An object is visible if it is either reflecting light or a light source itself. Furthermore, a reflecting object can have specular reflection (glossy finish, mirror-like), diffuse reflection (matte), or the combination of both.
Some flat-Earthers like to compare the appearance of the Moon to a glossy, metallic ball. Then, the difference in how the two objects look is taken as “proof” that the Moon is not reflecting light. They are wrong. A shiny metallic ball reflects almost all incoming light in mirror-like fashion, or that it has predominantly specular reflection. On the other hand, the Moon reflects incoming light to every direction, or that it has only the diffuse reflection.
Specular reflection is the mirror-like reflection. Incoming light is reflected at the same angle. Such reflection can be seen on objects like a water surface, most automotive paint, glossy photo papers, and mirrors.
Diffuse reflection is the reflection of light such that incoming light is scattered at many angles. Diffuse reflection occurs on objects like road surface, most wall paints, and papers.
An object can have both diffuse and specular reflection. Most automotive paint finish is specular, but it also has a diffuse reflection. The surface does not perfectly reflect all incoming light at the same angle as a mirror does. Ocean surface has ripples, and thus cannot be perfectly specular. Sunglint is the result of specular reflection on the ocean’s surface. It is only visible at a sufficient height where the local variation of the ocean’s surface is small enough compared to the height of the observer.
References
- Specular reflection – Wikipedia
- Diffuse reflection – Wikipedia
- Glossy – Wikipedia
- Paint sheen – Wikipedia