Ray Tracing and Combination of Lenses

[alingy1]

I was looking at this problem.
Does the problem assume that the focal point of the divergent lens is right next to it?
The answer is c, saying that the focus point will be further away: but that assumes that the focal point is not RIGHT NEXT to the divergent lens. If it were RIGHT NEXT (VERY CLOSE), the rays would diverge in the end rather than converge further down the road and this would prevent focusing.

 

[ehild]

I was looking at this problem.
Does the problem assume that the focal point of the divergent lens is right next to it?
The answer is c, saying that the focus point will be further away: but that assumes that the focal point is not RIGHT NEXT to the divergent lens. If it were RIGHT NEXT (VERY CLOSE), the rays would diverge in the end rather than converge further down the road and this would prevent focusing.

No, nothing is said about the location of the focal point of the divergent lens. You are right; the question is ambiguous.
ehild

 

[alingy1]

My mistake. The answer is b!
ehild, what I want to make sure is the following: what happens if the second lens, along with its focal point is much further away from the first lens. That is what I was saying with "the focal point of the divergent lens being right next to the divergent lens". What would seem to happen is that rays going through would diverge above and below by larger angles and prevent focusing. Am I not right?

Otherwise I pretty much get the problem.

The violet line represents what would happen to the rays shown in the problem if the focal point is too close to the diverging lens.
The red lines represent the focal points of the second lens.
The green lines represents two special rays to locate the image.

 

[alingy1]

Better illustration: http://imgur.com/Yi1Tm2Q

 

[ehild]

My mistake. The answer is b!
ehild, what I want to make sure is the following: what happens if the second lens, along with its focal point is much further away from the first lens. That is what I was saying with "the focal point of the divergent lens being right next to the divergent lens". What would seem to happen is that rays going through would diverge above and below by larger angles and prevent focusing. Am I not right?It can happen, yes. And the other thing can also happen, that the image moves farther. It depends on the focal distances and the distance between the lenses.

Can you derive the expression for the position of the image if you are given f1 and f2 and D the distance between the lenses ?


ehild