Focal point and length

This shows parallel beams from two helium-neon lasers converging to the principal focal point of a 30 cm double convex lens. The rays then enter a diverging lens of focal length -10cm on the right.

the point at which rays or waves meet after reflection or refraction, or the point from which diverging rays or waves appear to proceed.

the distance between the center of a lens or curved mirror and its focus.

the equivalent distance in a compound lens or telescope.