Understanding Two Convex Lenses: Ray Diagram and Focal Lengths

  • #1
waley

Homework Statement


I'm supposed to make a ray diagram of two convex lenses when light initially hits the first lens at parallel rays. When f(total) goes to infinity, the distance between the two lenses = the sum of the focal lengths of each lens.

Homework Equations


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The Attempt at a Solution


So the thing I'm confused about is, I'm assuming the focal lengths of the two lenses are different, or else there'd be no differentiation between f1 and f2. By just drawing these diagrams out on paper (imprecise, I know), I'm getting two different results - when the lens that has a smaller focal length is placed before the one with longer focal length, my image is magnified. But when I place the lens with larger focal length before the one with smaller, my image is magnified. Are both right? Are both wrong?? I couldn't find much info on two convex lenses online, are there any general rules for two convex lenses that I'm missing out on? Like can you both magnify and demagnify objects with different convex lenses, etc.
ps. is it right to think that because f(total) is just the distance from the object to the image, if this goes to infinity, then the object is so far away that the light rays are coming in parallel to one another?
 

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  • #2
Does this wordless picture help ?

upload_2017-11-19_11-21-21.png
 

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  • #3
waley said:
Are both right? Are both wrong?
Can't tell from the wording. Can you post the pictures ?
 
  • #4
BvU said:
Does this wordless picture help ?

View attachment 215249
That's one of the diagrams I drew. What if the focal length of the first lens is longer than the second lens?Would the image be magnified?
Sorry I can't post a picture atm, but I will if this doesn't clear things up.
 

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