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4 September 2013

Optical Phenomenon With a Glass Paperweight

Some time ago I saw a cubic glass paperweight for sale in Daisō. Everything in it was tagged with MYR 5 so I got myself a pretty deal knowing it could bring up some interesting optical tricks. It took me a while to get these photos posted up because I was looking in vain for an optimized camera setting to photograph a nice picture of total internal reflection demonstrated with this paperweight. 

Following the theory of geometrical optics based on Snell's law, a strong narrow beam of light (like from laser pointers) entering the cube should be totally reflected inside the cube, bouncing off at most at 3 internal surface and exit the cube to where it come from regardless of the angle of entrance. It is the basic operating principle of retro-reflectors

It worked physically when I tested it with a DPSS "green laser", but I find it difficult to capture it in photo for few reasons:

1. It is difficult to see the beam inside the cube because glass is transparent to light and thus did not provide media for light to scatter on. i.e. we can easily see the beam of a laser in a foggy day and not during clear summer nights. To overcome this problem is to increase the brightness of the laser beam or prolong exposure.

2. Some part of the laser light upon touching the glass cube will get reflected before entering the cube. these reflected light will be picked up by the camera under long exposure thus causing saturation. i.e. reflected light will cause saturation before the beam can be seen. 

3. To do long exposure photography, it is absolutely necessary to make sure the light source and the object doesn't move during exposure. Unfortunately, keeping the hand steady with laser pointer (I want to demonstrate non-normal angle of incidence) while holding your breath isn't going to work. 

So I gave up on that front, however I do have some other interesting photos coming from other optical phenomenon which is provided none other but this paperweight.

Diffraction pattern result from light transmitting through a microscopic air bubble trapped in glass

The picture above is one of them. I'd say it is a pretty interesting photo describing optical diffraction. What you are looking at was a beam of green light from a commercial laser pointer shining through an air bubble trapped inside the glass cube (I have digitally altered the hue). The spherical bubble is very small, I estimate its size no more than 0.5 mm, which became an object for the bright narrow beam of light to diffract on, thus creating the fringe pattern all around it. 

Actually, that singular photo shows multiple optical phenomena. Besides diffraction, we can also see refraction and also internal reflections. Can you find them all? 

*  *  *

Okay, laser beam aside. While I was trying to shoot the effect of a retro-reflector, I placed the paperweight on top of a piece of flat rectangular welding glass (from my previous experiment with haze particulates) in hope to increase the contrast for showing the beam reflecting inside the cube. Instead of the desired photographic effects, I found out there is actually a slight air gap between the cube and the welding glass even when the two objects are stacked together. 

White-light interference pattern created from path-difference between two interface 

Because both the surface of welding glass and glass cube are not optical-flats, the difference of thickness for the air gap between the two glass surface allows light wave to constructive and destructively interfere. And thanks to the black welding glass surface it dramatically brings up the interference pattern with bands of colour for each fringe indicating a polychromatic light source. Indeed, I was using typical fluorescent lamps to illuminate the setup.  

Under f/20 (f = 200 mm and small aperture for deeper depth-of-field), ISO-200, it took 3 second to expose the interference effect. This white-saturates the surrounding ugly table that supports the welding glass while keeping the object of interest in decent exposure.  

Inverse coloured fringes (top left) compared to previous image

It is interesting that when I orient the welding glass (without touching the glass paperweight on top) perpendicular to the previous orientation, the band of colour in each fringe got inverted but only for the case of "circle rings" and not the "X-bands" or "elongated-bands". I have yet to figure that out.

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