1. Chapter 18-2
Lenses

2. 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.

3. 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

4. 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

8. Why use a larger lens?
More light will go through
Brighter image

10. 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

11. 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

12. Chromatic Aberration
Light 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

13. How the Eye Works:

14. 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

15. 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

16. 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

17. 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

18. 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

19. What kind of lenses are these?

20. Key Equations from this chapter:

21. End 18-2