King E

It was about epsilon, in case of refraction.

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Let’s start out with a glass with no electric field in it. Now we we impose an electric field on the glass by sending light in it. Then, how do the charges in the glass set up their own counterbalancing electric field in opposite direction that results in reducing the total electric field?

I learned from a source that when external magnetic field is applied, parallel spin/spin up has lower energy than spin down. But why? How does this relate to two like poles of magnets facing each other and two unlike poles facing each other? Source:https://mriquestions.com/energy-splitting.html