TBR CBT 5 question 24

[crazyasian]

So I literally missed all but 1 question on this passage. Everytime I think I understand something I learn I'm wrong. question 24 asked what kind of images could be formed by appartaus 2. Apparatus 2 has a convex lens then a concave lens. the light would go through the convex lens 1st so it could be a real or virtual depending on where the object was. However as soon as it hits the concave lens it would always be a virtual, upright image. Am i wrong? BR says the imaged produced by the diverging lens can fall almost anywhere and it retains its upright or inverted orientation from the converging lens. Could anybody explain this to me? Thanks.

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[BerkReviewTeach]

So I literally missed all but 1 question on this passage. Everytime I think I understand something I learn I'm wrong. question 24 asked what kind of images could be formed by appartaus 2. Apparatus 2 has a convex lens then a concave lens. the light would go through the convex lens 1st so it could be a real or virtual depending on where the object was. However as soon as it hits the concave lens it would always be a virtual, upright image. Am i wrong? BR says the imaged produced by the diverging lens can fall almost anywhere and it retains its upright or inverted orientation from the converging lens. Could anybody explain this to me? Thanks.

24. What kinds of images are possible with Apparatus 2 (convex lens infront of a concave lens)?

A. Virtual, upright images
B. Real, inverted images
C. Virtual, inverted images
D. Real or virtual, upright or inverted images

The best thing to do with multiple lens systems is to consider each lens one at a time.

• convex lens: If the object is beyond the focal point of the first lens, then the first lens will form a real, inverted image that could be on either the left side or the right side of the second lens, depending on the exact numbers. If the object is inside of the focal point of the first lens, then the first lens will form a virtual, upright image.

The image of the first lens is the object of the second lens.

• concave lens: A concave lens is diverging, and diverging systems (lenses or mirrors) form SUVs (smauller, upright, virtual images). If the first image is an inverted real image, then the second image will remain inverted, but will be a virtual image. If the first image is an upright virtual image, then the second image will remain upright and virtual.

Without considering the cases where the first lens is the diverging lens, we have already found that A and C are both true, so D is our best answer. I'm a big believer in stopping at this point, because the best answer has been found and anything extra is wasted testing time on extraneous information. However, if you feel it will help for practice (when you're grading the exam), then consider what happens when the diverging lens forms an upright virtual first image that is (a) beyond the focal point of the converging lens and (b) inside of focal point of the converging lens.

 

[crazyasian]

Ok that helps alot thanks. I understand how A and C are possible, but I'm still having trouble seeing how you could form a real image. If it goes through a diverging lens then no matter what it has to be a virtual image right?

 

[BerkReviewTeach]

Ok that helps alot thanks. I understand how A and C are possible, but I'm still having trouble seeing how you could form a real image. If it goes through a diverging lens then no matter what it has to be a virtual image right?

That was my thought going through the question as well, which makes me not like D as an absolutely true answer, but I still would pick it as the best answer.

The only way I could see a real image forming from that lens system is light projecting through a smaller divering lens and then passing through a much bigger converging lens, which would bring the rays back together. The rays are really coming together, so the image should be real. I'm stretching this example to make an answer correct, which is a terrible test-taking habit to get into, but I need to do such things to keep sane. Questions with ambiguity bother me when I have time to contemplate them. I think in actuality the system I described would yield a distorted, double image. This is sort of how corrective diverging lenses work on objects that are really far away. I'm thinking that light really is striking my retina when I see a far away fire through my glasses. I could be backwards on my reasoning here, but the saving grace is that the answer choices allow me to make an ambiguous best choice.