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.
In blue, peripheral refraction or naked eye; in red, over-refraction with near-centre design contact lens; in yellow, over-refraction with distance-centre refraction.
