The short answer
A mirror is a smooth surface that reflects light so well that it forms an image. Most everyday mirrors are glass with a thin metal coating on the back. On a microscope, the mirror is the small tilting reflector under the stage. Its job is to catch light from a window or lamp and direct it up through the specimen and into the lenses.
Where the word comes from
“Mirror” came into English from Old French (mireor, mirour), which goes back to the Latin verb mirari, “to wonder at” or “to look at.” The same root gives us “admire” and “miracle.” So a mirror is, quite literally, a thing to look at yourself in.
How a mirror works
A mirror reflects light at the same angle it arrives. The shape of the surface decides what happens to the reflected light:
- Plane (flat) mirror: reflects light without converging or spreading it. It gives the familiar same-size image you see in a bathroom mirror.
- Concave mirror: curved inward like the inside of a spoon. It converges light toward a focus point. Shaving and makeup mirrors use this to magnify.
- Convex mirror: curved outward. It spreads light out and gives a wide, shrunken view. Car side mirrors and shop security mirrors use this.
What the mirror does on a microscope
Most specimens on a compound microscope are thin and see-through, and you view them by light passing up through them (transmitted light). Before built-in lamps were common, and for most of the history of the microscope, microscopes relied on a mirror to supply that light. You tilt it to catch light from a bright source and send it up through the hole in the stage.
Many student, vintage and field microscopes still use a mirror, since it needs no power. For how the mirror fits in with the condenser and diaphragm, see our guide to microscope illumination.
Plane side vs concave side
Microscope mirrors are often double-sided, and the two sides have different uses:
- Plane side: use this when the microscope has a condenser under the stage. The condenser is designed to take roughly parallel light and focus it onto the specimen itself.
- Concave side: use this when there’s no condenser. The curve concentrates the light onto the specimen, acting as a simple condenser.
Using the concave side together with a condenser usually gives uneven, overly concentrated light, so match the side to your microscope.
How to aim a microscope mirror
- Put the lowest-power objective in place, and open the diaphragm.
- Position the microscope facing a diffuse light source: a bright window with an overcast sky, or a desk lamp with a frosted bulb.
- Look through the eyepiece and tilt the mirror until the circle of light is bright and even.
- Place your slide and focus. Fine-tune the mirror angle if the field looks uneven.
Never aim the mirror at direct sunlight. Sunlight concentrated by the mirror and lenses can seriously damage your eyes.
Mirror vs built-in lamp
| Mirror | Built-in lamp | |
|---|---|---|
| Power | None needed | Mains, battery or USB |
| Brightness | Depends on surroundings; often weak at 400x | Consistent and adjustable |
| Color | Depends on the source | Fixed by the lamp (LEDs are usually near daylight) |
| Portability | Excellent for field use | Good with battery models |
| Best for | Low to medium power, outdoor or off-grid use | High power, photography, regular use |
A mirror works well at low magnification. At 400x and above, most people find a built-in LED much easier, because the light is brighter and more consistent. To see where the mirror sits among the other illuminating parts, see which are the illuminating parts of the microscope.