[modelboatmayhem (OP)]

Why are some objects and substances  transparent e.g. glass, diamonds, water. What do they have in common?  []

Thanks,
Martin - Peterborough  [O8)]

[eric l]

Most materials would be transparant if you could cut them into slices that were thin enough, but that does not answer your question.

In order to be transparent, the material must be massive or continuous, in the sense that it should not contain voids.  Glass is transparent, but the powder you obtain when you grind glass is just a white powder.  Big salt crystals are transparent, but table salt is a white powder;  ice is transparent, but snow is not; beer is (more or less) transparent, foam is not.  Why ?  because the refractive index of the material is different from the refractive index of the void (or the air in the void).  So on each interface between material and void (or air) light is diffracted and dispersed in different directions.

Milk is not transparent because it is a disperion of very small micelles of fat in water, fog is not transparent because it is formed by small droplets of water dispersed in air, and so on.  But a block of metal is not transparent either, even if it is massive.  Why ?

Light is electromagnetic radiation, and metals are good conductors for electricity because they have lots of free moving electrons.  Most transparant materials are bad conductors for electricity.  If a beam of light strikes such a material, like a glass plate, it does not find free movable electrons that can disperse the energy in all directions, so the energy goes through the material. 

I know this is a bit of an oversimplification, but if you stick to this two rules you can have a good idea whether a given material or object will be transparent or not :

• the material must be massive or continuous, i.e. no changes in refractive index within
• the material must be a bad conductor for electricity

[lightarrow]

Why are some objects and substances  transparent e.g. glass, diamonds, water. What do they have in common?  []

To put it even more oversimplified than eric I, the less light interact with the object, the more it is transparent.
The physical and chemical properties of those bodies are such to limit the light-matter interactions (for a lot of reasons; eric I wrote some of them).