- #1
jias
- 1
- 0
Can anyone explain to me the procedure that one would follow to you find the focal length of a concave lens using a convex lens? Thanks.
Find the focal length of a concave lens using a convex lens?
- Context: Undergrad
- Thread starter jias
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SUMMARY
The procedure to find the focal length of a concave lens using a convex lens involves placing the concave lens in front of the convex lens. The convex lens creates an image S' of an object S, and the concave lens then alters this image to a new position S''. The distances from the concave lens to these image points are denoted as 'a' and 'b', respectively. The focal length 'f' of the concave lens can be calculated using the lens formula: f = (ab) / (b - a), where 'b' is negative due to the nature of the image being virtual.
PREREQUISITES- Understanding of lens formulas and optics principles
- Familiarity with convex and concave lenses
- Basic knowledge of image formation in optics
- Ability to perform calculations involving distances and focal lengths
- Study the derivation of the lens formula in optics
- Learn about the characteristics of virtual and real images
- Explore practical experiments involving lens combinations
- Investigate the applications of concave lenses in optical devices
Students in physics, optical engineers, and anyone interested in understanding lens behavior and image formation in optics.
- #2
armis
- 103
- 0
Yeah there is a way
Assume the convex lens creates an image of S at the point S'. Now if you put a concave lens in front of the convex one then the image of S will move further and will be at point S''. Now consider a reverse path of light: S'' is now the object and you'll see it's image at S' which is created by the concave lens. Let the distance from the "center" of the concave lens to S'' be a and the distance from S' to the "center" of the concave lens be b. Then using the lens formula:
-\frac{1}{f}=\frac{1}{a}-\frac{1}{b} notice there is a minus near b and f. It's because the image is imaginary
Thus
f=\frac{ab}{b-a}
I hope I didn't make any mistakes
Assume the convex lens creates an image of S at the point S'. Now if you put a concave lens in front of the convex one then the image of S will move further and will be at point S''. Now consider a reverse path of light: S'' is now the object and you'll see it's image at S' which is created by the concave lens. Let the distance from the "center" of the concave lens to S'' be a and the distance from S' to the "center" of the concave lens be b. Then using the lens formula:
-\frac{1}{f}=\frac{1}{a}-\frac{1}{b} notice there is a minus near b and f. It's because the image is imaginary
Thus
f=\frac{ab}{b-a}
I hope I didn't make any mistakes
- #3
tiny-tim
Science Advisor
Homework Helper
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erm, armis … you're doing it again!
If you'd changed "Then using the lens formula:" to "Then use the lens formula!
", and stopped there, then that would have been a good hint. 
(The OP can always come back and ask for more help, if necessary, of course)
If you'd changed "Then using the lens formula:" to "Then use the lens formula!
", and stopped there, then that would have been a good hint. (The OP can always come back and ask for more help, if necessary, of course
)- #4
armis
- 103
- 0
Oh... I thought I was supposed to do that only in the homework section, sorry. But it does make a lot more sense your way. I think I finally got the idea
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