Microscopes and magnification · Year 8
Cells are far too small to see with the naked eye, so we use microscopes. A light microscope uses lenses to make an image bigger, up to about ×1,500. An electron microscope uses a beam of electrons and can magnify far more and show much finer detail. One formula links the size of the image, the real size of the object and the magnification.
Key facts
- Parts of a light microscope: eyepiece lens, objective lenses, stage, light source, and the coarse and fine focusing knobs.
- Total magnification = eyepiece magnification × objective magnification: a ×10 eyepiece and a ×40 objective give ×400.
- Magnification = image size ÷ real size. Rearranged: real size = image size ÷ magnification, and image size = real size × magnification.
- Use the same units for the image and the real size. 1 mm = 1,000 µm (micrometres) and 1 µm = 1,000 nm (nanometres).
- Resolution is the smallest distance between two points that can still be seen as separate. Electron microscopes have a much higher resolution, so they show ribosomes and the inside of mitochondria.
- Light microscopes are cheap and can show living cells in colour; electron microscopes need the specimen to be dead and in a vacuum.
- To make a slide: take a thin layer of cells, add a stain (iodine for onion cells, methylene blue for cheek cells), and lower a coverslip at an angle so no air bubbles are trapped. Start with the lowest-power objective.