# Focal length of the optical system

I understand this that when an object is kept at focus of convex lens, then image is formed at infinity.
So when we keep a convex lens above plain mirror and move object needle on the principal axis of convex lens and see a coinciding image with needle on lens and measure distance from lens then it is focal length of convex lens. As the Rays have retraced their path by being normal to plain mirror.

Now, when we pour some drops of water on plain mirror and again keep on top The convex lens, how in this case when the object and image coincides, on measuring the distance from convex lens this is the focal length of the system?
For some images see the link
http://www.seminarsonly.com/Enginee...ctive-Indices-of-Water-And-Turpentine-Oil.php

when you have water in between the convex lens and a plane mirror, the water will act like a concave lens.
now, for a system of lenses, for example, if you place 3 convex lenses in contact, with each lens having focal lengthe ##f_1,f_2,f_3##, then we difine the total power P of the configuration as $$P=P_1+P_2+P_3$$
hence the focal length of a single lens which can act like the given combination is obtained from :
$$1/f = 1/f_1 + 1/f_2 + 1/f_3$$
now try to use this relation.

This is a experiment. I guess you might have done it for physics practical?
I will give you a link so you may remember it
http://uotechnology.edu.iq/dep-laserandoptoelec-eng/English/laboratory/1/U p/Determination the refractive index of a liquid by a liquid l.pdf

I know that relation and all formulas, but
in 1/F=1/f1+1/f2 from where we will get F and f2? 2 variables here to solve. One would say that F we will get by seeing the image and object coincide again but how that distance measured is theoretically focal length of the convex and liquid lens system?
f1 we have determined by measuring the distance of the image and object needle being coincident.

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Thanks Aditya for giving me an explanation but got the answer of my query from blue leaf, if you see my Focus point thread in general physics.
I think sometimes way of expressing questions have a lot of impact on people (As I think I did there and kudos to the guy for explaining in a nice way).

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