Why does light slow down in water?

About

Fermi National Accelerator Laboratory is the premier particle physics lab in the United States. Founded as the National Accelerator Laboratory in 1967, it was renamed after physicist Enrico Fermi in 1974. Since its inception, Fermilab has been a world leader in particle physics, with discoveries including the bottom quark, top quark and tau neutrino, three of the elementary particles that make up our universe. Fermilab is home to approximately 1,700 scientists, engineers, technicians and support staff, and hosts about 2,500 visiting scientists from around the world. The laboratory is managed by the Fermi Forward Discovery Group for the U.S. Department of Energy. More information can be found at www.fnal.gov.