1. Ch. 30
Lenses

2. What is a lens?
A lens is a piece of glass or plastic that bends parallel rays of light so that they cross and form an image.

3. What kind of lenses are there?
There are two basic types of lenses:
Converging (convex in shape)
Diverging (concave in shape)

4. Converging lenses are thicker in the middle and cause light rays to converge after passing through this lens.
Ex. Magnifying glasses, reading glasses

5. Diverging lenses are concave (thinner in the middle) in shape and tend to cause light rays to diverge or separate.
Ex. Eyeglasses for nearsightedness

6. The principal axis of a lens is the line joining the center of curvature of its surfaces.

7. For a converging lens, the focal point is the point at which a beam of parallel light, parallel to the principal axis, converges.

8. What is the focal plane?
The focal plane is a plane perpendicular to the principal axis that passes through either focal point.

9. A lens affects light coming from the left in the same way as light coming from the right. Thus a lens has two focal points and two focal planes.

10. The focal length is the distance between the center of a lens and its focal point.

11. Virtual or real?
A virtual image is an image where light appears to come from, but the light does not actually originate there. This light cannot be projected on a screen.

12. A real image is an image that can be focused on a screen because light does originate from the image.

13. What is a ray diagram?
Ray diagrams show the principal rays that can be used to determine the size and location of an image.

14. What rays are useful for ray diagramming?
1. A ray parallel to the principal axis that passes through the focal point after refraction by the lens. (principal ray)
2. A ray through the center of the lens that does not change direction. (central ray)
3.A ray through the focal point in front of the lens that emerges parallel to the principal axis after refraction by the lens. (focal ray)

15. How do you know how much a material will bend light?
n =c/v
n = refractive index
c = speed of light in a vacuum
v = speed of light in other medium

16. How do you know where the image will be?
Thin Lens Equation:
1/f = 1/di + 1/do
f = focal length
di = image distance
do = object distance

17. What instruments use lenses?
The camera

18. Camera optics

19. How does it work?
The telescope

20. How does it work?
The projector

21. How does it work?
The compound microscope

22. How does it work?
The human eye

23. How does the eye work?

25. Farsightedness corrected

26. Nearsightedness corrected

28. Extreme nearsightedness

29. Farsightedness corrected with glasses