Khan Academy
Khan Academy: Photoelectric Effect
Article explains the experiments on the photoelectric effect and how these experiments led to the idea of light behaving as a particle of energy called a photon.
Science Struck
Science Struck: Basics of Quantum Mechanics for Dummies
A good explanation of basic quantum theory.
Science Struck
Science Struck: 8 Albert Einstein Inventions That Impacted the World
Describes the refrigerator invented by Einstein and many of his discoveries in physics.
National High Magnetic Field Laboratory
Magnet Academy: Robert Millikan
Robert Andrews Millikan was a prominent American physicist who made lasting contributions to both pure science and science education. He is particularly well known for his highly accurate determination of the charge of an electron via...
Wolfram Research
Wolfram Science World: Phillipp Lenard
This site from ScienceWorld mostly describes Lenard's most notable scientific discoveries and contributions. Includes minimal biographical information and links to additional information on related subjects.