on sight
as soon as you see someone

[TahlilGaran] Dictionary of Contemporary English ▲

on sight
as soon as someone or something is seen.
The law now requires all pigeons in the area to be shot on sight.

Even very young children can recognize certain words on sight.

[TahlilGaran] English Idioms Dictionary ▲

on sight
See: AT SIGHT.
{adv. phr.}
1. The first time the person or thing is seen; as soon as the person or thing is seen.
First graders learn to read many words on sight.

Mary had seen many pictures of Grandfather, so she knew him on sight.
Compare: AT ONCE(1).
2. On demand, on asking the first time.
The money order was payable at sight.

[TahlilGaran] English Idioms Dictionary ▲