Window glass has two sides, a front and a back. When the two sides are parallel to each other (both are flat), the refraction is minimized and the light will pass through the glass without changing direction very much.

However, if one or both sides of the glass are curved, light will bend as it passes through one side of the curved glass and then bend again as it passes through the other side of the curved glass.

There are two basic types of lenses: converging lenses and diverging lenses.

A converging lens takes light rays and brings them closer together to converge on a point.

A diverging lens bends the light rays away from each other - it diverges them. There are various shapes of each of these categories. You will hear about convex lenses, concave lenses, convex-concave lenses, plano-convex lenses, etc.

The point to remember is that whenever a lens is thicker in the middle than on the outsides, it is a converging lens. Whenever it is thicker on the outsides than in the middle, it is a diverging lens.

If you look at the diagram below and on the left, the light rays are passing through the glass, then being bent inward. This is a convex lens.

If you look at the concave lens, the light rays are passing though the glass and being spread out.