Reflecting on glass
I spent a long time thinking about what I wanted to write for my first post. Well actually, I know exactly what inspired me to set up this blog, but the post I had in mind was long and complicated and, well, life happened so I haven’t gotten around to it yet. But I still wanted to write something, so I figured I’d start small with something neat I learned recently.
It begins with a seemingly simple question. Imagine a piece of red glass. Now ask: why is it red? Depending on your knowledge of color and light you may already know the answer, but to me it seemed like a bit of a puzzle. For opaque objects, their color is dictated by the frequency of light they reflect. So, by that logic, red glass would be red because it reflects red light. Except… if you hold up red glass to a light source, the light that comes through is red, suggesting that it actually lets red light through. So which is it? Does it reflect red or transmit red?
Well, long story short, it’s the latter. There are actually 3 ways a substance can react to light: it can either absorb it, transmit it or reflect it. A substance that transmits 100% of light would be a glass so clear it was essentially invisible. An object that absorbs 100% of light would be a pure black even darker than vantablack (actually let’s call it carbon nanotube black because vantablack sucks). And an object that reflects 100% of light would appear as a perfect mirror. But in reality, objects usually exist between these poles.
While opaque objects absorb some of the light that hits it, the light it doesn’t absorb gets reflected, which gives it its color. And while clear glass mostly transmits light, most real world glass also absorbs some light, giving it a slight tint that makes it somewhat visible (to everyone except for birds and people transporting panes of glass in action movies). Going back to my original question, colored glass is created by mixing in particles that absorb specific frequencies such that only the color that you want gets through (for red glass, the particles are selenium dioxide).
But this made me think of another question. Glass is given color by the frequencies it absorbs, but could you theoretically give it color by reflecting light? Asked another way, opaque objects exist between reflect and absorb, glass exists between absorb and transmit, but does anything exist between transmit and reflect? And sure enough the answer is yes: dichroic glass.
Instead of filtering light by absorbing unwanted frequencies like traditional glass, dichroic glass operates by reflecting those frequencies. This gives it the somewhat trippy property of appearing one color when viewed head on, but another color when held up to light.
They operate using the same principle that causes rainbows to appear on the surface of soap bubbles and oil slicks, which is to say they have thin layers of film on the surface that reflect certain frequencies of light but allow others to pass through. People have been making dichroic glass for thousands of years (as seen in the Lycurgus Cup below), but the recent technique for mass producing it was developed by NASA in the 1950’s.
There are practical applications in lighting and projectors, as they absorb less heat and are more durable than traditional translucent filters, however I’m more fascinated by their visual and artistic properties. For example, I’ve seen it used to create outrageously expensive furniture (the side table below costs $2,500), but I feel like there must be other cool things you can do with it.
Has anyone out there played around with this stuff? I’d love to hear what other uses people have seen for it!