Microscopes and magnification

Light microscopes use lenses and visible light to form an image. They are cheap, easy to use, and can view living samples, but have a maximum useful magnification of about x1500 and relatively low resolution.

Electron microscopes use a beam of electrons instead of light. They give much higher magnification (up to x2,000,000) and much higher resolution, so far smaller structures — like organelles inside a cell — become visible. They are expensive, samples must be dead and placed in a vacuum, and images are produced in black and white.

There are two common types of electron microscope.

• A scanning electron microscope (SEM) scans a beam of electrons across the surface of a sample, producing detailed 3D-looking images of its surface.
• A transmission electron microscope (TEM) fires electrons through a very thin slice of sample. It gives higher resolution than an SEM and can reveal internal structures, such as organelles inside a cell, but the sample must be extremely thin.

Both need the sample to be dead and in a vacuum, unlike a light microscope.

Magnification is calculated with:

magnification = image size ÷ actual size

Rearranged, this also gives:

• actual size = image size ÷ magnification
• image size = actual size × magnification

All three values must be in the same unit before you divide or multiply — convert first, then calculate. Image size is usually measured with a ruler on a printed photo or on-screen image; actual size is the real, physical size of the specimen.

Microscopy uses very small units, so you need to convert confidently between them:

• 1 millimetre (mm) = 1,000 micrometres (µm)
• 1 micrometre (µm) = 1,000 nanometres (nm)
• 1 millimetre (mm) = 1,000,000 nanometres (nm)

To convert to a smaller unit, multiply by 1,000 (or 1,000,000 for mm to nm). To convert to a larger unit, divide. Most cells are measured in micrometres; most organelles and viruses are measured in nanometres.

1. Use a pipette to place a drop of water in the centre of a clean microscope slide.
2. Use forceps to peel a thin layer of epidermis (skin) from the inside of an onion layer.
3. Lay the epidermis flat in the water and add a drop of iodine solution to stain the cells.
4. Lower a coverslip onto the sample at an angle, using a mounted needle, to avoid trapping air bubbles.
5. Place the slide on the microscope stage and view under the lowest-power objective lens first, then increase magnification.
6. Focus using the coarse then fine focus wheel, and draw what you see, labelling the cell wall, nucleus and cytoplasm.