Creating the Illusion of Transparency
You can create the illusion of transparency in your drawings if you observe how light, shadow, and value behave on transparent objects under various conditions. For instance, both hollow and solid transparent objects have a primary highlight, and a secondary highlight opposite it. The difference is, on a solid transparent object the secondary highlight appears within the object, while on a hollow transparent object the secondary highlight is seen only in the object's walls.
Value contrasts are lessened when seen through transparent objects. For example, when you look through a window, the light values you see will be slightly darker and the darks slightly lighter than if the window was not there. A clean window lets you see stronger value contrasts than a dirty one.
The cast shadow of a hollow transparent object is usually a restatement of the object's transparency: transparent in the middle and opaque at the edges. We see darker cast shadows at the edges of a hollow transparent object because there, at its walls, the object is thickest and blocks more light.
The cast shadow of a transparent solid has a bright spot of light within it, caused by the secondary highlight that appears on the side of the object opposite the primary highlight.
The cast shadow of a hollow transparent object is light in the middle and dark at the edges.
The secondary highlight on a solid transparent object causes a bright area of light within the object's cast shadow.
Value contrasts are reduced when viewed through a transparent object. Here, a pane of glass floats in front of the curving stripes. We don't see the glass itself, but can see its effect in the reduction of contrast between the black and white stripes.
Each of the following elements will imply transparency. The more you can include in your drawing, the stronger the illusion you will create.
- A primary highlight that is hard-edged indicates that the surface it appears on is smooth (transparent objects are smooth).
- A secondary highlight within the object and opposite the primary highlight implies a transparent solid.
- A secondary highlight in the object's wall opposite the primary highlight implies a hollow transparent object.
- A cast shadow that is light in the middle and dark only on the edges implies a hollow transparent object.
- A cast shadow with a brightly illuminated area in an otherwise dark shadow implies a transparent solid.
- All transparent objects shift the values of what is seen through them toward middle values.
- Solid transparent objects distort (reverse, invert, or shift) things seen through them.
- Only the edges of transparent hollow objects significantly distort their background.
The iris of the eye (the colored port) is covered by a solid transparent lens called the cornea. See how the primary and secondary highlights on the cornea affect the light on the iris. The tear is a transparent solid and is similarly affected.