# Focus point

What happens if we keep the object at focus point of any lens system( like convex lens ,Plano convex lens etc) ?
Does image forms at infinity?
What is exact definition of focus point in optics?

blue_leaf77
Homework Helper
The image will be formed at infinity, in other words you can't see the image no matter how far you place the screen. In ray optics focus plane is a plane containing a point at which all incoming parallel rays will meet together, regardless of the angle those rays make with optical axis. In Gaussian optics, focusing is proportional to Fourier transforming the field just before the lens/mirror, this is true only under paraxial approximation.

So how in concave lens when we keep object at focus point, a virtual image at f/2 is formed but not at infinity?

blue_leaf77
Homework Helper
In diverging lenses, the image formed is virtual. This is because you can't find intersection of the output rays in the region behind the lens, but you can find one in a place in front of it (only that the intersection is made by the extension of output rays). That's if you see through the lens you will see the object as if it's placed at the plane where the virtual image was formed.
Different story happens in focusing lens, when the object stands in focal plane you can't find the intersection of output rays (or their extension) in either region, behind or in front of the lens.

So is Plano convex lens a converging lens? Is Plano concave a diverging lens?
How one determines that?

blue_leaf77
Homework Helper
It can be determined from the sign of lens focal length. Refer to "lens maker equation" for the calculation.

Thanks for all this. I have only two last major questions remaining that needed I think all this stuff. 1)So in all types converging lenses when we keep an object at focus point we get image at infinity?

2)Consider a plain glass and on it we pour some drops of water. Then we put a convex lens on it.
Is this a converging system?
In this as the object needle is placed at focus the Rays goes parallel or to infinity but because of plain mirror it retraces and forms a real image?

blue_leaf77
Homework Helper
1) Yes.

2) It is a converging system. But you can't say that all rays retrace the same path upon reflection from plain mirror. Only rays spreading from needle tip (assuming the tip touches the optical axis) do trace back its previous path. Other rays do not, as you can see the image is flipped.

• Raghav Gupta
Thank you very much blue leaf. Just a last one , how you are saying that in 2) it is a converging system when we don't have any data to determine that by lens maker equations?

blue_leaf77