Microscopy & Magnification

The Light Microscope

A light microscope uses visible light and lenses to magnify specimens. The main parts are:

Eyepiece lens

The lens you look through. It usually magnifies by ×10.

Objective lenses

A set of lenses with different magnifications (for example ×4, ×10 and ×40). The lens closest to the slide gives the highest magnification.

Focus knobs

• Coarse focus – large adjustments to bring the image into rough focus.
• Fine focus – small adjustments to sharpen the image.

Stage

The flat platform that holds the slide in place.

Light source (or mirror)

Provides or reflects light through the specimen so it can be seen.

Magnification

Total magnification is found by multiplying the eyepiece magnification by the objective lens magnification.

To calculate size from a micrograph, use:

$I=A\times M$

• I = image size (what you measure on the photo or drawing)
• A = actual size (real size of the specimen)
• M = magnification

Rearrange as needed: $M=\frac{I}{A}$ or $A=\frac{I}{M}$. Always use the same units for image size and actual size before calculating.

Units & Conversions

Microscope measurements are often given in different units. Convert along the chain:

$\text{nm}\underset{÷1000}{\stackrel{\times 1000}{\rightleftarrows }}\mu \text{m}\underset{÷1000}{\stackrel{\times 1000}{\rightleftarrows }}\text{mm}\underset{÷10}{\stackrel{\times 10}{\rightleftarrows }}\text{cm}\underset{÷100}{\stackrel{\times 100}{\rightleftarrows }}\text{m}$

Example

$\mathrm{2,500,000}\text{ nm}=2500\mu \text{m}=2.5\text{ mm}=0.25\text{ cm}=0.0025\text{ m}$

Light Microscope vs Electron Microscope

Light microscope

• Uses visible light.
• Maximum useful magnification about ×1,500.
• Lower resolution, so less detail is visible.
• Can view living cells.

Electron microscope

• Uses a beam of electrons instead of light.
• Much higher magnification (up to about ×2,000,000).
• Higher resolution, so much finer detail is visible.
• Cannot view living cells – the specimen must be in a vacuum and is often stained or coated.