When the film is exposed it records an image formed by light rays that are focused by the lens into many circles of confusion. This, however, cannot happen with the use of just one convex lens. A single convex lens bends the different colors of visible light in different degrees, which doesn't allow them to meet at a single focal point. The circles of confusion are too large to create a crisp image. In order to fix this, cameras use a simple system of many lenses which refocuses the light to meet at one focal point.
Refraction is the bending of light.
The speed at which light travels depends on the medium it is traveling in. In a gas like air, light travels faster than in a solid or liquid like glass and water. Once they hit the glass lens of the camera the rays slow down and bend. Look at the diagrams on the left and right. For certain kinds of cameras like the single reflex lens camera, one can interchange different types of lenses.