# 14.2: Lenses.

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14.2: Lenses

I. What is a lens? A lens is a transparent material with at least one curved surface that causes light rays to bend, or refract, as they pass through. The image formed depends on the shape of the lens.

II. Convex Lenses Thicker in the middle than the edges
Focuses light rays at the focal point; light rays converge Focal length depends on the shape of the lens Rays on the optical axis are not bent at all EX: magnifying glass

Convex Lens Imaging 2+ focal lengths = real, smaller, upside down
1-2 focal lengths = real, bigger, upside down <1 focal length = virtual, larger, upright

III. Concave Lenses Thinner in the middle than the edges
Light rays spread outward away from the optical axis; light rays diverge Image is virtual, upright, & smaller Used in some eyeglasses & telescopes

IV. Lenses & Eyesight Light enters through a transparent covering on your eyeball called the cornea. It causes the light to bend so it converges. The light then passes through pupil. Behind the pupil is a flexible convex lens that focuses the light rays to form a sharp image on your retina. The retina is the inner lining of the eye that has cells that convert light energy to electrical signals the optic nerve carries to the brain to be interpreted.

Seeing Near & Far The lens in your eye changes shape so you can focus on objects at different distances. The lens is flexible, and muscles attached to it change its shape and focal length. Far away = less convex Nearer = more convex

V. Vision Problems Farsighted:
See distant objects clearly, but can’t bring nearby objects into focus. Usually comes with age because the lens becomes less flexible Corrected by a convex lens that causes light rays to converge before entering the eye.

Nearsighted: Can see objects only when they are nearby
Caused by cornea and lens focusing light in front of the retina Corrected by a concave lens that spreads light rays so that they focus correctly on the retina.

Astigmatism: Occurs when the surface of the cornea is curved unevenly