Value and the Reflecting Surface
When considering how to render reflections, it is necessary to differentiate between polished surfaces and mirrored surfaces and between nat and curved surfaces. Each surface has its own rules. Reflections in mirrors and metallic surfaces are unique in that they will show all the values of the reflected object.
On a glossy surface that is not metallic, only those areas of an object that are of lighter value than the reflecting surface will be seen in the reflection. All values darker than the reflecting surface will be seen as the value of the reflecting surface. For example, the reflection of an object resting on the surface of a polished white tabletop will show less of the object's detail than the reflection of the same object resting on a polished black tabletop. A polished gray tabletop will reflect everything from gray to white; all other values will be seen as the gray of the surface.
When the reflecting surface is transparent, its value is determined by what is seen through the transparency. Window glass is a reflecting surface but it has no value of its own. If what you see through the window is dark, then the reflection you see in the window will have many values and much detail. If what you see through the window is light, reflections in the window will show few values and little detail.
Reflections are transparent except when they are very light in value. When the reflection is a very bright object or a light source, the reflection is opaque. The pattern in a polished marble tabletop, for example, will be visible through all but the lightest-valued reflections on its surface.
Reflections in a mirror are different from reflections in polished surfaces. A mirror reflects all the values of an object; a polished surface reflects only those values that are lighter than its own.
The reflection of an object in a polished (nonmetallic) surface will include only the values that are lighter than the reflecting surface itself. Here, the light side of the black cube and the front of the white cube are clearly seen in the reflecting surface, which is a light gray. The reflection of the front of the black cube is the same light gray as the reflecting surface, so we cannot see it. The gray cube is slightly darker than the reflecting surface, so we don't see its reflection either.
A clear glass window is a reflecting surface but has no value of its own. The value of the things you see through the window will determine how much detail you see in a reflection in the window. The most detail appears in a reflection when there are dark values behind the window.
Reflections are transparent except when they are very light in value. Here, the pattern of the checker- board is visible except where the light bulb is illuminated and where the reflection is very light.