Microscopes and magnification

Magnification is how many times bigger the image is than the real thing.

Resolution is how much detail you can see: the smallest gap between two points that still look like two points.

• Light microscope: cheap, small, works on living cells; magnifies up to about × 1500; shows nuclei but not ribosomes.
• Electron microscope: far higher magnification and resolution, so it shows ribosomes, the inside of mitochondria and viruses. It is expensive and large, and the specimen must be dead.

A light microscope's total magnification is the eyepiece times the objective: × 10 and × 40 gives × 400.

magnification = image size ÷ real size

There is no equation sheet in biology: learn it, and rearrange it with a triangle if that helps (real size = image size ÷ magnification, and image size = real size × magnification). Both sizes must be in the same unit.

1. Image 24 mm, real cell 60 μm

Tiny real sizes are often asked in metres in standard form: 2.5 mm image at × 50 000 gives 0.0025 m ÷ 50 000 = 5.0 × 10⁻⁸ m (50 nm).

Magnification has no unit. If you got × 0.4, you forgot to change the mm to μm.

Going the other way: a 6 μm nucleus at × 1500 gives an image 6 × 1500 = 9000 μm = 9 mm long.

A micrograph often has a scale bar: a line labelled with the real length it stands for. Measure the bar, then use it like any image: a bar 20 mm long labelled 5 μm gives magnification = 20 000 μm ÷ 5 μm = × 4000. A cell 36 mm long on the same micrograph is 36 000 ÷ 4000 = 9 μm long.

The field of view is the circle you see. It gets narrower as the magnification goes up: double the magnification and it is half as wide. A field 2 mm wide at × 40 is 2 × 40 ÷ 160 = 0.5 mm wide at × 160.

• Peel a thin layer of onion skin (light must get through) and lay it on a drop of water on a slide.
• Add a drop of iodine solution to stain it. A stain colours parts of the cell so they show up clearly: iodine makes the nucleus visible and turns starch grains blue-black.
• Lower a coverslip at an angle so no air bubbles are trapped.
• Start on the lowest-power objective (it shows a wider area, so the cells are easier to find), focus with the coarse wheel, then move to higher power and use only the fine wheel.
• Draw in pencil: clear outlines, no shading, labels with ruled lines, the magnification, and a scale bar if you can.

Safety: slides and coverslips are thin glass, so hold them by their edges and tell your teacher about any breakage. Iodine solution stains skin and stings eyes: wipe up drips straight away. A microscope with a mirror must never be pointed at the Sun.