Convex Lens

This is a lens in which both surfaces are curved so that a beam of light passing through it is brought to a point or focus. A convex lens can be used to start a fire by directing the sun's rays to a point (your tinder) so it should only be used on bright, sunny days.

The lens can come from binoculars, camera, magnifying glasses; even ice can be used if it is carved in a shape similar to convex lens. (The ice must be highly transparent to behave as if it's a convex lens).

Angle the lens to focus on your tinder. Hold the lens over the same spot until the tinder begins to smolder. Gently blow on the tinder until it turns into flame.