A question about different lens setups imaging a rotating stick

  • Thread starter roineust
  • Start date
  • #1
roineust
338
9
If i put a rotating stick behind lens of several types, so the stick center is behind the lens center, will the stick edges always appear to move at the same rate as areas closer to the stick center?
 
Last edited:

Answers and Replies

  • #2
A.T.
Science Advisor
11,649
2,946
If i put a rotating stick behind lens of several types,
Any possible lens?

so the stick center is behind the lens center, will always the stick edges appear to move at the same rate as areas closer to the stick center?
What is "rate"? Angular velocity? Instantaneous or average over a cycle?
 
  • #3
roineust
338
9
Any possible lens?


What is "rate"? Angular velocity? Instantaneous or average over a cycle?

What is the case for all the options that you have mentioned for main common types of lenses?
 
Last edited:
  • #4
33,662
11,233
I think he is trying to get you to make a better question, not volunteering to do it for you.
 
  • Like
Likes Vanadium 50 and roineust
  • #5
roineust
338
9
What is the case for all the options that you have mentioned and for all main different types of lenses?

By rate i mean, if there is always a straight line between the stick edge and the stick center.
 
Last edited:
  • #6
roineust
338
9
What is the case for all the options that you have mentioned and for all main different types of lenses?

I think i recall from visual memory, that for a certain lens type, if you look from the side of the lens at the stick, you will see the stick as a curved line and more curved as the stick gets closer to the lens edge, even if the stick is not rotating, is that correct?
 
Last edited:
  • #7
A.T.
Science Advisor
11,649
2,946
... even if the stick is not rotating...
So what's the point of the rotation?
 
  • #8
roineust
338
9
So what's the point of the rotation?

Correct, my question is also about the rate: Does there exist a common lens type, in which the stick seems to be curved toward the edges when the stick is not rotating and/or seems to be more/same curved toward the edges as a result of rotation, be it when looking from the side or from the center axis?
 
Last edited:
  • #9
A.T.
Science Advisor
11,649
2,946
Correct, my question is also about the rate: Does there exist a lens type, in which not only the stick does not appear to be straight, but it even seems to be more curved at the edge, as a result of rotation, be it when looking from the side or from the center axis?
Do you mean more curved than when not rotating at the same orientation? Are you asking about signal delay of the light rays due to the optical density of glass?
 
  • #10
roineust
338
9
Do you mean more curved than when not rotating at the same orientation? Are you asking about signal delay of the light rays due to the optical density of glass?

I am asking about such a phenomenon existence twice, once as a result of only the density of glass and again as a result of only the geometry of of the lens.
 
Last edited:
  • #11
A.T.
Science Advisor
11,649
2,946
I am asking about such a phenomenon existence twice, once as a result of only the density of glass and again as a result of only the geometry of the glass.
So no rotation then?
 
  • #12
roineust
338
9
So no rotation then?

As a result of rotation and also when there is no rotation.
 
Last edited:
  • #13
A.T.
Science Advisor
11,649
2,946
As a result of rotation and also when there is no rotation.
You already know that lenses can distort things. The rotation can create distortion due to different light travel times from different parts of the stick. But these are tiny non-noticeable effects at non-relativistic speeds.
 
  • #14
sophiecentaur
Science Advisor
Gold Member
27,805
6,323
If i put a rotating stick behind lens of several types,
Here's another example of a question that's so open that it just can't be answered usefully. We all read it slightly differently and so we can all come up with different answers. Wouldn't a DIAGRAM or even a selection of ("several") diagrams help us all to be talking about the same thing that the OP had in mind?
For a start, what is the axis of rotation of the stick?
 
  • #15
roineust
338
9
Here's another example of a question that's so open that it just can't be answered usefully. We all read it slightly differently and so we can all come up with different answers. Wouldn't a DIAGRAM or even a selection of ("several") diagrams help us all to be talking about the same thing that the OP had in mind?
For a start, what is the axis of rotation of the stick?

Here is a diagram and the question is if such a phenomenon happens in lens, first with no stick rotation and then with stick rotation. The apparent stick (in blue) is curved in one direction, but the question is about such a possible distortion, also in the opposite direction. The question is also about possible rate change, of the apparent stick curvature (a distortion that changes in time).

Rotatong Stick 2.jpg
 
  • #16
A.T.
Science Advisor
11,649
2,946
if such a phenomenon happens in lens, first with no stick rotation
Is the lens rotationally symmetrical around the view axis?
 
  • #17
roineust
338
9
Is the lens rotationally symmetrical around the view axis?

Yes
 
  • #18
sophiecentaur
Science Advisor
Gold Member
27,805
6,323
Here is a diagram
Brilliant, thanks. As they say "a picture is worth a thousand words". Sorry to be grumpy.
We have to assume total symmetry of any lens.
I'm sure you would not expect any measurable distortion. Even without the lens, the distance to the eye from various parts of the rod will be different and you would actually see the ends of the rod with a longer delay to the middle of the rod (which is nearest). So what you see would be different images, separated by much less than 1ns. What sort of rotation rate would you need in order to get a measurable difference in angle? This is along the same lines as early methods of measuring c with a rotating wheel [Fizeau's method] but he used a path distance of 8km and a toothed wheel, rotating at hundreds of rpm.
Using a lens would make the effect even less because the thickness of the glass is, in fact arranged to make the delay along all paths the same, from object to image (which is why it focuses).
 
Last edited:

Suggested for: A question about different lens setups imaging a rotating stick

  • Last Post
Replies
5
Views
361
Replies
17
Views
540
  • Last Post
2
Replies
58
Views
4K
  • Last Post
Replies
10
Views
444
  • Last Post
Replies
15
Views
474
Replies
4
Views
442
  • Last Post
Replies
10
Views
503
  • Last Post
Replies
7
Views
299
Top