Finding the focal length of a converging lens with VERY limited info

  • #1
Overdose_
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Homework Statement
A candle, a converging lens and a white screen are placed in a line with the lens between the candle and the screen. A distance of 72cm separates the candle and screen. As the lens is moved to all points between the candle and the screen, only one focused image of the candle can be made on the screen. What is the focal length of the converging lens?
Relevant Equations
The thin lens equation
Screenshot 2023-12-29 084440.png

This is the solution but I do not understand??? How could they just assume the object distance equals 2f? No magnification is given here.
We know the candle produces a real image and the distance is limited, so that means it must be somewhere between the principal focus, the center of curvature, and possibly a little beyond that. Doesn't that mean, then, that the object distance can be anywhere between 2 point something f to 1f? Only one image is produced, what if that ONE image isn't produced at 2f? Then wouldn't the assumption be invalid? What? Please help :cry:
 
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  • #2
Overdose_ said:
How could they just assume the object distance equals 2f?
In the equation relating ##f, d_i, d_o##, think about the symmetry.
 
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  • #3
Provided you have a 'simple' convex lens, be it Convex, Biconvex, Plano-convex, Positive meniscus or distributed 'Fresnel', there's a single focal length for each wave-length. ( Hence colour aberration !!)
A 'meta-material' may have dual or multiple focal lengths for same wave-length...
 
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  • #4
If you don't see the symmetry argument, consider this. Since ##d_i+d_o=L=72~##cm, you can write the thin lens equation as $$\frac{1}{d_o}+\frac{1}{L-d_o}=\frac{1}{f}.$$ This gives you a quadratic in ##d_o## which, generally, has two solutions. This means that, in general, there are two values of ##d_o## for a given focal length. What must be true for the quadratic to have only one solution?
 
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  • #5
Overdose_ said:
How could they just assume the object distance equals 2f?

The exercise becomes easier when you reason from the other end: If the 72 cm is greater than 4f, there will be two positions that give a sharp image (with the symmetry @haruspex mentioned). If it's less, there will be no such postion at all.

##\ ##
 
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  • #6
kuruman said:
If you don't see the symmetry argument, consider this. Since ##d_i+d_o=L=72~##cm, you can write the thin lens equation as $$\frac{1}{d_o}+\frac{1}{L-d_o}=\frac{1}{f}.$$ This gives you a quadratic in ##d_o## which, generally, has two solutions. This means that, in general, there are two values of ##d_o## for a given focal length. What must be true for the quadratic to have only one solution?
Thank you so much I understand now!! The two solutions are the same so object distance and image distance must be the same!!!
 
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  • #7
haruspex said:
In the equation relating ##f, d_i, d_o##, think about the symmetry.
Thank you! I finally found a video on the conjugate foci method and now I completely understand!
 
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  • #8
Nik_2213 said:
Provided you have a 'simple' convex lens, be it Convex, Biconvex, Plano-convex, Positive meniscus or distributed 'Fresnel', there's a single focal length for each wave-length. ( Hence colour aberration !!)
A 'meta-material' may have dual or multiple focal lengths for same wave-length...
All true, but, sadly, nothing to do with the question at hand.
 
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FAQ: Finding the focal length of a converging lens with VERY limited info

What is the simplest method to find the focal length of a converging lens with limited information?

The simplest method is to use the lens formula: 1/f = 1/v - 1/u, where f is the focal length, v is the image distance, and u is the object distance. If you can measure the distances of the object and the image from the lens, you can calculate the focal length.

Can I find the focal length with just a single measurement?

Yes, you can find the focal length with a single measurement if you place the object at twice the focal length (2f) from the lens. In this case, the image will also form at twice the focal length on the other side of the lens, making it easy to determine the focal length as half the measured distance from the object to the image.

How can I use sunlight to determine the focal length of a converging lens?

You can use sunlight by focusing the parallel rays of the sun onto a piece of paper through the lens. The distance from the lens to the point where the smallest, brightest spot of light is formed on the paper is the focal length of the lens.

Is it possible to find the focal length without any measurement tools?

It is challenging to find the exact focal length without measurement tools, but you can estimate it by using your eyes to gauge distances. For a more accurate result, some form of measurement, even a simple ruler, is recommended.

Can I determine the focal length using the magnification method?

Yes, you can determine the focal length using the magnification method. Place an object at a known distance from the lens and measure the size of the image formed. Use the magnification formula (magnification = image height / object height) along with the lens formula to calculate the focal length.

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