Understanding Two Convex Lenses: Ray Diagram and Focal Lengths

In summary, the conversation discusses making a ray diagram of two convex lenses with parallel light rays. The distance between the two lenses is equal to the sum of their focal lengths. There is confusion about the results when the lenses have different focal lengths and whether the image is magnified or demagnified. The possibility of different rules for two convex lenses is mentioned. There is also a question about the distance from the object to the image when f(total) goes to infinity. A diagram is shared and the possibility of a longer focal length for the first lens is raised. The question of whether the image would be magnified is asked.
  • #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.
 

FAQ: Understanding Two Convex Lenses: Ray Diagram and Focal Lengths

What is the purpose of a ray diagram for two convex lenses?

A ray diagram for two convex lenses is used to visually represent the path of light rays as they pass through the lenses. This diagram allows us to understand how the lenses work together to form an image.

How do you construct a ray diagram for two convex lenses?

To construct a ray diagram for two convex lenses, you will need to draw two parallel principal rays from the object to the first lens, then use the focal length and lens equation to find the position of the image. The second lens is then placed at the location of the image, and the rays are traced through the second lens to determine the final image location.

What is the difference between the focal length of a single convex lens and the focal length of two convex lenses?

The focal length of a single convex lens is the distance from the lens to the point where parallel rays of light converge. The focal length of two convex lenses is the distance between the two lenses where an object can be placed to form an image at infinity. This focal length is longer than the focal length of a single lens because the two lenses work together to form the image.

How does the placement of the object and lenses affect the image formed by two convex lenses?

The placement of the object and lenses can greatly affect the image formed by two convex lenses. If the object is placed closer to the first lens, the image will be larger and closer to the lens. If the object is placed further away, the image will be smaller and further away. Additionally, the distance between the two lenses can also affect the image size and location.

What happens to the image formed by two convex lenses if the lenses are moved closer together?

If the two convex lenses are moved closer together, the focal length of the system decreases and the image will become larger and closer to the lenses. If the lenses are moved further apart, the focal length increases and the image becomes smaller and further away from the lenses.

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