Thurs-Demo: The Next One Posted on Friday Because I Was Busy Thursday

This isn't a trend, but I seem to be rather busy on Thursdays. So much so that yesterday's demo didn't happen. Perhaps I'll describe what I was up to at some later point.

Today, however, I took advantage of the light output from an iPad. Many computing devices that have an LCD type screen emit polarized light. It's why I rarely wear sunglasses when I'm out taking pictures - the polarized glasses cut out too much - sometimes all - of the light from the review screen on the back. Wearing polarized sunglasses and looking at a computer can also be tricky.

But stick the iPad behind a transparent, isotropic material and then slap a polarized filter in front of the camera and you have an instant stress-field viewer. I redid the shot of a small cap gun charge through gelatin, but this time the only light was coming from an iPad.


This turned out much better than I had hoped. First, I didn't have a lot of light to work with. Second, many computer screens will produce scan lines because the screen is "refreshing" at some frequency (it's why video of older phosphor screen monitors "flicker"). But the iPad sent out a nice steady light (I made a blank white slide in keynote and just had that as the display).

Now I think I need to get my hands on some of those old fashioned 4"x4" rock thin sections and try photographing them...

Gelatin and High Speed Video

You may have seen something like this before - using polarized light to show how a material deforms under stress. I hadn't heard anyone using this technique on gelatin before, so I just had to try it out myself. What a lovely effect. There was som über geekery to be had when we noticed you could easily produce interference patterns. I'll be providing you some more details for tomorrow's demo, but here's a peek into what's going on in the dirt lab:


For all of you strain junkies out there: I'm thinking we are looking at longitudinal strain effects at first - these spread out to a maximum distance quickly and are then overtaken by the transverse surface waves as they radiate outward. Lockwood (Outside the Interzone) mentioned that he had disabled embedded video to reduce computer issues he was having, so here's a still from one of the experiments - this way even those without video capabilities can see what's going on.