Stress rainbows

A dull drippy day today!  

I have been dismantling a 30 year old self built wardrobe that will not be moving with us.  Tomorrow I shall take it to the tip.

Back in the 1960s, when I was studying for my Physics A level,  I did a project on the use of polarised light to analyse the stress in perspex beams.  This is a plastic sample box with a polarising filter resting on it and it shows the stress banding within the material resulting from the moulding process..