Properties of Glass: Facts & Misconceptions

[azaharak]

Dear all

I have pooled the following facts or misnomers that I have heard or been told over the years about glass. Please comment on whether they are indeed correct and factual depictions of what is happening.1) You can see through glass because it does not absorb visible radiation (or absorbs visible very light little). Glass is transparent to visible light, but not most forms of UV radiation.

2) Why can you see through certain glass clearly, but other types of glass that might exist on your shower door you can not. Even though glass doesn't absorb visible light much, doesn't it still get scattered in all crazy directions inside the glass?

Because Glasses are amorphous type solids, their atoms are randomly arranged (no crystalline structure). A visible light photon will get randomly scattered in all directions, all of these random kicks average out to about zero over the 10^8 atoms or greater thickness (10^23)^(1/3). An image of a checker board will have the same orientation when seen through the other side

Glasses on your shower door or glasses that permit fuzzy images have more ordering (crystalline structure). The ordering scatters light in particular directions.

3) If the above is true. Then why does glass allow for reflections to be seen off of the surface. Isn't reflection a function of crystalline structure, glass is not crystalline its amorphous. The reflection should occur within a thin layer of the glass as in metals.

Or does the reflection occur from everywhere inside the glass and a different feature.Thank you

 

[Plantis]

1) In general, it is true. However, you need to be more careful. There are a lot of different glasses. Regular glass is based on SiO2. We can added some metals and it will have different colors. Please, check this http://www.google.com/search?gcx=w&ix=c1&sourceid=chrome&ie=UTF-8&q=glass+absorption+spectrum
About UV:
quartz glass - the same SiO2, but another technology.
n.
A clear vitreous solid, formed by melting pure quartz, that can withstand high temperatures and is extremely transparent to infrared, visible, and ultraviolet radiations. Also called fused quartz, fused silica.

2)Because of scattering.
http://en.wikipedia.org/wiki/Frosted_glass

 

[chrisbaird]

Visible light is not scattered much by individual atoms. You have to get up into X-ray wavelengths for the light to scatter off the atomic arrangement (crystal lattice) in a meaningful way. That is why a perfect cube of pure polished quartz (crystal structure) will look the same to the naked eye as a perfect cube of glass (amorphous atomic structure), aside from the difference in refractive index, which is a bulk, macroscopic property. Glass that is frosted looks different because of macroscopic features placed on the surface or cut into the surface of the glass, that are large enough to scatter visible light. Frosted glass will look identical to frosted quartz if cut the same way, even through their atomic structure is vastly different.

 

[azaharak]

If what you are saying is true,Then why can you see a faint reflection in glass or total internal reflection. The reflection must be some form of scattering the incoming light.

And if scattering only occurs significant for near xray wavelengths, why do mirrors (metal) allow for scattering or reflection?

Thank you

From Wiki

"Most liquids and aqueous solutions are highly transparent. For example, water, cooking oil, rubbing alcohol, air, natural gas, are all clear. Absence of structural defects (voids, cracks, etc.) and molecular structure of most liquids are chiefly responsible for their excellent optical transmission. The ability of liquids to "heal" internal defects via viscous flow is one of the reasons why some fibrous materials (e.g., paper or fabric) increase their apparent transparency when wetted. The liquid fills up numerous voids making the material more structurally homogeneous.

Light scattering in an ideal defect-free crystalline (non-metallic) solid which provides no scattering centers for incoming lightwaves will be due primarily to any effects of anharmonicity within the ordered lattice. Lightwave transmission will be highly directional due to the typical anisotropy of crystalline substances, which includes their symmetry group and Bravais lattice. For example, the seven different crystalline forms of quartz silica (silicon dioxide, SiO2) are all clear, transparent materials.[1"
There must a good deal of scattering, the refractive index is a result of scattering. This obviously occurs for visible light.
If the light is emitted coherently in the same direction (preserves images), this obviously occurs for glass, air, still water, all of these are amorphous type materials. It must be a function of random scattering, or interference due to all atoms (in random orientations) that contribute to the overall outward direction of light.

If you shine visible light through a crystal (diamond) I do not believe you would be able to see straight through, the ordering here must be forcing the light in particular directions.

Thank you

 

[chrisbaird]

If what you are saying is true,


Then why can you see a faint reflection in glass or total internal reflection. The reflection must be some form of scattering the incoming light...

Let's be careful. I did not say that visible light rays are not scattered at all, just that they are not scattered much by individual atoms. They are scattered (refracted, reflected, etc.) by the material, but the wavelength is such that the ray interacts with many atoms at one. The typical atomic radius is 1 Angstrom. Visible light has wavelength of around 5000 Angstroms. As a result the visible light is not effected much by atomic arrangement. It is effected by macroscopic properties such as refractive index, surface roughness, etc.

 

[azaharak]

From Wiki

"With multiple scattering, the randomness of the interaction tends to be averaged out by the large number of scattering events, so that the final path of the radiation appears to be a deterministic distribution of intensity. This is exemplified by a light beam passing through thick fog. Multiple scattering is highly analogous to diffusion, and the terms multiple scattering and diffusion are interchangeable in many contexts. Optical elements designed to produce multiple scattering are thus known as diffusers. Coherent backscattering, an enhancement of backscattering that occurs when coherent radiation is multiply scattered by a random medium, is usually attributed to weak localization."

So you are saying, that the preservation of an image through glass has nothing to do with the fact that the glass is amorphous (randomly oriented), and the scattering would be averaged out to the say forward direction that it came in?

So if we were to look straight through a thick piece of glass, or diamond, we would still see an image preserved in the correct orientation with little blurring?

It is only when we fire through different wavelengths of light such as xrays through a diamond (which are on the order of atomic spacing), then the incoming may be scattered off in multiple directions such that the forward image would not be preserved?

What if we sent xrays through a glass which has random orientations of molecules. How would this scatter differently from xrays through a diamond?

I'm finding this helpful because a past professor has explained to me one of the reasons you can see through glass with an image preserved in the proper orientation is because of the random organization of the molecules scattering the light like random kicks that average out over the length of the glass.

 

[chrisbaird]

From Wiki

"With multiple scattering, the randomness of the interaction tends to be averaged out by the large number of scattering events, so that the final path of the radiation appears to be a deterministic distribution of intensity. This is exemplified by a light beam passing through thick fog. Multiple scattering is highly analogous to diffusion, and the terms multiple scattering and diffusion are interchangeable in many contexts. Optical elements designed to produce multiple scattering are thus known as diffusers. Coherent backscattering, an enhancement of backscattering that occurs when coherent radiation is multiply scattered by a random medium, is usually attributed to weak localization."

Fog particles are much larger than single atoms. Visible light does interact with fog particles on an individual basis because they are so large.

So you are saying, that the preservation of an image through glass has nothing to do with the fact that the glass is amorphous (randomly oriented), and the scattering would be averaged out to the say forward direction that it came in?

The preservation of a visible-light image through glass (or quartz crystal, or diamond crystal) occurs when:

1) The surfaces are flat and smooth, such that the surface roughness features are smaller than the wavelength of the light.

2) The material is optically non-lossy (there are no dye particles or metal impurities inside).

3) Any geometrical features inside the material (such as air cavities, trapped water bubbles, etc.) are smaller than the wavelength of the light used.

A great example is pure quartz versus milky quartz. Pure quartz is transparent and preserves an image even though it has regular crystal structure, while milky quartz is opaque white. The only difference chemically between the two is that milky quartz has small water and air bubbles inside it that are larger than the wavelength of visible light. Another example is ice versus snow. They are both made of H₂0, but ice will preserve an image through it and snow will not. This is because snow has surface features (snowflake structures) that are larger than the wavelength of visible light.

So if we were to look straight through a thick piece of glass, or diamond, we would still see an image preserved in the correct orientation with little blurring?

Yes. If you had a very pure, polished flat piece of glass, diamond, ice, or quartz, you would be very hard pressed to tell them apart using your naked eye alone. Any image would look the same through them. (They do have different indices of refraction, so the images would be slightly offset by different amounts for different flat materials. For this same reason, a lens made of diamond would magnify the image more than an identical lens made of glass. But index of refraction is a bulk property).

It is only when we fire through different wavelengths of light such as xrays through a diamond (which are on the order of atomic spacing), then the incoming may be scattered off in multiple directions such that the forward image would not be preserved?

Correct. You instead get an X-ray diffraction pattern:

Because the X-ray diffraction pattern is directly created by the arrangement of the atoms, it is used as a tool to find the atomic arrangement of a material. The helical structure of DNA was discovered using X-ray diffraction.

What if we sent xrays through a glass which has random orientations of molecules. How would this scatter differently from xrays through a diamond?

Diamond x-ray diffraction pattern:

For, glass, there would be rings instead of spikes.

I'm finding this helpful because a past professor has explained to me one of the reasons you can see through glass with an image preserved in the proper orientation is because of the random organization of the molecules scattering the light like random kicks that average out over the length of the glass.

Your professor was wrong. If that were true, then we could not see through pure diamond as it does not have a random atomic distribution.

 

[Claude Bile]

Dear all

I have pooled the following facts or misnomers that I have heard or been told over the years about glass. Please comment on whether they are indeed correct and factual depictions of what is happening.1) You can see through glass because it does not absorb visible radiation (or absorbs visible very light little). Glass is transparent to visible light, but not most forms of UV radiation.

2) Why can you see through certain glass clearly, but other types of glass that might exist on your shower door you can not. Even though glass doesn't absorb visible light much, doesn't it still get scattered in all crazy directions inside the glass?

Because Glasses are amorphous type solids, their atoms are randomly arranged (no crystalline structure). A visible light photon will get randomly scattered in all directions, all of these random kicks average out to about zero over the 10^8 atoms or greater thickness (10^23)^(1/3). An image of a checker board will have the same orientation when seen through the other side

Glasses on your shower door or glasses that permit fuzzy images have more ordering (crystalline structure). The ordering scatters light in particular directions.

3) If the above is true. Then why does glass allow for reflections to be seen off of the surface. Isn't reflection a function of crystalline structure, glass is not crystalline its amorphous. The reflection should occur within a thin layer of the glass as in metals.

Or does the reflection occur from everywhere inside the glass and a different feature.Thank you

1. Glass covers a wide range of properties, but generally, yes, glass is transparent in the visible and opaque in the UV. Most glasses possesses a sharp cut-off wavelength below which the glass is opaque. The cut-off wavelength depends on the energy band-gap between the valence and conduction bands of the glass; fused silica has the highest bandgap (around 7-9 eV) which corresponds to a cut-off wavelength around 200-250 nm.

2. Glass appears cloudy due to inclusions in the glass structure such as microcracks and air bubbles (basically anything which yields a strong refractive index boundary), which act as scattering centres.

3. Reflections occur off the glass surface and are therefore a function of the surface shape and are not affected by the bulk.

Claude.

 

[PAllen]

Minor note: most common glass is transparent to UVA, but opaque to shorter frequencies. This is useful in UV response testing of minerals, because moderately priced UVB/UVC sources include some UVA. If a mineral fluoresces slightly in such a source, but most of the response disappears with interposition of ordinary glass, you know the mineral was responding to residual UVA.