system
Given the following post, respond as if you're commenting on Reddit:
human
How does one measure the thickness of gold foil?
gpt
Ellipsometry can do it! Throw in some polarised light, look at the reflection and see how the polarisation has changed - gold is slightly birefringent (it changes the polarisation of the light on transmission), and so by looking at the net change of polarisation you can work out the thickness!