What is the Difference between imaginary part and imaginary number?

[SHASHWAT PRATAP SING]

I am a high school student and I have a doubt regarding Complex Numbers.
when we define a complex number we say that a number of the form z=a+ib ,where a and b are real numbers is called a complex number.
a is called the real part and b is called the imaginary part, but I have a doubt here we know that ib is an imaginary number then why we say only b is the imaginary part of z why not ib.
for example- In z=5+3i why we say 3 is the imaginary part of z why not 3i.
I am very confused for this please help me...

 

[hutchphd]

This is simply the way things are defined: the imaginary part of a complex number is a real number (the real number multiplying i .)
In fact the imaginary part of a pure imaginary number is a real number. These are just done to make the manipulations easier.

 

[PeroK]

I am a high school student and I have a doubt regarding Complex Numbers.
when we define a complex number we say that a number of the form z=a+ib ,where a and b are real numbers is called a complex number.
a is called the real part and b is called the imaginary part, but I have a doubt here we know that ib is an imaginary number then why we say only b is the imaginary part of z why not ib.
for example- In z=5+3i why we say 3 is the imaginary part of z why not 3i.
I am very confused for this please help me...

It's purely convention. The way things are defined we have: $z = Re(z) +iIm(z)$, where $Re(z), Im(z)$ are real numbers. You could define $Re(z) = a$ and $Im(z) = ib$. That wouldn't change anything except a detail about how you name things. For example, normally we have: $$|z|^2 = Re(z)^2 + Im(z)^2$$
Whereas, with your definition we would have: $$|z|^2 = Re(z)^2 + (\frac{Im(z)}{i})^2$$