You will need to replace the whole mirror. Just buy a replacement at auto zone and installe.
See link below
you take te old one out
To replace the driver side mirror on a 2003 Ford Focus you will need a socket and a Philips head screwdriver. Loosen the driver side door panel so that you can reach inside to unplug the wiring harness, and then remove the bolt and screw to remove the mirror. Put the new mirror in place and reattach everything you just removed.
since the convex mirror is curved outwards the the focus is behind the mirror
Use a vanity mirror and a torch so you see what you are doing.
It is called a Cassegrain. The small convex subreflector is a hyperboloid with one focus at the focus of the main mirror, and the other focus just behind the main mirror where the eyepiece is.
take the door panel off, take the plastic panle behind the side mirror off, and just remove the mirror. it should eather be held in by nuts, or mabe screws. hope that helps :D
It's a bit lengthy to recite word for word so . . . Follow the below link:
The mirror glass for a Ford Focus is available online at places like CariD and Parts Geek. You can also buy the entire mirror online on eBay for about $30.
The convex mirror diverges light rays, so if you draw the reflected rays in front of the mirror and continue drawing them at the back of the mirror the virtual light rays (at the back of the mirror) will join. This point is called a Virtual Focus Point.
A parabolic mirror would do the job.
there is an imaginary point in front of the concave mirror, called the focal point or focus, which is half the length of the radius of the sphere of which the mirror was a part of(radius of curvature). usually an object if seen in between the mirror and focus gets magnified, beyond the focus, it gets inverted and reduced.