How a fixed focal length camera Focus ?

I have a SLR fitted with a prime lens(a lens with fixed focal length - 50 mm). So now that focal length(dist b/w lens plane and focus screen) is fixed, how does the camera focus ?

What does the focus ring(manual focus) on this prime lens do other than change this distance - foc. length is fixed rite?

The lens does move on lenses with a fixed focal length, but only regarding focus. This is why most of the people I know call them "Prime" lenses instead of fixed focal length.

The focal length of 50mm is for an object at distance infinity. For all other object distance an image is formed by moving the lens, the lens is adjusted by moving away from the film plane.

The fixed focal length of the lens merely indicates that it is not a zoom lens. The focus ring does in fact move a lens that is located between the two lenses that you can see.

At least that is what I remember. Could be wrong though.

The focusing screen and film plane are at the identical distances from the lens ... that is how you are able to use the viewfinder to focus images ... I has to be that way. The only other method is the way rangefinder cameras work, because you are not looking through the lens, but a separate viewfinder with a rangefinder to help the photographer focus the subject.

And just so you know there are three type lenses still used today. Prime lenses (50mm f/1.4 is a sample), Zoom lenses (12-24mm f/4.0 is a sample) and what is sometimes called "B" for bellows mounted or "M" for bellows mounted marco lenses (the Leitz 125mm f/2.5 is one example)