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  • Investigate different types of microscopes.
  • Learn how image is formed in a compound microscope.

Although the eye is marvelous in its ability to see objects large and small, it obviously has limitations to the smallest details it can detect. Human desire to see beyond what is possible with the naked eye led to the use of optical instruments. In this section we will examine microscopes, instruments for enlarging the detail that we cannot see with the unaided eye. The microscope is a multiple-element system having more than a single lens or mirror. (See [link] ) A microscope can be made from two convex lenses. The image formed by the first element becomes the object for the second element. The second element forms its own image, which is the object for the third element, and so on. Ray tracing helps to visualize the image formed. If the device is composed of thin lenses and mirrors that obey the thin lens equations, then it is not difficult to describe their behavior numerically.

Image of a man viewing through the ocular lens and with his hand on the fine adjustment of the microscope.
Multiple lenses and mirrors are used in this microscope. (credit: U.S. Navy photo by Tom Watanabe)

Microscopes were first developed in the early 1600s by eyeglass makers in The Netherlands and Denmark. The simplest compound microscope    is constructed from two convex lenses as shown schematically in [link] . The first lens is called the objective lens    , and has typical magnification values from to 100× . In standard microscopes, the objectives are mounted such that when you switch between objectives, the sample remains in focus. Objectives arranged in this way are described as parfocal. The second, the eyepiece    , also referred to as the ocular, has several lenses which slide inside a cylindrical barrel. The focusing ability is provided by the movement of both the objective lens and the eyepiece. The purpose of a microscope is to magnify small objects, and both lenses contribute to the final magnification. Additionally, the final enlarged image is produced in a location far enough from the observer to be easily viewed, since the eye cannot focus on objects or images that are too close.

A ray diagram from left to right shows a virtual inverted enlarged final image of the object, a small object in upright position, a convex objective lens, inverted smaller image of the object, a large convex eye-piece and an eye on an optical axis. The object h’ is placed just outside F subscript O two, the principal focus of the objective lens. Rays from the object are passing through the objective lens, converging and forming an inverted magnified image h subscript I, which acts as an object for the eyepiece and passing at the eye. Dotted lines are joined backward from the rays entering the eyepiece at the tip of the virtual, magnified, inverted and final image of the object given as h subscript i. Distance of the object for the objective lens and distance of the image from it is given as d subscript o and d subscript I respectively.
A compound microscope composed of two lenses, an objective and an eyepiece. The objective forms a case 1 image that is larger than the object. This first image is the object for the eyepiece. The eyepiece forms a case 2 final image that is further magnified.

To see how the microscope in [link] forms an image, we consider its two lenses in succession. The object is slightly farther away from the objective lens than its focal length f o size 12{f rSub { size 8{o} } } {} , producing a case 1 image that is larger than the object. This first image is the object for the second lens, or eyepiece. The eyepiece is intentionally located so it can further magnify the image. The eyepiece is placed so that the first image is closer to it than its focal length f e size 12{f rSub { size 8{e} } } {} . Thus the eyepiece acts as a magnifying glass, and the final image is made even larger. The final image remains inverted, but it is farther from the observer, making it easy to view (the eye is most relaxed when viewing distant objects and normally cannot focus closer than 25 cm). Since each lens produces a magnification that multiplies the height of the image, it is apparent that the overall magnification m size 12{m} {} is the product of the individual magnifications:

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Source:  OpenStax, College physics. OpenStax CNX. Jul 27, 2015 Download for free at http://legacy.cnx.org/content/col11406/1.9
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