First, we will look at what happens when a ray travels through a piece of material with flat parallel sides. When the ray is perpendicular to the first side it slows down but does not change direction, so when it gets to the other side it is still perpendicular, so it will continue in the same direction as it speeds up again.

Then we look at what happens to a ray travelling through the same flat material at an angle. When it enters the material it is refracted. Since the sides of the material are parallel to each other, you may remember from the math class that you have equal alternate angles, so the angle of incidence at the second surface is equal to the angle of refraction at the first surface. No mater what angle you start with, Snell's law will give you the same angle of refraction for the second surface that you had for an angle of incidence at the first surface if the surfaces are parallel. So the ray of light emerges from the material travelling in the same direction as it was originally going - only it is offset slightly. 

Third, look at the material where the two surfaces are not parallel to each other. Refraction occurs at each surface. Even if the ray is perpendicular at one surface it will bend at the other surface.

Fourth, we only need to consider a curve to be a series of straight lines, each at a slightly different angle. In the diagram below we have a lens that has one flat surface and one "curved" surface. The diagram shows all of the rays perpendicular to the flat surface, so bending of the rays occurs only at the second surface.   The rays could come to the lens at an angle other than 90 degrees, and then each ray would bend at both surfaces, and each would still exit the lens going toward each other in the first diagram and away from the others in the second diagram..
 
 

A lens is a piece of material that transmits light, with two polished surfaces, at least one of which is curved.  Of the four diagrams in this part of the lesson, only two are diagrams of lenses.  Which diagrams are not lenses?