An optics question concerning pictures taken by camera

  • #1
guv
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Homework Statement
Here is the conceptual problem 23-3 (3rd example in chapter 23) on geometric optics from Giancoli's Physics textbook.

Q: Is the photo upside down? Close examination of the photograph on the first page of this Chapter reveals that in the top
portion, the image of the Sun is seen clearly, whereas in the lower portion, the
image of the Sun is partially blocked by the tree branches. Show why the reflection is not the same as the real scene by drawing a sketch of this situation, showing
the Sun, the camera, the branch, and two rays going from the Sun to the camera
(one direct and one reflected). Is the photograph right side up?
Relevant Equations
N/A
Answer from the book:
We need to draw two diagrams, one assuming the photo on p. 644
is right side up, and another assuming it is upside down. Figure 23–9 is drawn
assuming the photo is upside down. In this case, the Sun blocked by the tree
would be the direct view, and the full view of the Sun the reflection: the ray which
reflects off the water and into the camera travels at an angle below the branch,
whereas the ray that travels directly to the camera passes through the branches.
This works. Try to draw a diagram assuming the photo is right side up (thus assuming that the image of the Sun in the reflection is higher above the horizon than
it is as viewed directly). It won’t work. The photo on p. 644 is upside down.
Also, what about the people in the photo? Try to draw a diagram showing why
they don’t appear in the reflection. [Hint: Assume they are not sitting at the edge of
the pool, but back from the edge.] Then try to draw a diagram of the reverse (i.e.,
assume the photo is right side up so the people are visible only in the reflection).
Reflected images are not perfect replicas when different planes (distances) are involved.


I do not get the italicized part of the answer, especially the bold part. Not sure if it's because the description is too vague or if I overlooked anything in the answer. I have attached both the camera picture and the cartoon picture for the first part of the answer that makes sense.
 

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  • #2
I'm not sure what is puzzling you. Would it make more sense if it were to say "why they might not appear in the reflection"?
 
  • #3
Maybe... but then I don't get why you could change the wording from "don't" to "might". When do they show up? When do they not show up? What's the criteria?
 
  • #4
guv said:
Maybe... but then I don't get why you could change the wording from "don't" to "might". When do they show up? When do they not show up? What's the criteria?
Having established from the evidence of the sun and the obstructing tree branch that the picture is upside down, the question is: how come there are people on the far bank that you can see directly but not in the reflection?
That requires drawing a picture of how that can happen, in other words, why it could be the case. At least, that is how I would have worded it, because you are not required to prove that your drawing is the explanation; there could be another.
The last part asks, in effect, whether it could have been deduced that the photo is upside down from the evidence of the people alone.
 
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  • #5
Such is the power of one's visual cortex that a side-by side viewing of the profferred image with its parapatetic reflipped twin immediately points me to the veracity of the reflipped image. I think the sun in the trees is the driving force but no concious analysis is necessary for a firm conclusion!.
 
  • #6
Suppose there is a person sitting far from the edge of the water, when the point of reflection is over land, the person will not show up in the reflection.

Try to draw a diagram showing why
they don’t appear in the reflection. [Hint: Assume they are not sitting at the edge of
the pool, but back from the edge.] Then try to draw a diagram of the reverse (i.e.,
assume the photo is right side up so the people are visible only in the reflection).


For the 2nd part is the book referring to the same person far from edge? How does a person far from edge show up in the reflection in a upright picture? Does this have anything to do with the elevation of the camera? I am very confused by how the book is wording this.
 
  • #7
When the peoples SHADOW does not touch the water (because they are far back from the water), there is no reflection to show up in the water.
 
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  • #8
Thanks for the clarification! How does the shadow of a person show up in the up right picture if the shadow does not show up in the upside down picture? Or maybe that's not what the book is saying...
 
  • #9
guv said:
Thanks for the clarification! How does the shadow of a person show up in the up right picture if the shadow does not show up in the upside down picture? Or maybe that's not what the book is saying...
See post #7

When the peoples shadow does not touch the water (because they are far back from the water), there is no reflection to show up in the water.
 
  • #10
Right, I get that part. I am wondering now when the shadows would show up in reflection when the picture is taken up right as the book alludes to in the second part in post #6
 
  • #11
guv said:
Right, I get that part. I am wondering now when the shadows would show up in reflection when the picture is taken up right as the book alludes to in the second part in post #6
Well, if the people are standing right next to the water and the background doesn't keep the sun from hitting them there will be a reflection. Draw a picture !
 
  • #12
"right next to the water" part is what I am wondering...
 
  • #13
phinds said:
See post #7

When the peoples shadow does not touch the water (because they are far back from the water), there is no reflection to show up in the water.
The sun's reflection is further from the horizon than is the reflection of the people. Therefore the angle to the horizontal that the shadow takes from the heads of the people is greater than that of the reflection. This means the shadow can fall short of the shore but the line of the reflection does not.
 
  • #14
photo with reflection.jpg

In the diagram, the horizontal blue line is the water surface, the short thick vertical to the right is a person on the far shore. The angled solid lines are the light path from that person's head via reflection to my eye on the left.
Two possible land forms are shown for the far bank.
The dashed line rises steeply then runs level. I can see the person directly but not by reflection.
The dotted line rises slowly but continues up past the height of the person. I can see the person's reflection against the reflection of the sky, but viewed directly the person is invisible against the dark background.

Having proved from the evidence of the sun and intervening tree branch that the photo is upside down, we can infer that the dashed line may be correct but the dotted line cannot be.
 
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  • #15
I get what you are saying and the diagram, but is that what's asked in the original problem? I do not think there is a person in the picture taken by the camera.
 
  • #16
guv said:
I get what you are saying and the diagram, but is that what's asked in the original problem? I do not think there is a person in the picture taken by the camera.
There are five seven people in the photo. The question asks you to show why their reflections do not appear. As my diagrams prove, whether the people, their reflections, both or neither appear depends on the land form, which is unknown. So you can only show how, given that it is upside down, it is possible that the people appear and their reflections do not.

Edit: probably easier to spot them if you turn the picture the right way up.
 
Last edited:
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  • #17
guv said:
I do not think there is a person in the picture taken by the camera.
It took me an embarassingly long moment to find them.
1706740811954.png
 
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  • #18
Thanks so much! I can finally make sense out of this picture... :cool:
 
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  • #19
guv said:
Thanks so much! I can finally make sense out of this picture... :cool:
Reversed picture.jpg
 
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  • #20
DaveC426913 said:
It took me an embarassingly long moment to find them.
Don't overlook the person examining their cellphone while lounging against a wall!
1706764128232.png
 
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  • #21
DaveC426913 said:
It took me an embarassingly long moment to find them.

As I pointed out above in #5, our brains do a lot of subliminal "hard wired " processing. Here is the picture right side up. See the peoples? So much of this we learned in the early early timein life.
1706742358755.png
 
  • #22
hutchphd said:
As I pointed out above in #5, our brains do a lot of subliminal "hard wired " processing. Here is the picture right side up. See the peoples? So much of this we learned in the early early timein life.
View attachment 339535
That is NOT the picture right side up. What you have is the picture rotated 180 degrees.

THIS is it right side up
1706799391012.png
 
  • #23
Potato potaahto. Couldn't find the "invert" button, argument still holds for our brains.
 
  • #24
phinds said:
That is NOT the picture right side up.
Says who?

phinds said:
What you have is the picture rotated 180 degrees.
How is that not right side up?

Rotation works for a digital image but it also works for a physical photo. We don't actually know which one the photo is. Which is why I assumed the transformation that covers both possibilities.
 
  • #25
This is the question of why we don't see ourselves updside down in the mirror.
 
  • #26
hutchphd said:
This is the question of why we don't see ourselves updside down in the mirror.
AAAARRRRRGGGGHHHH and dammit ! You are obviously right. My bad. Thanks for the correction. For mine to be correct, you would have to be looking at the picture from the back while standing on your head :smile: (or you could flip the picture up/down and still have to be able to see through it to get my version.)
 
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  • #27
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  • #28
hutchphd said:
I love Feynman's lecture on parity
NIce. I hadn't run into that one before.
 
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