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.

Nova Delphini 2013

Okay, I gotta agree sometimes Facebook does provide useful news when it comes to niche interests. Recently an acquaintance from the astronomical society posted a star-map and prompt his readers to observe that particular patch in the night sky with a binocular or record it with a DSLR looking for the creation of a "new star".

It was a transient astronomical event called "nova" occurring within the border of constellation Delphinus (The Dolphin). To a light polluted sky, I have to admit it is quite difficult to pin down this constellation (hence its obscurity) but luckily the Dolphin was surrounded by three bright stars (an asterism astronomers called "the Summer Triangle" bounded by Deneb, Altair and Vega) and if we get the triangle in our viewfinder we are then able to capture the Dolphin and its associated new object.

My attempt in capturing Nova Delphini 2013

Initially designated PNV J20233073+2046041, Koichi Itagaki discovered this new event on the 14th of August, 2013 in Japan. When he first saw it with instruments, it was a dot in the sky not even visible to human eye glowing at magnitude 6.8. Barely days after discovery, the object now called "Nova Delphini 2013" is shining brighter at magnitude 4.5 on 16th of August. Now, magnitude 4.5 is bright enough for our eyes to perceive without optical aids but we need to get to dark observation sites; for the convenience of suburban dwellers, a small telescope or binocular would be enough to reveal this nova - provided we can find it around the Summer Triangle.  

My attempt on wide-angle higher magnification (f = 68mm). High ISO to compensate star tracking.  

At the time of my observation, photographic evaluation shows it was glowing bright yellow, spectroscopic data confirmed hydrogen gas present around the "new star" thus confirming a thermonuclear starburst from a nova. 

Novae are interesting evolutionary features in a binary star systems. It is one of the eventual outcome of two "average sized" stars gravitationally "locked" together much like two ice-skaters swiveling around each other tethered by a rope. Simply put, the sudden brightening of a star called "nova" is a beautiful process involving gravity and explosive nuclear fusion reaction in binary systems. I found a comprehensive lecture on the evolution of binary systems here, it should cover the reason for the occurrence of novae and also type Ia supernovae.

In the coming few weeks, I am interested to take another shot of this object under same photographic settings and compare its relative brightness to its surrounding stars. Finding this object with a telescope will prove to be challenging due to its small field-of-view, but darn my star-trackker was out of service (thanks to unmatching screws) just when I need it. Pfft. 

Homemade Spinthariscope

Recently I was lucky to stumble across a local online store which sells ion chamber module for homemade smoke alarm projects. Without much hesitation I bought two of these chambers knowing they contain tiny amount of radioactive materials.

Yes, you heard right. I was looking for an ionizing radiation source for some of my hobby experiments and Americium is the only radioactive man-made element (of the periodic table) entered the common household as ionization source inside smoke detector alarms.

Now, I am not going to elaborate how ionization chamber works inside smoke alarms because it will digress from our current topic. What I was interested was extracting the ionizing source inside the chamber; and so I did.

The source was Americium-241 (the small embedded golden disc at the top of the plastic Petri dish), with major part of the radiation being alpha particles spitting out of the yellow metal (trivial part in gamma emission) with average energy of 4.5 million electron volts (MeV), factory spectroscopic evaluation full wave half maximum at 0.7 MeV. Because Americium-241 has a relatively short half-life of 432.6 years, each source contains less than 0.5 micrograms of americium dioxide has an activity of 0.8 microcuries which is equivalent to about 30,000 nuclear disintegration per second.

See, radioactive decay is a relatively energetic process. Literally chunks of matter in the form of alpha particles are being tossed out into the air from each decaying Americium atom (remember about 30,000 atoms are decaying at any moment) inside the golden alloy. After so, the fragmented Americium atom will become a Neptunium atom (also radioactive) which subsequently decay into other more stable elements. As a matter of fact, a 15 year old Americium source will contain about 5 percent Neptunium-237, which has a half-life of 2.14 million years.

Can we see this flying-in-the-air nuclear chunk? No. Even if we take this radioactive source and put it next to our eyes, we cannot see anything. No green glowing stuffs like we so often see in science fictions and cartoons. Radioactivity is invisible to our naked eyes but fortunately we have some other ways to make these energetic flying particles appear.

When alpha particle travel in air at great speed, in this case about 15,000 kilometres per second (based on 4.5 MeV relativistic kinetic energy) it has high probability to collide with anything that comes in their way due to its relative huge size. Usually these particles doesn't travel far, about 4 centimetres at most in air and will lose their energy in scattering events. It is like smashing a cue ball into a pool table full of billiard balls - eventually the cue ball will stop as it deposits its kinetic energy into the surrounding ball during collisions.

When collided in just the right way with the right materials, the passage of an alpha particle can be "seen". This is done with a process physicist call "scintillation". A transparent material such as phosphor powder (the stuff you get from old TV screens) will emit flashes of light when it is being collided by energetic moving particles.

TV phosphor powder in plastic container

More than two years ago, a friend of mine gave me an old TV which I have dismantled to get its high voltage components. I also collected some phosphor materials by scraping off the powder inside the glass TV screen. I had to gently break the glass by covering the entire TV tube with a damp cloth to avoid vacuum implosion, a hit with a hammer near to the electron gun will crack the glass allowing air to enter the tube.

From a glance, it seems the grain size of the powder is quite coarse, and indeed they were flaked off from the glass surface. Among the materials that was scraped out, it is a mixture of three types of phosphor and there is no way to isolate them. The three types of powder corresponds to red, green and blue light emitted if energetic particles such as electrons collide on it. More specifically, typical TV phosphor components are denoted P22R, component: Y2O2S:Eu+Fe2O3 glows red at 611nm, P22G, component: ZnS:Cu,Al glows green at 530nm and P22B, component: ZnS:Ag glows blue at 450nm.

Using a cellophane tape, I stick some some phosphor material from the container making sure only the finest powder was glued on the tape, then the phosphor coated tape was mounted on a washer providing a hole for the alpha particles to strike from the bottom (refer to first photo on top, lower part of the Petri dish).

What I am making is essentially a spinthariscope, an early device to visualize radioactivity. It has only three components. A radioactive source (my Americium-241 alpha emitter), a phosphor screen for the alpha particles to scintillate on and lastly a magnifying glass powerful enough to collect the faint light coming out from the cellophane tape. Here's the schematic:

Before looking at the cellophane tape with a magnifying glass (binocular eyepiece), I had to test and see if the tape was scintillating. So I hooked up my DSLR, turned off the lights in my room, putting sensor sensitivity to the maximum and gave it a 25 second exposure, and amazing result shows:

The blue spot was a collective exposure of individual flashes happening on the phosphor coated tape. Of course, without a magnifying glass we cannot see those tiny individual flashes but as a whole, the blue spot corresponds to the surface of phosphor powder where it was radially exposed to an Americium alpha source directly below it. I was pretty excited to see the individual scintillations, and I was lucky yet again that my father have an old Russian binocular for me to disassemble.

The eyepiece of the binocular has short focal length and it is a compound lens which behaves like a convex high-power magnifying glass. This is ideal to be part of the spinthariscope component as the lens should be placed as close to the screen as possible to avoid losses of light and act as a "light gatherer".

So I cap on the lens and look through it. Nothing. I did not see anything at first but it was no surprise, because each scintillation only result in emission of a few light particles (i.e. photons) and therefore the light should be very faint. But when our eyes adapted to the dark, it has higher capability to detect those faint flashes of light and will usually take about 15 to 20 minutes to reach that kind of sensitivity in darkness.

I found a video in YouTube features what we are able to see through a spinthariscope. Please be noted that the video uses a movable alpha source, therefore the centre of the activity seems to move around the field-of-view.

It sparkled with beautiful flashes of light, where each spot represent one alpha particle striking through the phosphor powder on the cellophane tape. I here provide a simplified schematic of the mechanism:

The active area of the spinthariscope is this: radioisotope Americium-241 emits alpha particles travelling in straight line towards the phosphor coated tape. When an alpha particle collide with the phosphor materials, each atomic collision provides one photon (one quanta of light) and it is extremely faint. But I once read that human eyes are sensitive to even 6 photons per rod photoreceptor cell, (scientific citation here) I suppose each flash should compose of more than that amount of photons coming from each alpha particles, which implies it is a multiple collision event happening to one alpha particle. 

Enchanting views aside, light from scintillations in my spinthariscope is rather weak and it takes time for our eyes to adapt darkness for observation. Leslie Wright managed to use a photointensifier tube, something you can get from night-vision goggles to multiply the photons coming out from the scintillations and it was bright enough to be recorded with a compact camera.  

The next stop is for me is to take a controlled long exposure photograph in varying distance between the alpha source and the phosphored tape. Perhaps I am able to visualize the inverse-square law and do some elementary calculation based on brightness to validate it.