A number is an imaginary  number  when it multiplied by square root  of negative no. An imaginary number is the square root of a negative number and does not have a tangible value.”Complex” numbers have two parts, a “real” part (being any “real” number that you’re used to dealing with) and an “imaginary” part (being any number with an “i” in it). The “standard” format for complex numbers is ” a + bi “; that is, real-part first and i -part last.

Complex_Numbers

Imaginary No.