The camera lens is responsible for gathering the image, while the mirror is used to display it.
In camera anatomy, the lens and the mirror are two completely separate functions that work together to create and project an image inside of a camera. The lens is its own "appendage" while the mirror is part of the camera body itself. Both are critical components to a camera's function as the lens gathers the image and the mirror displays it.
Lens Refraction
The function of a lens is to gather the image for the camera to capture. In definition, a lens is simply a curved piece of glass that allows for a broader image to be taken in rather than only that which is directly in front of it. Refraction implies the collecting, bending and redistribution of light. The lens gathers all of the light coming in at different angles and directs them to form one single image, bending the beams and eventually projecting the correct image on either the film or sensor. Lenses are divided into categories of focal length and aperture. Focal length refers to the zoom, or how far the camera can extend in magnification (like a telescope). Focal length is measured in terms of millimeters (mm) such as a 28mm, 50mm, or 200mm lens. An aperture is the camera's equivalent to the iris in an eyeball; it allows more or less light into the camera during a given shot. The aperture is measured in f-stops, such as f3.5, f5, or f9.
Mirror Reflection
The lens is its own entity in a camera's construction and is usually removable. A mirror, however, is part of the camera's body itself. The mirror's sole function is to allow the photographer to view the image through the viewfinder. Without the mirror, the photographer would not know what they are taking a photograph of. When the image is gathered by the lens it is sent to the mirror and reflected upward and out of the viewfinder so the photographer can visualize the real image. This is the only purpose of the mirror, for when the photographer is ready to take the photograph, the mirror disappears, or rather "flips up" when the shutter button is pressed (that's why the screen goes black when you press the shutter, because with the mirror out of the way there is no way to view the image). Due to this"flipping" of the mirror when the shutter is pressed, the camera moves slightly causing a small amount of blur. To correct this matter, many high-technology cameras have "mirror-lock" features where the mirror is locked out of place before the shot is taken. In this case a tripod would be necessary for shooting.
Mirror Lenses
The mirror inside of the camera body will never change, however, there are a distinct breed of camera lenses known as mirror lenses that incorporate mirrors into the lens rather than glass. Mirror lenses contain two mirrors both at the front and back of the lens; the light that enters is reflected off of the back mirror onto the front mirror which is then reflected through a glass and onto the film or sensor. The advantages to mirror lenses are that they eliminate color shadows and can be shot with deep focal lengths. The downsides are that it is impossible to change the aperture or adjust the contrast, does not capture sharpness like refracting glass lenses and adds odd effects to the out of focus background objects.
