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14.2: Lenses

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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.

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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

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Convex Lens Imaging• 2+ focal lengths =

real, smaller, upside down

• 1-2 focal lengths = real, bigger, upside down

• <1 focal length = virtual, larger, upright

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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

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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 cellsthat convert light energy toelectrical signals the opticnerve carries to the brainto be interpreted.

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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

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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.

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• 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.

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• Astigmatism:– Occurs when the surface of the cornea is curved

unevenly