Classroom discussion I had in my Grade 10 class a few years ago. :)

1. boundless.com
Magnifying
Glass

2. wikipedia.org
Microscope

3. etsy.com
Telescope

4. bebusinessed.com
Telescope

5. wikipedia.org
Camera

6. bhphotovideo.com
Camera

7. What do these devices have in
common?

8. Image Formation on Lenses
Ray Tracing

9. Today
• Refraction (background)
• Converging Rays
– Applications
– Characteristics
– Image formation through ray tracing
– Exercises

10. Refraction
• Light also goes through some things
• The presence of material slows light’s progress
– interactions with electrical properties of atoms
• The “light slowing factor” is called the index of
refraction
– glass has n = 1.52 (meaning that light travels about 1.5
times slower in glass than in vacuum)
– water has n = 1.33
– air has n = 1.00028

11. glass; n2 = 1.5
air; n1 = 1.0
A
B
Refraction at a plane surface
Light bends at interface
between refractive
indices
Bends more the larger the
difference in refractive index
normal line

12. Convex Glass Surface
C
axis
A convex surface is called “converging” because parallel rays converge
towards one another
AIR (fast) GLASS (slow)
normal line
fast to slow bends
towards the normal

13. Convex Glass Surface
Caxis
The surface is converging for both air to glass rays and glass to air rays
AIRGLASS
normal line
slow to fast bends
away from the normal

14. Caxis
A concave surface is called “diverging” because parallel rays diverge
away from one another
Concave Glass Surface
AIR GLASS

15. C axis
Again, the surface is diverging for both air to glass rays and glass to
air rays
Concave Glass Surface
AIRGLASS

16. Lenses
converging lens
“bi-convex”
-has two convex
surfaces
diverging lens
“bi-concave”
-has two
concave
surfaces

17. CONVERGING LENS

18. Converging Lens
• The focal point of a curved
mirror was the image point
of a distant star
– It is the same for a lens.
– The focal point of a converging
lens is where the incoming
rays from a distant star all
intersect.
• A distant star is used to
guarantee that the incoming
rays are parallel
Focal point
Focal distance

19. F’F
Lenses
optic axis
2F 2F
principal axis
secondary focal point primary focal point

20. F
Similarly to a spherical mirror, incoming parallel rays
are deflected through the focal point

21. Thin Lenses
• Just as the ray tracing for mirrors is approximate and only
accurate for certain situations, the ray tracing for lenses is
accurate only for what are called “thin lenses”
F’F
thickness of lens
distance to focal point

22. How is the image formed by a
converging lens?

23. Converging Lens: Ray Tracing Rules
Rule 1:
Similarly to a spherical mirror, incoming parallel rays are deflected
through the focal point.
FF

24. Converging Lens: Ray Tracing Rules
Rule 2:
Rays passing through the center of the lens are undeflected, they
continue straight through without being bent. Several rays are
shown here as examples.
FF

25. Converging Lens: Ray Tracing Rules
Rule 3:
The reverse of Rule 1, rays passing through the focal point are
deflected to exit parallel to the axis
FF

26. Major Rays used in Ray Tracing

27. The incident light ray from the object
that is parallel to the principal axis will
be refracted passing through the
principal focal point after passing
through the optic axis.
Parallel Ray

28. F F’2F 2F’
Parallel Ray

29. The incident light ray that passes
through the secondary focal point will
be refracted parallel to the principal
axis.
Focal Ray

30. F F’2F 2F’
Focal Ray

31. The incident light ray that seems to
pass through the optical center will not
be refracted.
Optic Ray

32. F F’2F 2F’
Optic Ray

33. F F’2F 2F’
Image

34. Exercises

35. F F’2F 2F’
(1) Locate the image and
(2) Describe its characteristics
Focal length = 5 cm
Object’s location = 10 cm
Object’s height = 3 cm
Lens’ height = 8 cm

36. F F’2F 2F’
(1) Locate the image and
(2) Describe its characteristics
Focal length = 5 cm
Object’s location = 5 cm
Object’s height = 1 cm
Lens’ height = 8 cm

37. DIVERGING LENS

38. F’F
In diverging lens, parallel rays are deflected such that when extended
backwards, they appear to be coming from the focal point on the other side.
DIVERGING LENS

39. How is the image formed by a
diverging lens?

40. Diverging Lens: Ray Tracing
F’F
Parallel rays are deflected so they appear to be coming from the focal
point in front of the lens.

41. Diverging Lens: Ray Tracing
F’F
Just like for converging lenses, rays that pass through the center of
the lens continue undeflected (straight) through the lens.

42. Diverging Lens: Ray Tracing
F’F
Rays that, if extended, would pass through the focal point on the
other side of the lens, are deflected to be parallel to the axis.

43. Major Rays used in Ray Tracing

44. The incident light ray from the object
that is parallel to the principal axis will
be refracted as if it came from the
secondary focal point.
Parallel Ray

45. F F’2F 2F’
Parallel Ray

46. The incident light ray that seems to
pass through the principal focal point
will be refracted parallel to the
principal axis.
Focal Ray

47. F F’2F 2F’
Focal Ray

48. The incident light ray that seems to
pass through the optical center will not
be refracted.
Optic Ray

49. F F’2F 2F’
Optic Ray

50. F F’2F 2F’
Image

51. Diverging Lens: Image Formation
F’F
The image is virtual*, reduced, and right side up.

52. F F’2F 2F’
(1) Locate the image and
(2) Describe its characteristics
Focal length = 5 cm
Object’s location = 10 cm
Object’s height = 3 cm
Lens’ height = 8 cm

53. F F’2F 2F’
(1) Locate the image and
(2) Describe its characteristics
Focal length = 5 cm
Object’s location = 5 cm
Object’s height = 1 cm
Lens’ height = 8 cm

54. CORRECTIVE
LENSES

55. Corrective Lenses: Myopia
To correct myopia (nearsightedness), a diverging lens creates an
intermediate image of a distant star at your far point so that your eye
can see it even though the star is beyond your far point.

56. Corrective Lenses: Myopia
To correct myopia (nearsightedness), a diverging lens creates an
intermediate image of a distant star at your far point so that your eye
can see it even though the star is beyond your far point.
far point
image of distant object

57. Corrective Lenses: Hyperopia
To correct farsightedness your contact lens creates an (intermediate)
image of a book 25 cm away at your near point so that your
farsighted eye can see it even though the book is closer than your
near point
25 cmnear point

58. Corrective Lenses: Hyperopia
To correct farsightedness your contact lens creates an (intermediate)
image of a book 25 cm away at your near point so that your
farsighted eye can see it even though the book is closer than your
near point
near point 25 cm
focal point of corrective lens