Definition of Magnification in Microscopy

Objective Lens of Microscope
••• Image by, courtesy of Matthew Hine

A microscope has one basic purpose: to make objects that are very tiny in relation to the human eye appear larger, usually for the purpose of learning more about whatever is being studied or teaching others to do the same. (A telescope has a similar purpose in that it makes objects that look very tiny or can't be seen at all appear bigger; they do so, however, by in effect making large, very distant objects appear to be closer to you instead of magnifying objects in the same physical space.)

One magnification definition is "the process of making large," which is taken almost straight from the Latin; an idea that more properly captures magnification's meaning is "appearing to make something larger without actually doing so." But apart from magnification's specific definition as used in microscopy, the various instruments that classify as microscopes today feature combinations of lenses that allow users to achieve the necessary visualization.

Magnification: Definition and Related Terminology

Consider a very tiny yet extremely bright object, like an atom glowing at its maximum fluorescence (light that results from collisions with high-energy electromagnetic waves). You might be able to see it in some sense under a microscope, but you would not be able to make out any features or even necessarily place it precisely in space.

Resolution refers to the ability to discriminate between (i.e., visually separate) two adjacent objects. A resolution level in optics refers to the number of distinct pixels (picture elements) in a given area, such as dots per square inch.

Magnification, instead, is about details, typically those you could never see with the unaided eye simply because your eye is so large compared to things like molecules, bacteria and viruses. Using a magnifying device is akin to walking closer and closer to a sign and being able to make out more of the words and pictures as you approach.

Types of Microscopes

There are two basic kinds of light microscopes, the name given to microscopes that possess their own illumination source (most modern units do). Simple microscopes were the first microscopes manufactured, and these consist of a single, usually hand-held lens that curved outward on one or both sides. A compound microscope makes use of two lenses (or lens systems).

In a compound microscope, one of the lens systems forms an enlarged image of the object; the second lens system magnifies the image formed by the first lens. In the modern compound microscope, the two lens systems are the objective lens and the ocular (eyepiece) lens.

Levels of Magnification in Compound Microscopes

In most microscopes, the objective lens system offers more than one level of magnification. For example, by rotating a plate that puts different objective lenses on the user's viewing area, the objective magnification might be 4x, 10x or 100x. This simply means that the images created are 4, 10 and 100 times the size of the object itself.

The eyepiece lens is usually 10x, and there are often no other options. The total magnification gained in a compound microscope is just the product of the objective and eyepiece lens magnification values. So if you were looking at a specimen with an objective lens of 40x using a 10x eyepiece, the total magnification of the object would therefore be 10 times 40, or 400x.

A circular specimen with an actual diameter of 0.01 mm (1 × 10-5 m), far smaller than a period on a printed page, would appear 400 times larger using this level of magnification, making it look like a 4-cm-wide object (about 1.6-inch wide) from the same distance.

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