How Is Magnification Calculated with Two Thin Lenses?

Click For Summary

Homework Help Overview

The discussion revolves around calculating the magnification produced by a system of two thin lenses. The first lens has a focal length of 50mm, and the second lens has a focal length of 400mm. The setup involves a light source positioned at the focal point of the first lens, with the second lens placed 600mm from the first.

Discussion Character

  • Mixed

Approaches and Questions Raised

  • Participants explore the application of the magnification formula for each lens, questioning whether the magnification should be calculated separately for both lenses. There is discussion about the nature of the images produced (real vs. virtual) and the implications for the calculations.

Discussion Status

Participants are actively engaging with the problem, with some providing calculations and others questioning the assumptions made regarding the object and image distances. There is a recognition of the complexity introduced by the two lenses and the need for careful consideration of the lens equations.

Contextual Notes

Some participants note the potential misunderstanding of the object being a real object placed at the focal point, which affects the image distance for the first lens. There is also mention of the implications of using infinity in calculations, suggesting a need for caution in mathematical treatment.

avelon
Messages
4
Reaction score
0

Homework Statement




A light source placed in the focus of a (thin) lens with a focal length of 50mm.

A second (thin) lens (with focal length = 400mm) was placed 600mm after the focal point of the first one, an image of the light source was captured at the seonds lens focal point. How many times has it been magnified?
 
Physics news on Phys.org
welcome to pf!

hi avelon! welcome to pf! :wink:

Show us what you've tried, and where you're stuck, and then we'll know how to help! :smile:
 


tiny-tim said:
hi avelon! welcome to pf! :wink:

Show us what you've tried, and where you're stuck, and then we'll know how to help! :smile:

This is what I did

Magnification M = image_distance S / object_distance S'
M = 400 / 600 = 2/3
I don't think this is the right solution... any comments? :--)
 
there's two lenses …

won't you need to use the magnification equation twice?
 
tiny-tim said:
there's two lenses …

won't you need to use the magnification equation twice?

hmmm..Lens equation for lens1 would giva us
1/object_distance + 1/image_distance = 1/focal
"assuming it's a virtuall image", otherwise image_distance would be inf for lens 1
1/(-50)+1/i_d = 1/50
image_distance = 25

Magnification (lens1) = 25/50 = 1/5
Magnification (lens2) = 400/600 = 2/3

M = 1/5 * 2/3 ...still feeling I'm doing something worng here.
 
Last edited:
avelon said:
hmmm..Lens equation for lens1 would giva us
1/object_distance + 1/image_distance = 1/focal
"assuming it's a virtuall image" …

hmm … i think you're misunderstanding the question :redface: …

the object is a real object, so if it's placed at the focal point, then the image is at … ? :smile:
 
tiny-tim said:
hmm … i think you're misunderstanding the question :redface: …

the object is a real object, so if it's placed at the focal point, then the image is at … ? :smile:

When an object is placed at the focal point, the image will be at inf :-)

For lens1:
focal = 50 mm
Object_distance = 50 mm
Image_distance = inf (this will be object for the second lins)
Magnification = (inf/50)


For lens2:
focal = 400 mm
Object_distance = inf
Image_distance = 400 mm
Magnification = (400/inf)
----

Magnification for two lenses would be:
Magnification_lens1 * Magnification_lens2 = (inf/50) * (400/inf) = 8 times (I think I got it right this time)
 
hi avelon! :smile:
avelon said:
When an object is placed at the focal point, the image will be at inf :-)
…
Magnification for two lenses would be:
Magnification_lens1 * Magnification_lens2 = (inf/50) * (400/inf) = 8 times (I think I got it right this time)

that's right! :smile:

(but don't let the professor catch you multiplying or dividing by ∞ ! :wink:)

i'm not sure what the official way of proving this is :redface: …

i suspect you have to put the object a small distance x away from the focus, and let x -> 0
 

Similar threads

Focal length of the eyepiece lens in a compound microscope
  • · Replies 7 ·
Replies
7
Views
2K
Inserting thick lenses into a thin lens system and deducing values
  • · Replies 1 ·
Replies
1
Views
2K
Calculate a radius of a circle on a screen
  • · Replies 3 ·
Replies
3
Views
1K
Analyzing this 2-lens optical system
  • · Replies 5 ·
Replies
5
Views
2K
Finding the magnification of a ball lens
  • · Replies 4 ·
Replies
4
Views
4K
System of lenses with a light source at first focal point
  • · Replies 16 ·
Replies
16
Views
3K
How to measure the magnification of a microscope in the lab
  • · Replies 12 ·
Replies
12
Views
3K
Emergent light produced by two lenses
  • · Replies 15 ·
Replies
15
Views
2K
Understanding Two Convex Lenses: Ray Diagram and Focal Lengths
  • · Replies 3 ·
Replies
3
Views
3K
Splitting of a convex lens problem
  • · Replies 5 ·
Replies
5
Views
2K