sight for sore eyes, a

One whom it is a relief or joy to see, as in Linda, who had not seen him in 15 years, told him he was a sight for sore eyes. This idiom implies an appearance so welcome that it heals ailing eyes. [First half of 1700s]

The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.