A modern optical microscope A microscope is an instrument that magnifies very small objects, allowing the viewer to see detail in the object that is invisible to the naked eye. Microscopes are used mostly—but not only—in biology and medical research. There are two main types of microscope: optical microscopes and electron microscopes. In an optical microscope, the image of the object is created by light. In an electron microscope, the image is created by a beam of tiny subatomic particles called electrons. Microscopes were first used for scientific research in the 17th century. The first electron microscopes appeared in the 1930s.
In an optical microscope, the image of the object is created by light. The simplest optical microscope is a magnifying glass, which contains a single lens. The lens gathers and bends light coming from the object, making the object look larger than it really is. Compound microscopes have more than one lens. A standard compound microscope has two groups of lenses. The first group, called the objective, gathers light from the object and focuses it to create a magnified image of the object. The second group, called the eyepiece, magnifies this image.
Parts of an optical microscope
The eyepiece of a microscope is a cylinder containing two or more lenses. Typical magnifications for eyepieces are 2 times, 10 times and 50 times. The eyepiece brings the image into focus on the eye. The objective is another cylinder containing a compound lens. There will usually be three or four objective lenses fixed to an objective turret, or revolving nosepiece. This can be rotated manually to select one of the objectives, and thus obtain different magnifications. Some objective lenses on research microscopes have a magnification of 200 times.
The magnification of a compound optical microscope is the product of the magnification of its eyepiece and objective lens (e.g. 10 x 200 = 2000).
The platform below the objective where the specimen, fixed to a transparent slide, is placed is called the stage. A hole in the centre allows light to pass through it to illuminate the specimen from below. The eyepiece or objective barrel is moved up and down for focus by using adjustment knobs: one for coarse and one for fine focusing. Most microscopes have their own light source, usually a halogen lamp, LED or laser. The light is focused on to the specimen by a condenser.
Micrograph of a snow crystal
The image from an optical microscope can be photographed to produce a micrograph. In a digital camera, a charge-coupled device (CCD) allows the capture of digital images. A digital microscope is a microscope fitted with a digital camera which allows observation of a specimen using a computer: the image is shown directly on a computer screen without the need for an eyepiece.
The first compound microscope was probably built by Dutch spectacle-maker Zacharias Janssen in about 1590. Early microscopes had poor-quality lenses and gave blurred images. In the 1670s another Dutchman, Anton van Leeuwenhoek, began making simple, single-lens microscopes. He was the first person to see micro-organisms, such as bacteria and amoebae.
Microscopes for research
Scientists first began to use microscopes for research in the 17th century. English scientist Robert Hooke (1635–1703) published a book called Micrographia in 1665 which contained a series of accurate drawings of specimens he had studied under a microscope, including insects, fungi and snowflakes. Using his microscope he was able to discover, for example, how nettles sting and the existence of cells—the building blocks from which all living things are made. By the 19th century, microscopes were much improved, with a far greater degree of magnification. Scientists already suspected that there was a connection between "germs" (micro-organisms such as bacteria) and disease. French scientist Louis Pasteur (1822–1895) was able to observe bacteria through a microscope and prove beyond doubt that bacterial infection caused diseases in both animals and humans.
Louis Pasteur studying bacteria under a microscope
Consultant: Chris Oxlade
Zacharias Janssen builds the first compound microscope.
English scientist Robert Hooke publishes Micrographia, a collection of his observations using a microscope.
Anton van Leeuwenhoek develops a high-precision microscope.
The first electron microscope is built by Ernst Ruska and Max Knoll. It is a transmitting electron microscope (TEM).
The first scanning electron microscope (SEM) is built by Ernst Ruska.
German physicist Gerd Binnig and Swiss physicist Heinrich Rohrer develop the first scanning tunnelling electron microscope (STM).
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