# Plastic Protractor Polarized 05375

- By: Nevit Dilmen
- Date: 2004
- Classification: Photograph
- Credit line: Self-photographed
- Museum: [Wikimedia Commons](https://commons.wikimedia.org/wiki/File:Plastic_Protractor_Polarized_05375.jpg)
- Rights: [CC BY-SA 3.0](http://creativecommons.org/licenses/by-sa/3.0/)
- Picture: https://invisiblenetwork.ai/thumbs/commons/thumb/e/e4/Plastic_Protractor_Polarized_05375.jpg/1920px-Plastic_Protractor_Polarized_05375.jpg
- Who can see it: Teens profiles and older (museums give no ratings; placed by Invisible Network from the museum's own words)

Tension lines in plastic protractor seen under cross polarized light. When a ray of plane polarised light is passed through a photoelastic material, it gets resolved along the two principal stress directions and each of these components experiences different refractive indices. The difference in the refractive indices leads to a relative phase retardation between the two component waves. The birefringence of the plastic layer shows colored fringes. Also called Photoelasticimetry or photoelasticity. Not seen does not mean No difference.