1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

B Invisibility cloaking using 4 lenses

  1. Jan 17, 2017 #1
    So, here's an article about invisibility cloaking

    What I don't understand is how's the object between the lenses is cloaked. Is it because the rays are focused to a point after it passed through the first lens? (hence, there will be a region where rays do not pass through?) assuming that's the answer, but then in that case, isn't that, only rays that are parallel to the optical axis come into focus at one point? (which is the focal point) how about the rays that pass through optical center and the rays that pass through the first focal point and become parallel? if we draw a diagram, wouldn't these two principal rays disturb the cloaking region? i tried to draw a ray diagram to understand how this works. the final image of the object would be magnified and upright, but i still couldn't understand why can't we see the image, I really hope anyone here clear up my confusion because i have been thinking about this for days and my physics teacher couldn't help me that much. I have a very basic knowledge in optics such as constructing ray diagrams and solving problems using the thin lens equation. I'm sure there must be some concepts that i didn't know about. Here, I attach two diagrams i constructed to explain in a clearer way-hopefully- of what i don't understand. thank you in advance

    Attached Files:

  2. jcsd
  3. Jan 17, 2017 #2


    User Avatar
    Gold Member

    You seem to be overthinking it. They show very clearly how it's done. The rays from the first lens converge to a point BETWEEN the first two lenses and as long as you don't put anything in front of that exact focus point, it won't be seen.
  4. Jan 18, 2017 #3
    Yes, as you say, if you put the ruler at the focus point of the beam that is imaging the grid in the distance, then the image of the grid would be blocked and you would see the presence of the ruler. With the ruler just outside this focus point, you are not blocking the grid imaging beam, so you see the grid. But you don't see the ruler, as you might by just placing it behind a lens (although not in focus). Well, it's a narrow field of view optical system - notice only about an inch of the distant grid is viewed through the lenses - so once the ruler is off-axis enough, all rays from it are vignetted and won't reach your eye. That's my interpretation of what's happening and I'm having trouble understanding what is unique about it.
  5. Jan 18, 2017 #4


    User Avatar
    Gold Member

    EDIT: OOPS ... I should have said, as long as you don't put anything in front of the converging rays from the first lens since you COULD put them there but not block the focal point itself.
  6. Jan 21, 2017 #5
    It's too bad this thread didn't attract more attention. You might consider, in your first diagram at it is drawn, that the rays between the middle two lenses (at image position 2) would completely miss the next lens. What you drew is correct for finding image locations if you assume all the lenses are big enough to transfer the rays, but in reality, depending on the size of the lenses, this might not be the case.
  7. Jan 21, 2017 #6


    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    It's a shame that the demonstration is done like a conjuring trick, rather than demonstrating its limitations - i.e. that the illusion doesn't work all over the field of the lens. They use spread fingers and an offset ruler etc. which implies more than the true situation. It's a smart trick but hardly Lord of the Rings or Star Trek.
  8. Jan 21, 2017 #7
    oh, i think I understand it now, so if the lenses are not big enough, we wouldn't even see the image? is that correct? thanks for the replies
  9. Jan 21, 2017 #8
    The reduction of image intensity for off-axis points is referred to as vignetting.
  10. Jan 21, 2017 #9
    ouhh,i think i got it, thank you so much
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted