• Reversibility of light and returning lens backward

    From Eric@21:1/5 to All on Sat May 26 20:05:41 2018
    Dear all,

    I have quite a naive question but I can't manage to really figure the explanation and even if my assumption is true.

    let's suppose you have an object, a lens and the object image ;
    with the object at the exact same location if you turn the lens backwards so so frontal face is now the back, will the image be the same as before ?

    I presume that for a thin simple lens this is true, but I also guess that it's not true for more complex optical systems which do not behave the same if you return them backwards.

    But I also think that the reason why it is so is due to reversibility of light, so that I'm confused because thsi principle should work in both situations wether with a simple thin lens or with a complex systems with multiple elements (like a microscope
    or a telescope).

    Can you help me and tell where I'm wrong ?

    Thanks

    Fred




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  • From Lu Wei@21:1/5 to Eric on Sun May 27 08:38:58 2018
    On 2018-5-27 2:05, Eric wrote:
    Dear all,

    I have quite a naive question but I can't manage to really figure the explanation and even if my assumption is true.

    let's suppose you have an object, a lens and the object image ; with
    the object at the exact same location if you turn the lens backwards
    so so frontal face is now the back, will the image be the same as
    before ?


    Not in the general case.

    I presume that for a thin simple lens this is true, but I also guess
    that it's not true for more complex optical systems which do not
    behave the same if you return them backwards.

    You are right. For complex "thick" systems there is a only special point
    (maybe called origin point) by which you can turn the system backwards
    and the image remains at the same place.

    But I also think that the reason why it is so is due to reversibility
    of light, so that I'm confused because thsi principle should work in
    both situations wether with a simple thin lens or with a complex
    systems with multiple elements (like a microscope or a telescope).

    No, the reason is the symmetry of imaging formula:
    1/od + 1/id = 1/f
    in which object distance and image distance is exchangeable -- if turned
    at the origin point.

    --
    Regards,
    Lu Wei
    PGP key ID: 0xA12FEF7592CCE1EA

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