Here it's written that : 
Aperture of the mirror is assumed as small. 
What does it mean ????
The surface of the mirror from which reflection can take place is called its aperture.
Generally the aperture of the mirror is assumed as small.
The central point on the surface of the mirror is called its pole 

It means that when we do the numericals we would take the diameter of the reflecting surface of the mirror(aperture) as very small.

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