Anatoly A Khripov what is the experiment you refer to?
- What is the optical configuration?
- Is there a single lens?
- Where is the source located with respect to the lens?
- Is it a conventional lens, zone plate, diffractive or holographic lens?
It is very difficult to make constructive comments without more information about your problem.
Why do you expect an infinite number of real images? With a single conventional converging lens there is usually no more than a single real image.
Dear Alan Robinson
I mean very simple setup - Bessel's method for finding focal length. So we have 1 converging lens, source and screen. In the method we have two real images as the source to screen distance is more than 4 focal lengths of the lens. In the experiment the source to screen distance is constant, we move only the lens. First we adjust the lens-to-source distance to get one clear larger real image. We write the lens position. Then we move the lens to get the second clear smaller real image. We write the lens position. Then we can calculate the focal length using some formula (it simply is deduced from thin lens formula for the setup).
So why we have only two real images or even one as you told? I would be appreciate if you give me the references.
There can only be two real images maximum. Given a fixed distance from the object to the screen, the location of the lens occurs at the two conjugate points, one providing an enlarged image of the object and the other a diminished image of the object at the two positions respectively
Book Basics of Photonics and Optics
Dear Srini Vasan,
I told the same - two real images. But why? Everybody solved the physics textbook problems, like the following. E. g. you can geometrically draw some light rays passing through the lens and you can draw the image. The drawn image will be reversed and enlarged or dimished, real or imaginary depending on the source to lens distance. So according this drawing technique yoeu get infinite number of real images. Could you tell about the method of building the image of real lens in practice?
If you have a source separated by distance u from the principal plane of a converging lens of focal length, f, then an image is formed at distance v from the lens.
The lens equation requires that 1/u + 1/v = 1/f
https://en.wikipedia.org/wiki/Lens#Imaging_properties
If the subject distance, u, is greater than the focal length, then there is a single real image formed at distance v on the opposite side of the lens. For u < f, the image is virtual and on the same side of the lens as the subject.
In Bessel's method, the source is separated from a screen by a distance D > 4f. The screen and source are fixed, and the lens position is adjusted until a real image is focused on the screen. For a thin lens, the image distance v = D - u. For a thick lens or a compound lens, v = D - u - h, where h is the separation of the front and rear principal points of the lens.
The lens equation becomes: 1/u + 1/(D-u) = 1/f.
This is a quadratic equation in u, with solutions: u = D/2 ± sqrt(D²/4 - Df)
For D > 4f there are precisely two solutions - two lens positions at which an image of the source is formed at the screen. This follows from the properties of quadratic equations. The source-lens separation for the second solution is equal to the screen-lens separation for the first solution.
Dear Alan Robinson,
Thank you very much for clear answer and spending your time on it.
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