"Stained Glass" demo

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Stained Glass Polarisation Demo

This is a simple demonstration which contains physics far beyond what is studied to “A” level. The simplicity does, however make for a good demonstration when talking about light or crystal properties. It may even be useful for a geology demo.

Requirements

A small (10 cm square but this is not critical) plate of glass
Sticky back plastic (“sellotape”)
Polarising filters (ideally two but one if using an LCD display as a light source)
Craft knife (sharp)
Washable pen (and possibly a stencil or template).


Method

Clean the glass first and ensure that there are no sharp edges. If necessary tape all around the edge with insulation tape to reduce possible cut hazard.

Write IN REVERSE the lettering you want to display on the back of the glass. Or use a template underneath the glass.

Each letter should then be marked with a line to indicate the orientation of the sticky back plastic to be applied. The orientation for may need to be tested first to suggest the most effective angle.


In the example. It has been found that there is one orientation that has a minimal change compared to the background colour. In the video link it obvious in the O and S of optics.

Letters are then cut from the sticky backed plastic using the marks as a guide. This can be helped by placing a polarising filter below the glass as it makes the plastic easier to see.

When all the letters are cut out and excess tape scraped off, wipe any guide marks from the rear of the glass.

The second polarising filter can be secured to the back of the glass with sticky backed plastic as the film behind the plate will not affect, to any great extent, the transmitted light.

View the result with this “youtube” video:

  • {{#ev:youtube|PoMgqgkMyms|320}}


Simpler Method

A quicker version can be made using an off-cut of acrylic to support strips of adhesive tape. These can be viewed against an LCD monitor through a polarising filter

tape held on a an acrylic sheet viewed through a polarising filter

Theory (speculative)

It is suggested that the tape acts as a 1/4 wave plate, this would change the linearly polarised light to circularly polarised. Circularly polarised light has a rotating electric vector. However, a simpler explanation would be that the tape acts as an optically active film that happens to rotate the plane of polarisation through 90 degrees. It is thought, personally, that the coincidence of film thickness with the 90 degree rotation is unlikely.

--D.B.Ferguson 20:14, 3 May 2011 (BST)


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