What is the red paint stain on the back of the mirror?

The plated mirror is of poor quality and corroded by oxidation. The quality of plated mirrors is different. The mirror in the bathroom is easy to be oxidized and blackened after a long time. After a long time, it will be like the phenomenon in your photo, with black spots in the middle.

In English, a mirror is an object with a smooth surface and can reflect light. At first, the ancients used polished bronze as mirrors.

There are two kinds of mirrors: flat mirror and curved mirror. Flat mirrors are often used to tidy up the appearance. Curved mirrors are divided into concave mirror and convex mirrors, which are mainly used as dressing mirrors, furniture accessories, architectural decoration parts, optical instruments, solar cookers, reflectors for headlights and searchlights, reflecting telescope, car rearview mirrors, etc.

Application:

In science, mirrors are often used in telescopes, lasers, industrial instruments and other instruments. Metal devices with regular reflective surfaces and glass or metal products with metal reflective films are often inlaid with metal, plastic or wooden frames.