# focal length multiplied by wavelength

by Aziza
Tags: focal, length, multiplied, wavelength
 P: 172 Simple question: is there any physical significance of the quantity (focal length of lens)*(wavelength of laser light passing through the lens) ? How does it relate to the size of an image on the image plane? According to my professor, (focal length)*(wavelength)/(constant) is the length of the image. Also, we are using an SLM to create the object (at the focal plane), if that makes any difference..
 P: 372 I suspect that your prof is talking about the Rayleigh Criterion, which describes the resolution of a lens - the minimum angular separation of two point sources such that they will be recognisable as two sources in the image. The expression you are quoting isn't quite right for that - it's the f-number (the focal length of the lens divided by its diameter), not the focal length itself that you use. Wikipedia on the subject. Born and Wolf is the bible for this kind of thing, if your library has a copy.

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