Official websites use .gov
A
.gov website belongs to an official
government organization in the United States.
Secure .gov websites use HTTPS
A lock (
) or https:// means you've safely
connected to the .gov website. Share sensitive
information only on official, secure websites.
(a) Photograph of a paintbrush in a transparent glass half filled with water. Refraction by the water produces a discontinuity in the brush. (b) The photograph is altered to remove the effect of refraction, but transparency is still perceived.
