A mirror is an optical device which can reflect light. Usually, however, only those devices are meant where the angle of reflection equals the angle of incidence. This means that diffraction gratings, for example, are not considered as mirrors, although they can also reflect light.
Mirror surfaces do not need to be flat; there are mirrors with a curved (convex or concave) reflecting surface.
Dielectric Mirrors & Metallic Mirrors
Dielectric mirrors and metallic mirrors are two types of mirrors that differ in their reflective coatings and properties. Here are some of the main differences:
- Dielectric mirrors are composed of multiple thin layers of dielectric material, while metallic mirrors have a single layer of metal (such as aluminum, silver or gold) on a substrate.
- Dielectric mirrors offer higher reflectivity (over 99%) over a narrow spectral range (a few 100 nm), while metallic mirrors have lower reflectivity (around 90% to 98%) over a broad spectral range (from visible to infrared) .
- Dielectric mirrors have more durable coatings that are easier to clean and more resistant to laser damage, while metallic mirrors have softer coatings that are more susceptible to damage and require special care when cleaning .
- Dielectric mirrors are sensitive to polarization, angle of incidence, and phase dispersion, while metallic mirrors are insensitive to these factors . This makes dielectric mirrors suitable for applications that require high reflectivity and low loss, such as laser cavities and interferometers, while metallic mirrors are suitable for applications that require broad bandwidth and low dispersion, such as ultrashort-pulse lasers and general-purpose optics .