Here's why:
* Highly reflective: Both silver and aluminum are excellent reflectors of light. They have a high reflectivity in the visible spectrum, which is why they make good mirrors.
* Durable: The coating needs to be durable to withstand handling and cleaning. Silver and aluminum, when applied in a thin layer, are strong enough for this purpose.
* Cost-effective: While other materials like gold can be used, they are much more expensive. Silver and aluminum are more affordable options.
The process:
The reflective coating is usually applied to the back of the glass through a process called vacuum deposition. This involves heating the silver or aluminum in a vacuum chamber, causing it to evaporate and coat the surface of the glass. A thin protective layer of paint or lacquer is often applied over the reflective coating to prevent tarnishing and oxidation.
Note:
* Some mirrors might use other reflective coatings, such as gold (for specialty applications like astronomical mirrors) or dielectric coatings (for specific wavelength reflection).
* The back of some mirrors, particularly older ones, might have a copper coating instead of silver. However, copper is less reflective and more prone to tarnishing than silver.