LENSES

What is Lenses?

Lenses are smoothly curved transparent materials, usually made of glass that refract light.

LENSES

The degree of bending depends on the curvature and index of refraction of the lens.

Two Main Types of Lenses

CONVEX ( Converging Lens)

CONCAVE ( Diverging Lens)

Also known as Converging Lens

CONVEX

Convex Lens

It is thick in middle and thin at the edges.

 Lens such that a beam of light passing through it is brought to a point or focus.

Also known as Diverging Lens.

CONCAVE

Concave Lens

It is thin at the middle and thick at the edges.

Lens such that a parallel beam of light passing through it is caused to diverge or spread out.

Different Kinds of Eye Defects

Myopia (nearsightedness) This is a defect of

vision in which far objects appear blurred but near objects are seen clearly. The image is focused in front of the retina rather than on it usually because the eyeball is too long or the refractive power of the eye’s lens too strong. Myopia can be corrected by wearing glasses/contacts with concave lenses these help to focus the image on the retina.

Hyperopia (farsightedness) This is a defect

of vision in which there is difficulty with near vision but far objects can be seen easily. The image is focused behind the retina rather than upon it. This occurs when the eyeball is too short or the refractive power of the lens is too weak. Hyperopia can be corrected by wearing glasses/contacts that contain convex lenses.

AstigmatismThis defect is when the light rays do not all

come to a single focal point on the retina, instead some focus on the retina and some focus in front of or behind it. This is usually caused by a non-uniform curvature of the cornea. A typical symptom of astigmatism is if you are looking at a pattern of lines placed at various angles and the lines running in one direction appear sharp whilst those in other directions appear blurred. Astigmatism can usually be corrected by using a special spherical cylindrical lens; this is placed in the out-of-focus axis.

PresbyopiaPresbyopia is part of aging. When

someone gets into their forties, and is caused by the hardening of the crystalline lenses in the eye. People with this will find that their arms are “getting shorter” as they have to hold reading material farther and farther away.

Strabismus  Strabismus, more commonly

known as crossed-eyes, is when the two eyes do not look toward the same object together. One eye is normal, while the other one points in some other direction. Strabismus can be corrected with prisms. The prisms bend the light so the light goes to the correct spot.

CataractA cataract is when the lens of the

eye or its capsule is opaque. It is the leading cause of blindness in the world. The word “cataract” means waterfall. For someone with a serious cataract, it's like seeing through a waterfall to them.

Sample Problem #1 A 4.00-cm tall light bulb is placed a distance of 45.7 cm from a double convex lens having a focal length of 15.2 cm. Determine the image distance and the image size. Like all problems in physics, begin by the identification of the unknown information.

ho = 4.00 cmdo = 45.7 cmf = 15.2 cmNext identify the unknown quantities that you wish to solve for.

di = ???hi = ???To determine the image distance, the lens equation must be used. The following lines represent the solution to the image distance; substitutions and algebraic steps are shown.

1/f = 1/do + 1/di1/(15.2 cm) = 1/(45.7 cm) + 1/di

0.0658 cm-1 = 0.0219 cm-1 + 1/di

0.0439 cm-1 = 1/di

di = 22.8 cm

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