Microscopes

A light microscope shines light up through a thin specimen and uses two lenses to make an image that looks bigger. Total magnification = eyepiece × objective.

Part	Job
Eyepiece lens	The lens you look through. It magnifies the image again, usually ×10.
Body tube	Holds the eyepiece the right distance above the objective lens, so the two lenses line up.
Nosepiece	Turns to click a different objective lens into place.
Objective lens	The lenses nearest the slide, usually ×4, ×10 and ×40. Always start with the lowest.
Stage clips	Hold the slide still on the stage.
Stage	The platform the slide sits on, with a hole for light to come up through.
Diaphragm	Controls how much light reaches the specimen.
Light source	Shines light up through the specimen. Older microscopes use a mirror instead.
Arm	Joins the top of the microscope to the base. Carry the microscope by the arm.
Coarse focusing wheel	Each turn moves the stage a long way. Use it on low power to get the image roughly in focus.
Fine focusing wheel	Each turn moves the stage a tiny way, for a sharp image. The only focusing wheel to use on high power.
Base	The heavy bottom that keeps it steady. Support it with your other hand when you carry it.

How to focus:

1. Put the slide on the stage, hold it with the clips and switch the light on.
2. Turn the nosepiece to the lowest power objective lens.
3. Looking from the side, use the coarse wheel to bring the stage up close to the lens.
4. Look through the eyepiece and turn the coarse wheel slowly, moving the stage down, until the image appears.
5. Use the fine focusing wheel to make the image sharp.
6. Switch to a higher power objective lens and refocus with the fine wheel only.

Focusing makes the image clearer. A higher power lens makes it bigger, shows a smaller area and lets less light through. The image is upside down and back to front. Carry the microscope by the arm, with a hand under the base.

Scientific drawings: a sharp pencil; smooth, unbroken lines; no shading or colouring; labels with straight lines drawn with a ruler; a title and the magnification.

Stretch A light microscope shows cells, nuclei and chloroplasts. Ribosomes and viruses are far too small: an electron microscope magnifies over a million times, but it is huge, expensive, and the specimen must be dead.