A very interesting article on why the sky is dark at night: https://arachnoid.com/sky/index.html

Study The Theoretical Minimum at https://theoreticalminimum.com/courses for a series of courses taught at Stanford by Leonard Susskind:
“These courses collectively teach everything required to gain a basic understanding of each area of modern physics including all of the fundamental mathematics.”

https://ophysics.com/index.html interactive simulations
http://hyperphysics.phy-astr.gsu.edu/hbase/index.html hyperlinked topic map