Every body contains a natural lens that uses a focal point, the human eye. The exact date of the invention of lenses is not known, but archeological evidence found lenses over 2700 years old in Nimrud, the capital of the former Assyrian empire, according to the New York University physics department's lens lab. The first definite evidence of the deliberate use of focal points occurred in corrective eyeglasses in 13th century Italy (See References 1; Page 1).
Focal points are the converging point of light in lenses with a curvature, reports NYU. The focal length is a measurement from the center of a lens to the focal point. You can locate a focal point by taking the inverse of the illuminating object, added to the inverse distance of the image from the lens. The distance of a focal point can be on either side of a lens, depending on the location of illuminating light (See References 1; Page 1).
Focal points can be negative or positive, claims Georgia State University's physics department. Convex lenses, which have a bulge, focus light at the focal point. Concave lenses, which are thin in the middle, have a "negative focal point" on the side of the lens with the illumination. Concave lenses give the appearance of light emanating from the focal point (See References 2).
Focal points are essential for optometrists to create corrective eyeglasses. The inverse of the focal point distances gives you the magnifying power of a lens, reports Georgia State University. People who are farsighted need closer focal points to increase the size of distant images. Nearsighted people need a concave lens that focuses the images closer to the lens (See References 3).
All lenses have some focal point errors, called "chromatic dispersion," reports Georgia State University. This happens because the different colors of light have separate wavelengths and thus refract differently when emerging from a lens. You can see this occur in nature with rainbows. The solution to chromatic aberration requires using shining light through convex and then a concave lens, called a doublet (See References 2).