Refraction and convex lenses

For a convex lens, where light travels from a fast medium (e.g. air) to a slow medium (e.g. glass), the light is bent towards the normal.

See below:

Light bends towards the normal line at surface one of a convex lens

NOTE: See Mammoth Memory, Refraction, for how to use the term "FAST SOFA" to recall that, when going from fast medium to slow medium, the light bends towards the normal.

For a convex lens, when light travels from a slow medium to a fast medium, the light is bent away from the normal.

See below:

Light bends away from the normal line at surface two of a convex lens

NOTE: Again, see Mammoth Memory, Refraction and use the term "FAST SOFA" to mean, here, that when passing from a slow medium to a fast medium, the light bends away from the normal.

The overall impact of refraction on a lens is:

Light ray bending at the two surfaces of a convex lens

We can see that the light has been bent twice. In some books this diagram is summarised as:

Convention for drawing a light ray passing through a convex lens

But technically this is not correct.