Concave Lens _ Image Formation

[-Castiel-]

A concave lens always forms a virtual image (for a real object), will it always form a real image for a virtual object? (that is, when rays converge on it)

If yes, why?

If no, why?

 

[sophiecentaur]

Ask yourself what happens to the rays leaving an object when they pass through a concave lens. Would they ever be brought together to form a real image?
There's your answer.

 

[-Castiel-]

Ask yourself what happens to the rays leaving an object when they pass through a concave lens.

They Diverge?

Would they ever be brought together to form a real image?
There's your answer.

Yes?

 

[sophiecentaur]

If two rays are, as you say, 'diverging' then how can they meet at a point to form a real image? They are surely getting further and further apart. No?

btw, what is a "virtual object"? (I missed that when I first read it).

 

[sophiecentaur]

However . . . . if you have a convex lens, producing a real image and you put a concave lens between the first lens and the image it forms, then the resulting (real) image will be formed a bit further away than the original (real) image.

 

[-Castiel-]

btw, what is a "virtual object"? (I missed that when I first read it).

if you have a convex lens, producing a real image and you put a concave lens between the first lens and the image it forms,.

This precisely. If you put a Concave Lens in front of an image (make the converging rays that form the said image, fall on the concave lens before they can intersect and form he image) then that image should act as an object (virtual?) for the concave lens.

So basically an actual object that you place in front of a lens, is a real object.

then the resulting (real) image will be formed a bit further away than the original (real) image

the original (real) image as you put it is no longer real because it was never formed, right? The rays were supposed to intersect but the lens intercepted them before that could happen. That is what I mean by a virtual object.

So again, for a virtual object, will a concave lens Always form real image?

That is if two converging rays were to fall on a concave lens could it diverge them so much that they appear to come from behind the mirror as opposed to only shifting a bit further.

 

[sophiecentaur]

You are talking of an intermediate image and not a virtual object here, I think.

In answer to your original question, I would say that you can produce a real or virtual image, depending on the focal lengths of the two lenses and the position.
A Galilean telescope has a convex lens with a concave eyepiece lens - producing a virtual image at a distance which you can actually focus you eye on. The optics inside some camera lenses produces a shifted real image because of the presence of a concave element. So there is no general rule.

 

[JeffKoch]

So again, for a virtual object, will a concave lens Always form real image?

Are you familiar with the thin-lens equation? If so, you can answer this question for yourself.