What is Magnification and resolution? — Biology

Definition 2.4 High School Biology · Chapter 2 — Cells: The Common Unit of Life

The magnification of an instrument is the ratio of the size of the image to the size of the object. Its resolution is the smallest distance between two points that it still shows as separate. Magnifying an image beyond what the resolution justifies enlarges the blur, not the detail.

Sizes on a logarithmic scale: each step is ten times larger. The naked eye resolves about 0.1\, mm; the light microscope reaches 0.2\, µ m and sees whole cells and bacteria; the electron microscope resolves nanometres and shows the parts inside. — Sizes on a logarithmic scale: each step is ten times larger. The naked eye resolves about $0.1\,\mathrm{mm}$ ; the light microscope reaches $0.2\,\text{µ}\mathrm{m}$ and sees whole cells and bacteria; the electron microscope resolves nanometres and shows the parts inside.

Examples

Example 2.7 (Cheek cells)

A cheek cell drawn $24\,\mathrm{mm}$ wide from a $\times 400$ image is $24/400 = 0.06\,\mathrm{mm} = 60\,\text{µ}\mathrm{m}$ across, and its nucleus, drawn $3\,\mathrm{mm}$ , is about $8\,\text{µ}\mathrm{m}$ . A red blood cell is $7\,\text{µ}\mathrm{m}$ ; the bacteria of the mouth, $1\,\text{µ}\mathrm{m}$ long, appear as barely resolved dots at the same magnification.

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Definition 5.8 University Biology — Year 1 · Chapter 5 — The Cell: Unit of Life

A microscope magnifies (enlarges the image) and resolves (separates neighbouring points). Only the second limits what can be seen: the resolving power is the smallest distance $d$ between two points that still appear distinct,

d = \frac{0.61\,\lambda}{\mathrm{NA}},

where $\lambda$ is the wavelength of the illumination and $\mathrm{NA}$ the numerical aperture of the objective (the sine of its half-angle of light collection times the refractive index of the medium, at most about $1.4$ with oil). Magnifying beyond the resolving power enlarges the blur, not the detail.

The compound light microscope: the objective forms an enlarged, inverted intermediate image, which the eyepiece magnifies again for the eye. The objective’s numerical aperture, not the total magnification, sets what can be resolved.

A compound light microscope: lamp and condenser below the stage, a turret of objectives above it, binocular eyepieces. Resolution 0.2\, µ m, magnification up to 1000\,, living specimens possible. — A compound light microscope: lamp and condenser below the stage, a turret of objectives above it, binocular eyepieces. Resolution $0.2\,\text{µ}\mathrm{m}$ , magnification up to $1000\,$ , living specimens possible.

Examples

Example 5.11 (A bacterium and a mitochondrion)

Under the light microscope at $1000\times$ , an E. coli cell is a rod $2\,\mathrm{mm}$ long on the retina’s scale, with no visible interior; a mitochondrion is a dot at the limit of resolution. In a TEM section the same bacterium shows its two membranes, its wall, its nucleoid as a pale fibrous region and its ribosomes as dark grains; the mitochondrion shows its double membrane and the folds of the inner one. Neither is alive.

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