin specimen, revealing internal structure. A scanning electron microscope (SEM) scans a focused beam across the specimen and records signals such as secondary or backscattered electrons. SEM images commonly emphasize surface form; detectors can also collect characteristic X-rays for elemental analysis. Conventional operation requires vacuum, and biological specimens often undergo fixation, dehydration, sectioning, or coating. Low-vacuum and cryogenic methods permit different preparation conditions. (jeol.com)

Cryo-electron microscopy examines specimens preserved at low temperature, commonly in vitreous ice. Combining images of many particles can reconstruct the three-dimensional structures of biological molecules, including protein complexes. Its development was recognized by the 2017 Nobel Prize in Chemistry. (nobelprize.org)

Scanning probes and super-resolution

A scanning tunneling microscope maps electrically conducting surfaces by measuring a tunneling current between a sharp tip and the sample. An atomic force microscope measures tip–sample forces through the response of a cantilever and can operate under fluids. These methods generate maps of surface properties rather than ordinary optical photographs and can provide atomic-scale information under suitable conditions. (research.ibm.com)

Super-resolution microscopy comprises optical techniques that distinguish features beyond conventional diffraction-limited imaging. Stimulated-emission depletion uses controlled fluorescence suppression to restrict the emitting region. Single-molecule localization methods estimate the positions of sparsely emitting labels across successive images. Their performance depends on labeling, detected photons, specimen motion, and reconstruction assumptions; exceeding the conventional limit does not mean unlimited resolution. (nobelprize.org)

Specimens and image interpretation

Microscopic images represent specific physical signals, not necessarily an object’s natural appearance. Fluorescence intensity reflects emission, SEM brightness depends on detected electron signals, and probe images reflect tip–surface interactions. Display colors may be assigned computationally. Preparation can alter specimens, while illumination or electron irradiation can cause damage. Meaningful interpretation therefore connects the image to its acquisition method, specimen preparation, and spatial scale. (microscopyu.com)