Microscopes and magnification
Light and electron microscopes
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.
The magnification calculation
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). Both sizes must be in the same unit.
- Image 24 mm, real cell 60 μm
- 24 mm = 24 × 1000 = 24 000 μm
- magnification = 24 000 ÷ 60 = × 400
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.
Required practical: looking at cells
- 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.
- Lower a coverslip at an angle so no air bubbles are trapped.
- Start on the lowest-power objective, 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.
To estimate a cell's size, count how many cells fit across the field of view and divide its width by that number.
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