Microscopy · GCSE Biology

Light and electron microscopes

Light microscopes use light and lenses. They are cheap, easy to use and can show living cells, but only magnify up to about ×2000.
Electron microscopes use a beam of electrons. They have a much higher magnification (up to about ×2 000 000) and a much higher resolution.
Electron microscopes let scientists see sub-cellular structures in detail, such as mitochondria, ribosomes and plasmids.
A transmission electron microscope (TEM) gives flat, detailed images of the inside of cells; a scanning electron microscope (SEM) gives 3D images of surfaces.

Magnification is how many times bigger the image is than the real object.
Resolution is the ability to tell apart two points that are close together; higher resolution gives a sharper, more detailed image.
Magnification = size of image ÷ size of real object. Rearranged: real size = image size ÷ magnification.
Always use the same units for image size and real size before dividing.

1 millimetre (mm) = 1000 micrometres (µm); 1 µm = 1000 nanometres (nm).
1 µm = 1 × 10⁻⁶ m and 1 nm = 1 × 10⁻⁹ m.
Cells are usually measured in micrometres; ribosomes and viruses in nanometres.
Standard form writes very big or small numbers as A × 10ⁿ, where A is between 1 and 10.

Put a thin layer of the specimen (such as onion epidermis) on a slide and add a drop of water or stain, like iodine, to make structures visible.
Lower a coverslip at an angle to avoid trapping air bubbles.
Start with the lowest-power objective lens, focus with the coarse focusing wheel, then use the fine focusing wheel.
Switch to a higher-power objective and refocus to see more detail.
Draw what you see with clear, unshaded pencil lines, label structures and write the magnification. A scale bar shows the real size.

Total magnification = eyepiece lens magnification × objective lens magnification.
Example: a ×10 eyepiece with a ×40 objective gives a total magnification of ×400.

Magnification: How many times larger an image is than the real object: image size ÷ real size.
Resolution: The ability to distinguish between two points that are close together; the smallest detail that can be seen.
Light microscope: A microscope that uses light and glass lenses; it can view living cells up to about ×2000.
Electron microscope: A microscope that uses a beam of electrons; it has much higher magnification and resolution.
Micrometre (µm): One thousandth of a millimetre (1 × 10⁻⁶ m).
Nanometre (nm): One thousandth of a micrometre (1 × 10⁻⁹ m).
Objective lens: The lens nearest the specimen; changing it changes the magnification.
Eyepiece lens: The lens you look through, usually ×10.
Stain: A dye, such as iodine or methylene blue, that makes parts of a cell easier to see.
Standard form: A way of writing numbers as A × 10ⁿ, where 1 ≤ A < 10.