Chapter 18-2

Lenses

Lenses

• A lens is made of transparent material, such as glass or plastic,

• with a refractive index larger than that of air.

• Each of the lens’s two faces is part of a sphere and

• can be convex, concave, or flat.

Convex Lenses

• Thicker at the center than at the edges

• Converging lens (they refract parallel light rays so that the light rays meet)

• see 6 cases for do in convex lenses worksheet

Real Images from Convex Lens

• To form a real image:• Object must be beyond 2F (equivalent to C for

mirrors)• Image is on other side of lens• Ray diagrams drawn the same as for mirrors

Why use a larger lens?

• More light will go through

• Brighter image

Concave Lens

• Thinner in the middle than at the edges• Diverging lens

(rays passing through it spread out)• The image is on the same side of the lens as

the object• The image is virtual, erect, reduced in size

(no matter how far object is from lens)• Focal length is negative

Defects of Lenses

Spherical Aberration (as found also in concave mirrors)

• Light rays that pass through the extreme edges of lenses do not meet at focal point

Fix: • in cameras, use only the center of the lens; • in telescopes—use a combination of convex and

concave lenses—Hubble Telescope had to have a fix for spherical aberration of its main mirror—images were fuzzy from launch in 1990 to fix in 1993

Chromatic AberrationLight rays that pass through

the extreme edges of lenses disperse as the edge of the lens acts as a prism

Fix:

join a convex lens with a concave lens with different index of refraction (this combination is called an achromatic lens)—used in all precision optical instruments

How the Eye Works:

Vision Defects

Nearsightedness:

• (a) Light focuses before the retina

• Cannot see distant objects

• Gets worse as the body grows

• (b) Fix: concave lens which will focus the light back on the retina

Vision Defects (cont)

• Farsightedness:• (c) Light focuses behind

the retina• Cannot see close objects• Gets worse as the body

ages (40+): lens is less flexible

• (d) Fix: convex lens which will focus the light back on the retina

Contact Lenses• Rests on a layer of

tears between it and the cornea

• Produces the same result as eyeglasses

• Most refraction occurs at air-lens surface where change in refractive index is greatest

Other Optical Instruments: Microscope

Use 2 convex lenses

• Object is put very close to lens with short focal length (the objective lens)

• Real image is produced between the 2 lenses

• Ocular or eyepiece produces greatly magnified virtual image

Other Optical Instruments: Refracting Telescope

• 2 convex lenses

• Objective lens has long focal length

• Light from distant star forms real image at the focal point of objective lens

• Eyepiece lens has short focal length

• Refracts light into parallel beam

• Viewer sees virtual, enlarged, inverted image

What kind of lenses are these?

Key Equations from this chapter:

End 18-2