FOCUS AND FOCAL LENGTH OF SPHERICAL MIRRORS

 Principal focus of concave mirror 

The ray of light incident on a concave mirror which is parallel to the principal axis, passes through a fixed point on the principal axis after reflection. This point is the principal focus of a concave mirror.
Focus of concave mirror is real, can be focused to a screen and always be in front of the mirror.

Principal focus of a convex mirror 

Ray of light parallel to the principal axis incident on a convex mirror appears to diverge from a fixed point on principle axis from other side of a mirror. This is the principal focus of a convex mirror.
Focus of convex mirror is virtual because it is assumed to be at the back of the mirror (behind the mirror) and is not possible to focus them on a Screen 

Focal length (f)

Focal length of a mirror is the distance from pole of a mirror to its principal focus. Denoted by ‘f’.
Focal length(f) of a spherical mirror is always half the radius of curvature(R) of that mirror.
f = R/2

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