# Photoelasticity - Spiked castor cup

- By: Colin
- Date: 2017-03-12
- Classification: Photograph
- Museum: [Wikimedia Commons](https://commons.wikimedia.org/wiki/File:Photoelasticity_-_Spiked_castor_cup.jpg)
- Rights: [CC BY-SA 4.0](https://creativecommons.org/licenses/by-sa/4.0)
- Picture: https://upload.wikimedia.org/wikipedia/commons/thumb/3/3b/Photoelasticity_-_Spiked_castor_cup.jpg/1920px-Photoelasticity_-_Spiked_castor_cup.jpg
- Who can see it: Kids profiles and older (museums give no ratings; placed by Invisible Network from the museum's own words)

A spiked castor cup made of clear hard plastic, placed onto a mirror. The plastic is birefringent and demonstrates internal stress as coloured patterns (photoelasticity) when photographed using cross polarisation.