It's not clear to me, Part II

Yesterday I did a post on glassware - specifically, how people in my family tend to grab the same glasses when they come to our house. In the process I took pictures of a variety of glass objects. I called the post It's Not Clear To Me as a pun, but then I started to think about the terms transparent, translucent and opaque.

I did some research and learned about refraction too. Here are the differences in these terms:

1. Opacity is the measure that a substance is impenetrable to (absorbs) visible light
2. Translucency is the property of allowing light to pass but diffusing it (you cannot see clearly through it)
3. Transparency is the property of allowing light to pass through a material without diffusion
4. Refraction is the degree to which light gets redirected or bent as it passes from one material (air) into another (glass)

In the process of taking the pictures, I noticed some glass is sharper, clearer, more sparkly than other glass. It is more transparent, more refractive, perhaps. Why?

These two glasses are both from Germany - high quality glass - washed in the same dishwasher, polished with the same dish towel - yet they are definitely different.

Some glasses are known as lead crystal - glass with up to 25% lead added to the molten silica (sand) instead of calcium. Addition of lead changes the properties of glass:

• makes glass heavier (as lead is 5x heavier than calcium)
• lowers the melting point of glass
• makes liquid glass less viscous (more runny) which makes it easier for air bubbles to escape during cooling
• results in a softer surface which is easier to cut
• gives it a higher level of refraction - it is more "sparkly"
• makes a ringing sound when struck or rubbed

Any kind of glass slows down the light passing through it. The refractive index is a scale that allows us to measure this drag on light (making it bend, or turn). Here are some materials and their refractive index numbers:

0.00 = vacuum
1.00 = Air at standard temperature and pressure
1.33 = water (liquid)
1.38 = human eye (cornea)
1.50 = ordinary glass
1.58 = polycarbonate (plastic)
1.70 = lead crystal glass
1.76 = sapphire crystal
2.15 = cubic zirconia
2.41 = diamond
3.02 = mercury ore
3.96 = silicon

No wonder a diamond sparkles! Here are some well-known companies making lead crystal glass objects: Arc, Baccarat, Mikasa, Steuben, Swarovski, Waterford.