How a concave lens bends light

The surfaces of a concave lens are parts of spheres too, but they curve the other way — and so does the light.

Figure to be added

Figure 5.43 — two dashed circles with centres C1 and C2 whose inner edges form the two surfaces A and B of a concave lens, the principal axis through both centres.

As with a convex lens, a ray along the principal axis, or through the optical centre O, goes straight through without bending.

The focus of a concave lens

Rays parallel to the axis spread out — they diverge — after a concave lens. The point they seem to come from is the focus of the lens.