1. Geometric Optics
Geometric Optics: The process of using light rays to determine how light behaves when it strikes an object.
Light travels in a STRAIGHT LINE

2. ANOTHER WAY TO THINK OF LIGHT
Light leaving the candle travels in all directions, but we only SEE the light that travels in to our Eye-Brain

3. ANOTHER WAY TO THINK OF LIGHT (CONT’D)
We don’t see light that doesn’t enter our eyes.

4. ANOTHER WAY TO THINK OF LIGHT (CONT’D)
If the light going away from eyes is reflected in to our Eye-Brain, then we will see it.

5. SIDE NOTE
When we look at diagrams, we will ignore light that doesn’t enter our Eye-Brain.
Don’t forget that the other light exists, we are just ignoring it to simplify our lives!

6. RAYS & DIAGRAMS
Ray Diagrams show how light moves from the object to the eye.
Light Ray – a line on a diagram representing the direction and path that light is travelling.

7. LUMINOUS SOURCES
Atoms in LUMINOUS OBJECTS emit light rays in all directions produced from other energy sources.
Atoms in NON-LUMINOUS OBJECTS scatter the light rays from luminous objects in all directions.

8. NOTE
Atoms in all objects produce or scatter light rays. This diagram only shows light rays from atoms at the top and bottom of the objects.

9. KEEP IT SIMPLE
To make diagrams simpler, we only show one ray of light from the top and bottom of objects.
We simplify it even more and just show only the rays coming from the top of the object.

10. MEDIUMS
A TRANSPARENT MEDIUM allows nearly all rays to pass straight through unaltered. (Ex. Air)
An OPAQUE MEDIUM absorbs or scatters all the rays. (Ex. Textbook, wall, etc.)

11. MEDIUMS (CONT’D)
A TRANSLUCENT MEDIUM transmits and scatters the rays. Wax paper causes light rays to bounce off and scatter, so you can see light coming through but no clear image.

12. The Laws of Reflection

13. Reflection - Images in Plane Mirrors

14. Where can we see an image of ourselves?
a mirror. 
calm water surface. 
a piece of shiny metal wall. 
a piece of window glass.

15. What are mirrors?
A mirror is any polished surface that allows an image to be produced through reflection. An image is a reproduction of the original object in front of the mirror that is produced through the use of light.

16. Mirror Normal Angle of incidence Angle of reflection Reflected ray
Incident ray
Reflected ray
Angle of incidence
Angle of reflection
Mirror

17. Properties of Light that Lead to Reflection
Light reflects off of ALL surfaces
A beam of light can be thought of as a group of individual rays of light all travelling together
Make sure to stress that light reflects off all surfaces – it is a common student misconception that light only reflects off shiny surfaces

18. Laws of Reflection Investigation
Page 482

19. The Two Laws of Reflection

20. Angle of incidence = Angle of reflection
LAW #1
Angle of incidence = Angle of reflection
In other words, light gets reflected from a surface at the same angle it hits it.
normal
incident ray
reflected ray
mirror

21. LAW #2
The incident ray, the reflected ray, and the normal all lie in the same plane.

22. Question 1 The angle between an incident ray and the mirror is 20.
The angles of incidence and reflection are ____ and ____.
20

23. Question 1
The angle between an incident ray and the mirror is 20. 70 70
The angles of incidence and reflection are ____ and ____.
20

24. P.O.E
Predict!
Will there be a difference if I shine light off of a smooth piece of aluminum foil compared to if I shine light off a crumpled piece of aluminum foil? VS

25. P.O.E
Observe!

26. P.O.E
Explain!
This can be explained with the terms specular reflection and diffuse reflection!

27. Specular Reflection Light reflects off a smooth, shiny surface
Examples: plane mirror, still water, smooth piece of aluminum foil
Produces a clear reflection
parallel incident rays
parallel reflected rays

28. Diffuse Reflection
Light reflects off a rough, not perfectly smooth surface
Examples: cover of a book, waves, a crumpled piece of aluminum foil
Produces a diffuse (unclear) reflection
parallel incident rays
reflected rays in different directions

29. Does the angle of incidence = angle of reflection?
True or False?
Reflection of light by a rough surface does not obey the 2 laws of reflection.
Does the angle of incidence = angle of reflection?
Does the incident ray, normal, and reflected ray lie in the same plane?

30. Does the angle of incidence = angle of reflection?
True or False?
Reflection of light by a rough surface does not obey the 2 laws of reflection.
Does the angle of incidence = angle of reflection?
Does the incident ray, normal, and reflected ray lie in the same plane?

31. Even in diffuse reflections, each incident ray is reflected back at the same angle it strikes the object! Angle of incidence = Angle of reflection
Additional example:
Textbook page 485
Figure 5

32. Specular Reflection
A specular reflection is reflection off a smooth surface
The light bounces off in a predictable and consistent way

33. Specular Reflection
A specular reflection is reflection off a smooth surface
The light bounces off in a predictable and consistent way

34. Diffuse Reflection
A diffuse reflection occurs off a surface that is not smooth
Light rays are scattered and reflect in many different directions
Even microscopically rough surfaces will produce this effect

35. Diffuse Reflection
A diffuse reflection occurs off a surface that is not smooth
Light rays are scattered and reflect in many different directions
Even microscopically rough surfaces will produce this effect

37. Parabolic (curved) Mirrors
Types of Mirrors
Plane Mirrors
Parabolic (curved) Mirrors

38. Properties of Plane Mirrors
The image is right-side-up (not inverted)
Left and right are reversed
The image is the same size as the object (unmagnified)
Light reflects with the properties of angles discovered earlier this class
May choose to wait until Day 10 when you explore mirror ray diagrams to outline the remainder of the properties of plane mirrors (e.g. image is virtual, virtual image distance = object distance)

39. Challenge Question: Which of these appears on the front of an ambulance?

40. The writing on an ambulance is reversed because…
The image in a plane mirror is reversed
Therefore, this is what we see when we look in the rearview mirror

41. More examples…

42. A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
Question
What letters of the alphabet will appear the same in a plane mirror?
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

43. A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
Question
What letters of the alphabet will appear the same in a plane mirror?
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

44. Using Light Rays to Locate an Image
We know that light travels in straight lines!
Our brain doesn’t understand that light reflects
When our eyes detect reflected light from a plane mirror, we project these rays backwards in a straight line

45. Source and Apparent Source
Light Source
Apparent Light
Source
Virtual rays are shown using dotted lines

46. Using Equal Perpendicular Lines to Locate an Image
3 cm
3 cm
Two Helpful Hints:
The distance from the object to the mirror is the same as the distance from the image to the mirror
2.7 cm
2.7 cm
If the tip of the pencil is 3 cm away from the mirror, the reflection of image will also be 3 cm away

47. Using Equal Perpendicular Lines to Locate an Image
Two Helpful Hints:
The object-image line is perpendicular to the mirror surface

48. Diagram
The Ray Diagram shows that:
ON = IN
OI ⊥NM N I O M

49. So how do we draw ray diagrams?
Locating Images
So how do we draw ray diagrams?

50. Drawing the virtual image
Step One:
Choose several points on the object and draw a perpendicular line to the mirror

51. Drawing the virtual image
Step Two
Measure the distance of this line
1 cm
1.5 cm
2 cm

52. Drawing the virtual image
Step Three
Extend this line an equal distance behind the mirror
1 cm
1.5 cm
2 cm

53. Drawing the virtual image
Step Four
Repeat these steps until you have enough lines to accurately locate your image
1 cm
1.5 cm
2 cm
Virtual image = an image in which light does not actually arrive at

54. Drawing the virtual image
Step Five
Draw in the reflected ray
1 cm
1.5 cm
2 cm
Virtual image = an image in which light does not actually arrive at

55. Drawing the virtual image
Step Five
Draw in the incident ray
1 cm
1.5 cm
2 cm
Virtual image = an image in which light does not actually arrive at

56. Practice Question 1
Mirror

57. Practice Question 1
Mirror

58. Practice Question 2
Mirror

59. Practice Question 2
Mirror

60. Going back to the beginning…

61. Four Characteristics of an Image
But….
How are we suppose to remember this?
SALT

62. S stands for SIZE What is the size of the object?
Larger, Smaller, or the Same Size as the object?
LARGER
SAME
SMALLER

63. A stands for ATTITUDE How is the image oriented?
Is the image upright or inverted?
UPRIGHT
INVERTED

64. L stands for LOCATION ? Is the image in front or behind the mirror?
Is the length from the object to mirror the same as the image to mirror? ?

65. T stands for TYPE Is the image real or virtual?
A real image is formed when light is actually arriving at the image location
A virtual image is formed from an apparent light source
VIRTUAL
REAL

66. Question: What is the SALT for plane mirrors?
Same Size
Upright
Behind the Mirror
(the same distance behind)
Virtual