1. The Study of Mirrors and Lenses
Ch 19 Optics
The Study of Mirrors and Lenses

2. Mirrors
Optics includes the study of how mirrors and lenses form images.
To study this… we focus on Light traveling in rays as straight lines.
Ray Diagram - Shows how rays change direction when they strike mirrors and pass through lenses.
We draw these to show how mirrors and lenses work
Here are examples of ray diagrams we’ll be learning about

3. Law of Reflection Incident Ray - the “Incoming Ray”
Let’s define some terms…
Incident Ray - the “Incoming Ray”
Angle of Incidence (i) - The angle that the incident ray makes with a line drawn perpendicular to the surface of the mirror.
Angle of Reflection (r)- The angle the reflected ray makes with the perpendicular line.
The law of reflection states that the angle of reflection is equal to the angle of incidence.

4. Plane Mirrors How do Mirrors work?
Rays of light strike you, then reflect onto the mirror, and then reflect back into your eyes
The rays appear to come from behind the mirror
Plane Mirror - A mirror with a flat surface
You see a reversed (Right-to-Left) image of yourself
An image is a copy of an object formed by rays of light.
Your image is the same distance behind the mirror as you are in front and the image is right side up.
A plane mirror always produces a virtual image… Let’s discuss more about this…

5. L.O.S.T
L- Location: location of the image (in front or behind the mirror).
O- Orientation: which way the image is oriented compared to the original object (upright or inverted).
S- Size: compared to original object is it same size, smaller or bigger?
T- Type: is the image a real image or virtual image?

6. Plane Mirror Ray Diagrams
First, we draw an image of the object on the other side of the mirror
Distance A is equal to distance B and the image size is the same size as the object size.
Intersections will give you the image location!
Draw any small object you’d like under the left side “object” arrow

7. Intersections will give you the image location!
Plane Mirror Ray Diagrams
Second, we draw light rays from the image to the eye
The image is virtual. Broken lines from the image to mirror indicate virtual rays. Virtual image: Light rays do not actually meet at the image position. Because of that, a virtual image cannot be projected on a screen.
Continuous lines from the mirror to eye indicate the reflected rays.
Intersections will give you the image location!

8. Intersections will give you the image location!
Plane Mirror Ray Diagrams
Third, we join the light rays from the mirror to the object
Lines joining the object to the positions of the reflected rays on the mirror represent the incident rays by following the law of reflection.
Intersections will give you the image location!

9. Plane Mirrors con’t.
Plane mirrors always produce what is calleda virtual image
They only exist inside (behind) the mirror
These images cannot be projected onto any surface..
Virtual Image - a copy of an object formed where the light rays appear to come from. (The image of this object is not REAL: Rays do not really come from behind the mirror)

10. Section Mirrors
Let’s try! L: O: S: T: L: O: S: T:

12. Section Mirrors
Let’s try! L: O: S: T: L: O: S: T:

13. Plane Mirrors Characteristics of a plane mirror image:
L:Object distance from mirror = image distance from mirror
O: Orientation is ALWAYS upright
S: Object size = Image Size
T: ALWAYS forms a virtual imageImage is reversed- left to right

14. Plane Mirrors – we’ve just discussed…
types of mirrors
Plane Mirrors – we’ve just discussed…
Next up…
Concave Mirrors (ie: inside of a spoon)
Convex Mirrors (ie: outside of a spoon)

15. Concave Mirrors Concave Mirrors
Sometimes, mirrors show you very distorted images. (look at both sides of a metal spoon)
Concave Mirrors
The inside surface of the curved mirror is the reflecting surface
Focal Point- the point where the light rays meet
Concave mirrors reflect light rays that are parallel to the optical axis through the focal point.

16. Concave Mirrors
Real Image- A copy of an object formed at the point where light rays actually meet.
Real images can be viewed on a surface or screen.
The image is in front of the mirror.
Occur when the object is further away from the focal point. Reflected rays meet in front of the mirror
Virtual images occur when the object is closer to the mirror than the focal point. Reflected rays spread out and appear to be coming from behind the mirror
Concave mirrors can form real OR virtual images

17. Concave Mirrors
Concave mirrors can form real OR virtual images

18. Concave Mirrors When does it make a real image?
Object has to be farther away (behind) from the focal point
When does it make a virtual image?
Object has to be closer to (in front of) the focal point

19. Intersection will give you the image location!
Concave Mirror Ray Diagrams
Pick a point on the top of the object and draw two incident rays traveling towards the mirror.
Using a straight edge, accurately draw one ray so that it passes exactly through the focal point on the way to the mirror. Draw the second ray such that it travels exactly parallel to the principal axis. Place arrowheads upon the rays to indicate their direction of travel.
Intersection will give you the image location!

20. Intersection will give you the image location!
Concave Mirror Ray Diagrams
Once these incident rays strike the mirror, reflect them according to the two rules of reflection for concave mirrors.
The ray that passes through the focal point on the way to the mirror will reflect and travel parallel to the principal axis. Use a straight edge to accurately draw its path. The ray that traveled parallel to the principal axis on the way to the mirror will reflect and travel through the focal point. Place arrowheads upon the rays to indicate their direction of travel. Extend the rays past their point of intersection.
Intersection will give you the image location!

21. Intersection will give you the image location!
Concave Mirror Ray Diagrams
Find the location of the bottom of the object
If the bottom of the object lies upon the principal axis (as it does in this example), then the image of this point will also lie upon the principal axis and be the same distance from the mirror as the image of the top of the object. At this point the entire image can be filled in.
Intersection will give you the image location!

22. Concave Mirrors Let’s try! L: O: S: T: L: O: S: T:
What image was formed? L: O: S: T:
Let’s try! L: O: S: T:

23. Concave Mirrors L: O: S: T: L: O: S: T:

24. Convex Mirrors The outside surface is the reflected surface.
Reflected rays spread out.
Convex mirrors always cause light rays to spread out and can only form virtual images.
Rays appear to be coming from a point behind the mirror. This is where the image appears.
The image is always upright and smaller than the object.

25. Characteristics of a Concave mirror image:
Location of Object
Location of Image
Orientation of Image
Size of Image
Type of Image
Past F
Between mirror and F

26. Smaller or same size as object
Concave Mirror
Characteristics of a Concave mirror image:
Location of Object
Location of Image
Orientation of Image
Size of Image
Type of Image
Past F
In front of mirror
Inverted
Smaller or same size as object
Real
Between mirror and F
Behind mirror
Upright
Larger than object
Virtual

27. Intersection will give you the image location!
Convex Mirror Ray Diagrams
Pick a point on the top of the object and draw two incident rays traveling towards the mirror.
Using a straight edge, accurately draw one ray so that it travels towards the focal point on the opposite side of the mirror; this ray will strike the mirror before reaching the focal point; stop the ray at the point of incidence with the mirror. Draw the second ray such that it travels exactly parallel to the principal axis. Place arrowheads upon the rays to indicate their direction of travel.
Intersection will give you the image location!

28. Intersection will give you the image location!
Convex Mirror Ray Diagrams
Once these incident rays strike the mirror, reflect them according to the two rules of reflection for convex mirrors.
The ray that travels towards the focal point will reflect and travel parallel to the principal axis.
The ray that traveled parallel to the principal axis on the way to the mirror will reflect and travel in a direction such that its extension on the other side of the mirror passes through the focal point. Align a straight edge with the point of incidence and the focal point, and draw the second reflected ray. Place arrowheads upon the rays.
Intersection will give you the image location!

29. Intersection will give you the image location!
Convex Mirror Ray Diagrams
Locate and mark the image of the top of the object.
The image point of the top of the object is the point where the two reflected rays intersect. Since the two reflected rays are diverging, they must be extended behind the mirror in order to intersect. Using a straight edge, extend each of the rays using dashed lines. Draw the extensions until they intersect. The point of intersection is the image point of the top of the object. Both reflected rays would appear to diverge from this point.
Intersection will give you the image location!

30. Intersection will give you the image location!
Convex Mirror Ray Diagrams
Find the location of the bottom of the object
If the bottom of the object lies upon the principal axis (as it does in this example), then the image of this point will also lie upon the principal axis and be the same distance from the mirror as the image of the top of the object. At this point the complete image can be filled in.
Intersection will give you the image location!

31. Convex Mirror L: O: S: T: L: O: S: T:

32. Convex Mirror PROCESS BOX
Let’s try!
Convex Mirror L: O: S: T:
PROCESS BOX
How does the LOST description compare for both of the images produced by convex mirrors

33. Characteristics of a convex mirror image:
L: Image is ALWAYS behind mirror
O: Orientation is ALWAYS upright
S: Object size > Image Size
T: ALWAYS forms a virtual image

34. Mirrors: to Recap…
Convex mirror = virtual Concave = real OR virtual

35. Mirrors – reflect light SUMMARY TABLE OF ALL MIRRORs
Types of Mirrors
Images Formed
Virtual or Real
Direction of Image
Plane / Flat
Convex
(Curves outward)
Concave
(curves inward)
Object – Focal- Mirror
Focal- Object- Mirror

36. Mirrors – reflect light SUMMARY TABLE OF ALL MIRRORs
Types of Mirrors
Images Formed
Virtual or Real
Direction of Image
Plane / Flat
Virtual
Upright/opposite
Convex
(Curves outward)
Upright/smaller
Concave
(curves inward)
Object – Focal- Mirror
Real
Upside down
Focal- Object- Mirror
Upright

37. Do Now:
Complete Page 15 in your HW Packet
Agenda
Check HW
Go over HW
Teach Lenses Slide 19-28
HW: pgs 2, 6, 12, 13, 14, 16

38. Mirrors Finished……. onto lenses!

39. Lenses
When light enters a new medium at an angle, the change in speed causes the light to bend, or refract.
Index of Refraction – measure of how much the light bends. This is the ratio of the speed of light in a vacuum to the speed of light in the material.
A low index of refraction (near 1) causes light to slow and refract very little.
Air =
Diamond = 2.42
(This is why diamonds sparkle – lots of refraction)

40. Concave and Convex Lenses
Lens - an object made of a transparent material that has 1 or 2 curved surfaces that can refract light.
Light is bent while it passes through
The curvature and the thickness affect the way it refracts light.
Lenses work the opposite way mirrors do.

41. concave lens = divergent lens
Concave Lenses
concave lens = divergent lens
Concave Lenses are curved inward at the center and are thicker at the outside edges
Concave lenses cause incoming parallel rays to spread out or diverge (refraction caused moving through a new medium – air to glass)
Always form Virtual images
The image is formed at the point from which the refracted rays appear to come
The diverging rays appear to come from a single point, the focal point, on the same side of the lens as the object

42. Intersection will give you the image location!
Concave lens
Concave Lens Diagrams
Pick a point on the top of the object and draw two incident rays traveling towards the lens.
Draw one ray so that it travels towards the focal point on the opposite side of the lens; stop the ray at the point of incidence with the lens.
Draw the second ray such that it travels exactly parallel to the center line.
Add arrowheads
Intersection will give you the image location!

43. Intersection will give you the image location!
Concave Lens
Concave Lens Diagrams
Once these incident rays strike the lens, refract them according to the rules of refraction
The ray that travels towards the focal point will refract through the lens and travel parallel to the center line.
The ray that traveled parallel to the principal axis on the way to the lens will refract and travel in a direction such that its extension passes through the focal point on the object's side of the lens.
Place arrowheads upon the rays to indicate their direction.
Intersection will give you the image location!

44. Intersection will give you the image location!
Concave Lens
Concave Lens Diagrams
Locate and mark the image of the top of the object.
Since the refracted rays are diverging, they must be extended behind the lens in order to intersect.
Using a straight edge, extend each of the rays using dashed lines. Draw the extensions until they intersect.
The point of intersection is the image point of the top of the object.
Draw the image in
Intersection will give you the image location!

45. Intersection will give you the image location!
Concave Lens
Concave Lens Diagrams
Draw in the image
If the bottom of the object lies upon the principal axis (as it does in this example), then the image of this point will also lie upon the principal axis and be the same distance from the mirror as the image of the top of the object. At this point the complete image can be filled in.
Intersection will give you the image location!

46. Concave Lens
Let’s try! L: O: S: T:

47. Concave Lens
Let’s try! L: O: S: T: L: O: S: T:

48. convex lens = converging lens
Convex Lenses
Convex lenses are curved outward at the center and are thinnest at the outer edges
Convex lenses cause incoming parallel rays to come together and converge. (These lenses are sometimes called “converging lenses”)
These rays meet at a single point, the focal point, on the side of the lens opposite to the object
convex lens = converging lens

49. Convex Lenses, cont… Convex lenses form either real or virtual images
Real images are produced when an object is closer to the focal point than the lens (The object is also inverted)
Virtual images are produced when an object is closer to the lens than the focal point (The object is upright and larger)

50. Object Behind Focal Point Object in front of Focal Point

51. Intersection will give you the image location!
Convex Lens
Convex Lens Diagrams
Pick a point on the top of the object and draw two incident rays traveling towards the lens.
Using a straight edge, accurately draw one ray so that it passes exactly through the focal point on the way to the lens.
Draw the second ray such that it travels exactly parallel to the center line.
Add arrowheads
Intersection will give you the image location!

52. Intersection will give you the image location!
Convex Lens
Convex Lens Diagrams
Once these incident rays strike the lens, refract them according to the rules of refraction for converging lenses.
The ray that passes through the focal point will refract and travel parallel to the principal axis.
The ray that traveled parallel to the principal axis on the way to the lens will refract and travel through the focal point.
Place arrowheads upon the rays to indicate their direction
Intersection will give you the image location!

53. Intersection will give you the image location!
Convex Lens
Convex Lens Diagrams
Mark the image of the top of the object.
If the bottom of the object lies upon the principal axis (as it does in this example), then the image of this point will also lie upon the principal axis and be the same distance from the mirror as the image of the top of the object. At this point the entire image can be filled in.
Intersection will give you the image location!

54. Convex Lens L: O: S: T:
Let’s try! L: O: S: T:

55. Convex Lens L: O: S: T:
Let’s try! L: O: S: T:

56. Convex (curves outward)
Lenses – refract light
Types of Lenses
Images Formed
Virtual or Real
Direction of Image
Concave
(Curves inward)
Virtual
Upright/smaller
Convex (curves outward)
Object – Focal- Lens
Real
Upside down
Convex
(curves outward)
Focal- Object- Lens
Upright

57. Convex (curves outward)
Lenses – refract light
Types of Lenses
Images Formed
Virtual or Real
Direction of Image
Concave
(Curves inward)
Convex (curves outward)
Object – Focal- Lens
Convex
(curves outward)
Focal- Object- Lens

58. Total Internal Reflection
Fiber optics carry information as light and cable TV signals because the rays are unable to exit… …they simply bounce off the walls and continue moving down the cable
Critical Angle- Angle of incidence that produces an angle or refraction of 90°
Total Internal Reflection- Complete reflection of a light ray back into its original medium.

59. Section Review Questions
Which type of mirror is used in car headlights (convex or concave)?
Which type of mirror is used in movie projectors(convex or concave)?
THIS type of mirror always makes images that are virtual, smaller, and upright.
Explain how the Angle of Incidence and Angle of Reflection are related.
How does the location of “real” and “virtual” images differ for mirrors vs lenses?
Predict (specifically!) what would happen to the light transmission if a fiber optic cable were damaged by digging underground with a shovel.

60. Practice drawing ray diagrams
Type of mirror:
Image size:
Image Orientation:
Image Type:
Type of mirror:
Image size:
Image Orientation:
Image Type:

61. Practice drawing ray diagrams
Type of lens:
Image size:
Image Orientation:
Image Type:
Type of mirror:
Draw the image: