Hollow vs. Solid Transparent Objects
Transparent objects come in two varieties: hollow, such as an empty glass jar, and solid, such as a glass jar filled with water. Because their walls are solid, hollow objects are subject to the same rules that govern the way we depict transparent solid objects, but the rules apply only to the hollow object's walls. When a jar is filled with water, the transparency of the glass and that of the water combine in such a way that the jar is perceived as if it were solid glass
There are two areas of light on a transparent solid the primary highlight, which appears just where it would on a nontransparent object; and a secondary highlight within the object, which appears opposite the primary highlight and is often brighter than it. This secondary highlight is one feature that indicates a transparent solid. Translucent objects will also have a secondary highlight.
A hollow transparent object, like the empty jar at left, is visible at its edges. What is seen through it remains relatively unchanged. One way we can tell that a transparent object is solid, like the water-filled jar at right, is by the way it distorts the image of what is behind it.
Transparent solid objects have two highlights: a primary highlight, which appears in the same place it would on any nontransparent object, and a secondary highlight, which appears within the object on the side opposite the primary highlight. The secondary highlight is one clue for identifying a solid transparent object.