1. Viewing Images in Plane Mirrors
Seeing Things
Viewing Images in Plane Mirrors

2. When we look in the Mirror we see an Image of the Object.

3. In the previous slide show you saw that the Image appears to be the same distance behind the Mirror as the Object is in front of it.
You also saw that a line connecting the Object and Image intersects the Mirror at 90 °

4. The Image appears to be behind the Mirror
The Image appears to be behind the Mirror. But the light rays can not come through the Mirror. Mirrors are Opaque.

5. Question 2) How can the light rays appear to go through an opaque Mirror?

6. Get this graphic from your notes .

7. Place a MIRA or Reflect View on it as shown .

8. Place a MIRA or Reflect View on it as shown .

9. Place a MIRA or Reflect View on it as shown .

10. Notice you can see an Image in the “Mirror” If you look over the top, behind the Mirror there is no Image. The light rays must come from the Object and reflect off the Mirror . How do they do this?

11. On the graphic in your notes, the back of the Mirror is identified by diagonal lines. This is where light is reflected on glass mirrors
The Arrow over the candle is there because it’s easier to draw as an Image

12. First, find the location of the Image by drawing a line from the top of the Object through the Mirror so that it intersects at 90°.
This line should be drawn very lightly so that it does not confuse the diagram later.
The Arrow over the candle is there because it’s easier to draw as an Image

13. Measure the distance from the top of the Object to the Mirror (dO).
The Arrow over the candle is there because it’s easier to draw as an Image

14. Copy this distance (dOtop) to the other side of the Mirror
Copy this distance (dOtop) to the other side of the Mirror. It becomes (dItop).

15. Repeat this process for the bottom of the Object.
This line should be drawn very lightly so that it does not confuse the diagram later.
The Arrow over the candle is there because it’s easier to draw as an Image

18. These two points give the location of the top and bottom of the Image
These two points give the location of the top and bottom of the Image. For any point dO = dI
The Arrow over the candle is there because it’s easier to draw as an Image

19. To make the diagram less cluttered I’ve removed the unneeded part of the dotted line

20. Rays of light must be entering the eye as if they had come straight from the Image. That is the way the eye sees things.

21. The line is dotted behind the mirror because light rays can not through an opaque Mirror. They travel in this direction but could not come from the Images location.

22. The light rays that appear to come from the top of the Image really came from the top of the Object and reflect off the Mirror into the eye.

23. The light rays that appear to come from the bottom of the Image really came from the bottom of the Object and reflect off the Mirror into the eye.

24. The light rays reflect most strongly off the back of glass mirrors
The light rays reflect most strongly off the back of glass mirrors. A metallic coating applied to the back of the glass causes the reflection.

25. Reality Check Use a MIRA or Reflect View to see if the Predicted Image you drew and the real Image, overlap. If they do the light rays must reflect as predicted

26. Your Turn
Answer the questions in your notes,
Note page

27. Your Turn
Answer the questions in your notes,
Note page

28. Question Answers follow
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29. Your Turn In your notes,
Show how light is reflected from the mirror so that the eye sees the Image behind it.
Note page

30. A) should look like. Try B)
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31. B) should look like. Try C)
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32. C) should look like. Try D)
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33. D) should look like. Try the next questions
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34. Which Eye/ Brains can see the Object?

35. Which Eye/ Brains can see the Object?
C) and D) only. Light travels in a straight line from the object to the Eye/Brain. Rectilinear Propagation is the term for this

36. Which Eye/ Brains can see the Image in the Mirror?

37. Which Eye/ Brains can see the Image in the Mirror?
A) C) and D) only. Light appears to travel in a straight line from the Image to the Eye/Brain. Rectilinear Propagation is the term for this

38. Full Height Mirror.
How big does a Mirror have to be in order to see yourself “Head to Toe”? (Hint You see your image)
Understanding occurs at many levels. First have a students attempt a theoretical (academic) solution like the one that follows. They must also understand it at an experiential (emotional) level.
Stars Indicate activities that should be done before continuing. Do the Full Body Mirror Activity. Make sure the mirror is perpendicular to the floor.
The Full Body Mirror Activity is in the Plane Mirror Folder
Reference I highly recommend listening to the following podcast from the Exploratoriums Teacher Institute Teaching Tips: Through the Looking Glass by Thomas Humphrey
Exploratorium | The museum of science, art and human perception
at the Palace of Fine Arts, 3601 Lyon Street, San Francisco, CA 94123

39. As usual find the location of the Image, by measuring the perpendicular distance from the Object to the Mirror.

40. Copy this distance to the other side of the Mirror.

41. This gives the location of the Image.

42. Draw a ray of light, entering the eye as if they had come straight from the top of the Image.

43. This light ray really came from the top of the Object.

44. Draw a ray of light, entering the eye as if they had come straight from the bottom of the Image.

45. This light ray really came from the bottom of the Object and reflected off the Mirror.

46. The large orange triangle has a height which is the distance from the Object to the Image (dI+dO = 2dO ) Its base is the height of the person (two arrows)

47. The smaller light orange triangle has a height which is the distance from the Object to the Mirror (1 dO ) or half the height of the larger triangle. From similar triangles the size of the mirror is half the height of the person

48. Does how far away you stand affect the size of the mirror
Does how far away you stand affect the size of the mirror? You still want to see yourself “Head to Toe”.

49. As usual find the location of the Image, by measuring the perpendicular distance from the Object to the Mirror.

50. Copy this distance to the other side of the Mirror.
Understanding occurs at many levels. Some students will have to see this. Hold a large mirror against a wall to make sure it is perpendicular to the floor. Have a skeptical student view their Image in the mirror and use a piece of Bristle board to block off the bottom of the mirror until the student can “just” see their feet.
Ask the student to walk toward the mirror keeping an eye on their feet. Ask them if they can still Just see their feet.

51. This gives the location of the Image.

52. Draw a ray of light, entering the eye as if they had come straight from the top of the Image.

53. This light ray really came from the top of the Object.

54. Draw a ray of light, entering the eye as if they had come straight from the bottom of the Image.

55. This light ray really came from the bottom of the Object and reflected off the Mirror.

56. The large orange triangle has a height which is the distance from the Object to the Image (dI+dO = 2dO ) Its base is the height of the person (two arrows)
Understanding occurs at many levels. Some students will have to see this. Hold a large mirror against a wall to make sure it is perpendicular to the floor. Have a skeptical student view their Image in the mirror and use a piece of Bristle board to block off the bottom of the mirror until the student can “just” see their feet.
Ask the student to walk toward the mirror keeping an eye on their feet. Ask them if they can still Just see their feet.

57. As before the smaller light orange triangle has a height equal to the distance from the Object to the Mirror (1 dO ) or half the height of the larger triangle. From similar triangles the size of the mirror is half the height of the person
Understanding occurs at many levels. Some students will have to see this. Hold a large mirror against a wall to make sure it is perpendicular to the floor. Have a skeptical student view their Image in the mirror and use a piece of Bristle board to block off the bottom of the mirror until the student can “just” see their feet.
Ask the student to walk toward the mirror keeping an eye on their feet. Ask them if they can still Just see their feet.

58. So where is the Image - Really Below are two eye charts, one is inverted so that it is easily read in a mirror. The other is normal. Each line is exactly half the size of the line above it
Understanding occurs at many levels. First have a students attempt a theoretical (academic) solution. They must also understand it at an experiential (emotional) level.
Stars Indicate activities that should be done before continuing. Before explaining the theoretical (academic) solution do the group labs.
Have a sceptical student hold an inverted Eye Chart and view it’s Image in a mirror. Have them read smaller and smaller lines until they make more than one mistake. Repeat with another student standing beside the mirror holding an identical but normal Eye Chart. Make sure there is enough room behind the mirror.
The Eye Charts are in the Plane Mirror Folder

59. A Inverted Eye Chart is held. Its Reflection is viewed in a Mirror.
How difficult will it be to read the Image of an Eye Chart held by the student? Explain your Choice
Understanding occurs at many levels. Do the next two problems as group labs first. Have a skeptical student hold an inverted Eye Chart and view their Image in a mirror. Then have them read smaller and smaller lines until they make more than one mistake. Repeat with another student standing beside the mirror. Make sure there is enough room behind the mirror.

60. Would it be the as difficult as reading the an Eye Chart held half the distance to the Mirror?
Understanding occurs at many levels. Do the next two problems as group labs first. Have a skeptical student hold an inverted Eye Chart and view their Image in a mirror. Then have them read smaller and smaller lines until they make more than one mistake. Repeat with another student standing beside the mirror. Make sure there is enough room behind the mirror.

61. Would it be the as difficult as reading the an Eye Chart held at the distance of the Mirror?
Understanding occurs at many levels. Do the next two problems as group labs first. Have a skeptical student hold an inverted Eye Chart and view their Image in a mirror. Then have them read smaller and smaller lines until they make more than one mistake. Repeat with another student standing beside the mirror. Make sure there is enough room behind the mirror.

62. Would it be the as difficult as reading the an Eye Chart held twice the distance to the Mirror?
Understanding occurs at many levels. First have a students attempt a theoretical (academic) solution. They must also understand it at an experiential (emotional) level. Before explaining the theoretical (academic) solution do the group labs.
Have a sceptical student hold an inverted Eye Chart and view it’s Image in a mirror. Have them read smaller and smaller lines until they make more than one mistake. Repeat with another student standing beside the mirror holding an identical but normal Eye Chart. Make sure there is enough room behind the mirror. The Eye Charts are in the Plane Mirror Folder

63. Answer and Explanation follows Before looking at the theoretical (academic) solution do the group lab.
Understanding occurs at many levels. First have a students attempt a theoretical (academic) solution. They must also understand it at an experiential (emotional) level. Before explaining the theoretical (academic) solution do the group labs.
Have a sceptical student hold an inverted Eye Chart and view it’s Image in a mirror. Have them read smaller and smaller lines until they make more than one mistake. Repeat with another student standing beside the mirror holding an identical but normal Eye Chart. Make sure there is enough room behind the mirror. The Eye Charts are in the Plane Mirror Folder

64. The Image of an Eye Chart held by a student would be as far behind the Mirror as the student is in front.

65. Therefore it would be as difficult as reading the an Eye Chart held twice the distance to the Mirror

66. If you can just read the Middle line of an Eye Chart taped onto a Mirror
Which line of the same sized Eye Chart could you be able to read when viewed in a Mirror? Would it be, a) one Line up (2X’s Larger) b) the same line c) one line down (Smaller by half)

67. The size of the Image viewed in a Mirror is given by the angle between the rays coming from the top and bottom of the Eye Chart Image. This angle is the same as that of an Eye Chart One half as high at the Mirror. The lines would all be shrunken by one half. Therefore you would could only read the larger lines (one up or 2X’s Larger, on the Image)
Each line is double the one below. Viewed in the image they are shrunken by one half