material-science
I was reading the internets and I saw an image of of random facts. Yes, I know, its a humor site, but based on the image item #23 I have some questions.
Does glass really get stronger the longer its submerged? If so, why and whats the upper limit of its strength? Does depth matter?
Infra-red radiation will pass through a polarised medium just like visible light does i.e. the component at right angles to the plane of polarisation will be blocked. So there's nothing special about the fact the glasses are polarised.
However most materials are only transparent over a restricted range of wavelengths. Optical glass only transmits light with a wavelength from about 300nm to around 2500nm. There's a good article on the optical properties of glass here.
However I think passive 3D glasses are made from conventional polarisers, which are made from a polymer called polyvinyl alcohol. I struggled to find much about the spectra of PVA polarisers, though I found this article that lists some polarisers that work down to 3000nm. So it seems PVA polarisers will transmit at least the near infra-red. However the term infra-red covers a huge range of wavelengths from 750nm - 1mm. I can't think of anything, even air, that is transparent right across this range of wavelengths so everything absorbs infra-red light to some extent.
Generally speaking, ultra-violet light is absorbed because it has enough energy to excite the electrons in atoms and infra-red light is absorbed because it has the right energy to excite molecular vibrations. Visible light has too much energy to excite molecular vibrations, but too little to excite electrons in atoms, so that's why most things are transparent to visible light (and probably why organisms like humans evolved to use it). Most things that are opaque, like say paper or chalk, don't absorb the light but scatter it by multiple reflections at air/solid interfaces. The exceptions are things like transition metal complexes and organic dyes.
Suppose you bend a perfect, i.e. unscratched, piece of glass, the forces on it look like:
The top of the glass is in tension and the bottom in compression, but the stress is spread over a large area of glass so the local stress at any point isn't enough to break the glass.
Now put a scratch in the top surface and bend it again:
This time the stress is concentrated at the leading edge of the scratch so you get a high local stress and the glass breaks at the tip of the scratch. This means the crack grows until it breaks through the glass. That's why putting a scratch on the glass causes it to break when bent. The scratch focusses the stress at the scratch.
Best Answer
As far as I remember, there is some truth behind this statement: glass is inherently extremely strong, but it is fragile in practice because of microcracks on its surface. In water, glass dissolves to some extent. As a result, microcracks partly disappear, and glass becomes much stronger (at least for a while). I remember reading that A. F. Ioffe (I guess his name was sometimes written as Joffe) held a thin glass stick in water for a few days, and the stick became flexible. Maybe I'll try to find a reference later.
EDIT: the following article confirms that holding glass in water can make the glass stronger, but the authors used hot water: