Why a convex lens?

Question

What type of lens would be helpful in starting a campfire?

--my guess would be a convex lens (like a magnifying glass)

But why? Why would this type of lens be helpful in starting a campfire?

Answer 1

A convex lens can converge the rays of the sun that fall on the entire lens into a small area at the focus, thus producing a hot spot. A concave lens does not do that.

Answer 2

A biconvex lens (convex on both sides) bends parallel rays (from distant light sources) to a single point, called the focal point of the lens.

The image created at the focal plane is basically a dark circle with all the light (that would have filled the circle) concentrated into a point at the center.  The light intensity at this point is very high (over 100 times as high as ordinary sunlight), and can heat small objects to the point of self-ignition.