They are read and written by lasers, but mirrors are components.

Lasers can be used to treat wrinkles by a laser resurfacing procedure. It can be done with two types of lasers: one that removes thin layers of skin and the other type that stimulates collagen growth.

what is the thin gas and clusters of stars surrounds the disk of the Milky Way Galaxy.

For eye surgery they are specially useful. Since lasers are very focused and thin, but still have heaps of energy, they can be used to cut thin layers of a patient's cornea, if you have myopy for instance, with lots of precision.

the beam of light is very weak

Information is stored in the computer's hard drive, which is a magnetic disk read by lasers.