Let’s consider glass and transparency. Why is glass transparent and why is it sometimes not? What does it mean to be translucent or opaque? The ability to transmit, absorb, or reflect light is a characteristic that makes glass very appealing in art applications so artists choose to make use of it in a variety ways.
In a work of art we look at both formal (having to do with the shape or form of the object) and conceptual (having to do with the story being told by the object) characteristics. When that artwork is made of glass, notable formal aspects include not only shape, size, texture, and color which we see in most art objects, but also an enhanced relationship with light because of the range of transparency to opacity that is potential in the material.
While the concept of an artwork may be communicated by way of image, the formal aspects of the object can also play a role in storytelling. Glass, because of its inherent characteristic of translucency (allowing more or less light to pass through it) stirs the imagination of the viewer in unique and poignant ways. Light has long been associated metaphorically with concepts of goodness and epiphany in human culture. Varying the levels of translucency in glass allows an artist to modulate the viewer’s experience of light in relationship to form in an object, evoking emotional response through the material.
Innerland: The brilliant clarity of lead crystal in Eric Hilton’s sculpture draws the viewer close to examine the engraved details within. We first experience the surface and shape of this sculpture, then travel visually through the transparent glass, deeper into the mysterious landscape-like compartments.
Red Pyramid: Varying levels of light and shadow in this translucent geometric cast glass sculpture by Libensky & Brychtova evoke a mood of introspection. We see a monumental form with a hard surface that would be cool to the touch, yet appears warm as if it burns from within.
Window with Hudson River Landscape: The luminous color in this Tiffany Window stirs the viewer’s imagination. We know we are looking at an image on a hard glass surface, but the opacity of the window bars contrasts with translucent glass panels, allowing us to feel as though we are seeing far into the distance.
Artist in Residence Andrew Erdos at The Studio 2012: The mirrored surfaces of Andrew Erdos’s blown glass sculptures reflect back the face of the viewer. While we examine his combination of peculiar, animal-like forms, we also see ourselves. Erdos uses the reflective quality of glass to pull us into his work.
Light and the translucent characteristics of glass are of great interest to the artist, but are also fascinating from a scientific viewpoint. We are used to looking through glass. We’re familiar with the beautiful and varied colors of light shining through stained glass windows. We rely on the reflective quality of glass in our mirrors every day, but what makes these common experiences possible? Why is glass transparent or opaque? Reflective or absorbing of light? What exactly do these words mean?
Let’s begin with some definitions because although we have all probably heard these terms before, it’s useful to pin down specific meanings since the same words can convey different things depending on how they are being used.
Opaque: means not able to be seen through; not transparent. Light does not pass through something that is opaque. The historical meaning of opaque is “darkened” and so metaphorically, opacity might infer something that is dark, unclear, or hard to understand.
Translucent: A translucent object is not completely clear but does let some light pass through it. It might have color or be cloudy, and it diffuses light. So an artist might use translucency to convey a feeling of mystery, or something that is only partially revealed.
Transparent: An object is said to be transparent when light passes through (or is transmitted through) it so that you are able to see objects on the other side clearly. Transparent also means being free from pretense or easily understood and is historically derived from Latin terms meaning to show through or to show oneself. Crystal clarity in an art object might indicate purity or enlightenment.
When glass is clear light will be transmitted through it. This means that light energy, or photons, pass through the glass rather than being reflected or absorbed by it. So why do they do that?
Glass is made up of a loose, non-uniform (in other words not crystalline) network-like arrangement of atoms of different elements (including silicon, oxygen, calcium, sodium). Atoms include a nucleus and various numbers of electrons. Electrons are negatively charged particles occupying different energy levels or shells in the space surrounding, and at specific distances away from the nucleus. The lower energy levels are closest to the nucleus and the higher energy levels are farther away. When electrons are able to gain energy they will move from a lower to a higher energy level within the atom. Light is made up of photons, which are packets of energy. When a photon gets close enough to an atom it may excite an electron (transfer some of its energy) enough to allow it to move to a higher energy level, in which case the photon is absorbed and ceases to exist as light. There are many different wavelengths of light, some of which are visible to the human eye. If the visible wavelengths of light are absorbed by a material, then that material will appear to be dark or opaque to the viewer.
So in order for a material to appear opaque, its atoms must have their energy levels and electrons arranged in such a way that the photons of visible light interact with or excite the electrons enough to move to higher energy levels and so be absorbed by the material. It takes specific amounts of energy for an electron to travel from one level to another. Different levels have different amounts of space between them, so they require more or less energy for an electron to pass from one to the next.
In glass, the space between these energy levels is broad enough that electrons require more energy than is provided by a photon of visible light to be moved from one level to the next. Since photons of visible light don’t have enough energy to excite these electrons to move to higher energy levels, they are not absorbed and so they pass (or are transmitted) through the glass. Once light passes through the glass it may then be reflected from the surface of an object on the other side of the glass allowing us to see that object through the glass! And THAT is how we experience transparency in glass.
Of course that brings up another interesting question. In order to experience glass as transparent, we pretty much have to see something through it, which means that thing on the other side of the glass has to be reflecting light from its surface for us to perceive. So next we need to examine reflectivity and what that means. We’ll be doing just that in my next blog entry: Reflection & Light in Glass & Art
See other posts in this series: Where Art and Science Happily Meet.