Brief description

A mirror is a polished surface that reflects light specularly, producing a virtual image of objects in front of it. It is an essential tool for personal care, scientific observation, and signaling.

Use / Function

Its practical purpose is to provide a visual reflection of objects and light:

Personal: Grooming, dressing, and self-inspection.
Optics: Used in reflecting telescopes, periscopes, microscopes, and lasers to redirect light.
Signaling: Heliographs use mirrors to send messages via reflected sunlight over long distances.
Energy: Concentrating solar power using parabolic mirrors to generate heat or electricity.
Decoration: Increasing the sense of space and improving interior lighting.
Scale: Domestic (personal mirrors), Industrial (telescopes, solar plants).

Glass
Silver
Tin
Obsidian
Bronze

Operating principle

It relies on the physical phenomenon of specular reflection:

Reflection: When light hits a smooth surface, it bounces off at the same angle it arrived (angle of incidence = angle of reflection).
Coherence: Unlike a rough surface that scatters light (diffuse reflection), an extremely smooth surface preserves the parallel nature of light rays, forming a clear image.
Virtual Image: The brain perceives the reflected rays as if they were coming from a point behind the mirror, creating a “virtual” version of the object.

How to create it

Polished Metal (Basic): Take a flat plate of bronze, silver, or copper. Use progressively finer abrasives (sand, silt, then fine cloth) to polish the surface until it becomes highly reflective.
Silvered Glass (Advanced):
1. Preparation: Thoroughly clean a flat sheet of glass.
2. Sensitization: Apply a tin chloride solution to help the metal stick to the glass.
3. Silvering: Pour a mixture of silver nitrate and a reducing agent (like glucose) over the glass. The silver precipitates out of the solution and bonds to the glass surface.
4. Protection: Once dry, paint the back of the silver layer with a dark, opaque paint to prevent oxidation and scratches.
Technical Level: Intermediate (polished metal) to Advanced (chemical silvering).

Materials needed

Essential:
- Metal: Silver, tin, or aluminum for the reflective layer.
- Substrate: Flat glass for modern mirrors or a solid metal plate for ancient ones.
- Chemicals: Silver nitrate, ammonia, and glucose (for the chemical silvering process).
Tools: Polishing cloths, fine abrasives, level surface for pouring chemicals.
Possible Substitutes: Still water in a dark container, polished obsidian, or highly polished stainless steel.

Variants and improvements

Ancient Mirrors: Made of solid polished bronze or obsidian. They required constant maintenance as they tarnished quickly.
Mercury-Tin Mirrors: (16th-19th century) A tin foil was coated with mercury to create an amalgam on glass. Very clear but highly toxic to make.
Silvered Mirrors: Modern standard using chemical precipitation of silver. See Silver Mirror for the detailed chemical process. More reflective and durable.
Aluminized Mirrors: Uses vacuum deposition of aluminum. Common in telescopes and industrial applications due to high durability and cost-effectiveness.

Limits and risks

Oxidation: The reflective metal layer (especially silver) can tarnish if exposed to air or moisture, causing dark spots.
Fragility: Glass-based mirrors are highly susceptible to impact and thermal shock.
Toxicity: Antique mirror-making processes involving mercury pose severe health risks (mercury poisoning).
Surface Quality: Any imperfection or scratch on the surface will distort the reflected image.