You might find two different types of mirrors (or looking glasses) in your car.
Generally, the mirror mounted to the front window is a simple flat mirror. It probably wouldn't be described as a lens. The same is true for the mirror on the driver's side.
On the passenger's side of the car, one often finds a mirror which has words printed on it saying, "Objects in mirror are closer than they appear." This is a curved mirror which acts to broaden the field of view seen behind you. This makes everything smaller. This mirror is a convex mirror.
Some cars, and most large trucks, will have some additional convex mirrors. These are very curved and distort images a lot. But these gives the driver a very large field of view.