Microscope

A **microscope** is an instrument used to examine objects that are too small to be seen by the naked eye.

The earliest known use of simple microscopes or magnifying glasses is in the 13th century. Galileo Galilei built an improved compound microscope in 1625 after various other inventers claimed to have made one. Giovanni Faber coined the name *microscope* for this device that Galileo had called "*little eye*".

### Compound microscopes

Around 1665, Robert Hooke’s used a compound microscope (a microscope with more than one lens) which was an important optical and mechanical forerunner of the modern microscope. Hooke’s book *Micrographia* featured breathtaking microscopic observations and biological speculation on specimens that ranged from plants to fleas. The book also discussed planets, the wave theory of light, and the origin of fossils. Hooke was first to discover biological cells and coined the word cell to describe the basic units of all living things. The word cell was motivated by his observations of plant cells that reminded him of “cellula,” which were the quarters in which monks lived.

### Light and optical microscopes

Antonie van Leeuwenhoek achieved up to 300 times magnification using a simple single lens microscope. He re-discovered red blood cells and spermatozoa, and helped popularize microscopes to view biological structures. In 1676, van Leeuwenhoek reported the discovery of micro-organisms. In 1893 August Köhler developed a key principle of sample illumination, Köhler illumination, which is central to achieving the theoretical limits of resolution for the light microscope. Also in the 1800s Carl Zeiss and Ernst Abbe revolutionized optical theory and the practical design of microscopes.

The modern optical microscope or light microscope uses visible light and a system of lenses to generate magnified images of small objects. The object is placed on a stage and may be directly viewed through one or two eyepieces on the microscope. A camera is typically used to capture the image.

The optical microscope has more recently evolved into the digital microscope. In addition to, or instead of, directly viewing the object through the eyepieces, a type of sensor similar to those used in a digital camera is used to obtain an image, which is then displayed on a computer monitor.

### Electron microscopes

In the early 20th century a significant alternative to the light microscope was developed, an instrument that uses a beam of electrons rather than light to generate an image in a much higher resolution. The German physicist, Ernst Ruska, working with electrical engineer Max Knoll, developed the first prototype electron microscope in 1931. The first commercial scanning electron microscope was developed by Professor Sir Charles Oatley in 1965.

The two major types of electron microscopes are transmission electron microscopes (TEMs) and scanning electron microscopes (SEMs). They both have series of electromagnetic and electrostatic lenses to focus a high energy beam of electrons on a sample. With a 100 nm level of resolution, organelle membranes and proteins such as ribosomes are seen. With a 0.1 nm level of resolution, detailed views of viruses (20 – 300 nm) and a strand of DNA (2 nm in width) can be obtained.

In 1970 Professor Albert Crewe used the scanning transmission electron microscope (STEM), which was a hybrid of TEM and SEM, to publish his *Visibility of Single Atoms* in the journal Science, which presented photographic evidence of atoms of uranium and thorium.

### Scanning probe microscopes

From 1981 to 1983 Gerd Binnig and Heinrich Rohrer studied quantum tunneling and created the scanning probe microscope (SPM) from quantum tunneling theory, that read very small forces exchanged between a probe and the surface of a sample. The probe approaches the surface so closely that electrons can flow continuously between probe and sample, making a current from surface to probe. They received the Nobel Prize in Physics for the SPM.

### Fluorescence microscopes

Fluorescence microscopy in biology led to techniques for fluorescent staining of cellular structures and this drove the development of the confocal microscope. In 1978 the first practical confocal laser scanning microscope was built.

### Super resolution laser microscopes

Structured illumination can improve resolution by around two to four times and techniques like stimulated emission depletion (STED) microscopy are approaching the resolution of electron microscopes. Stefan Hell was awarded the 2014 Nobel Prize in Chemistry for the development of the STED technique.

### X-ray microscopes

X-ray microscopes are instruments that use electromagnetic radiation usually in the soft X-ray band to image objects. They are often used in tomography to produce 3d images of objects like biological materials.

### Quantum microscopes

Laser microscopes face a major problem of intensity, using light billions of times brighter than sunlight on Earth. Quantum entanglement can be used with a less intense laser to produce the same microscope performance. This is done by concentrating photons into laser pulses that are a few billionths of a second long, producing entanglement that is 1,000 billion times brighter than ever before.

### Superlens

A superlens uses metamaterials to achieve resolution beyond the diffraction limit and may approach infinite resolution within reality, in order to one day image objects as small as molecules, atoms, and beyond.