Compounding a foldscope

Here is a problem that might adorn entrance examination tests someday. It originated in a dream that I had a few days ago. I suspect this dream was inspired by the SLR hack of the foldscope.

The low power lens of the foldscope can magnify objects ~140 times. The low power of a typical compound microscope has a magnification of 40x, derived from a 4x objective lens and a 10x eyepiece. What would be the magnification of an image formed by the 140x foldscope lens and further viewed under the compound microscope?

My answer in my dream was (140 x 40) 5600x, which is way more than any compound microscope can provide. Using a higher power objective (10x) would give an additional 100x magnification over the 140x to give me a total of 14000x (140×100). It occurred to me that I not only solved the oil immersion problem, but I now potentially have a setup more powerful than most laboratories around me. You can imagine my excitement of seeing an object magnified 5000/14,000x with my compound microscope and a foldscope.

So I did the experiment to see if I was right. I used a scale micrometer to see how many pixels correspond to a particular distance at various magnifications, and then calculated effective magnification using the 140x foldscope as base. I also used Daffodil pollen to get a feel for how it would look.

Experimental setup Scale micrometer at maximum digital magnification Pixels/micron  (Scale) Daffoldil pollen
Foldscope with 140x lens scale-foldscope-rot 6.647 p/m

Foldscope 140x +Compound microscope (4x obj, 10x eye)
scale-40x-foldscope-rot3 12.241 p/m



Foldscope 140x + Compound microscope (10x obj, 10x eye)
scale-100x-foldscope-rot 19.69 p/m

*Notes:  1) I either used the whole foldscope for the 4x objective of the compound microscope, or took the lens out and stuck it to the slide with tape when using the 10x objective of the compound microscope. I noticed that the foldscope lens has to be well pressed to the sample to make good images. 2) Pixels/micron were computed in ImageJ. 3) Scale is computed using the foldscope magnification as a base with the formula:  

(Pixels/micron from Column3)/6.647*140.

This means that combining a foldscope image with 40x compound approximately doubles the magnification, and under 100x compound the magnification is approximately tripled. And so I learnt that not all dreams come true. However, I think this is telling us something about the principles of magnification and I am sure what I observed can be derived from first principles. Any takers for an interesting pen-and-paper problem?




8 Comments Add yours

  1. Manu Prakash says:

    @Laks: this is an incredible composition of dreams and ideas and a real experiment. To solve the puzzle requires knowing parts of both the regular objective of a microscope and a good understanding of a Foldscope. An absolutely wonderful question for an entrance exam 🙂

    I will wait for community to rally around and propose possible hypothesis. This is a great way to also teach hypothesis generation. As @Laks said – any takers.


  2. mherring says:

    Great Idea Laks! I would be very interested to know whether this has an effect on resolution, since that is not typically improved with digital zoom. Current resolution of the Foldscope is ~2 micons.

  3. laksiyer says:

    @Marie. It definitely improves it for the few other samples I tested. I need to give it some really hard tests.

  4. Matthew Rossi says:

    @Laks, this is a question I’ve been wondering myself for a while, so you’ve more or less answered it for me. However, I want to add a further question to it: using the lenses provided in a single Foldscope kit, is it possible to effectively compound the Foldscope as you have done here. You’ve set a fire under me to begin playing with hacking my scope a bit. That is, after all, what got me into this in the first place.

  5. Niramay Gogate says:

    @Laksiyer , I know it is been a long time since you uploaded these photos , but anyway , this is what I feel : without going into mechanism of a regular compound microscope , we can say that it is an instrument which magnifies an object at a distance of (say) 1-2mm from its objective by an amount of X . X is a variable with its value depending on objective and eye piece. So it makes perfect sense that , at 1-2mm from objective , if you place a 140 times magnified image of an object or the object itself , your compound microscope should work the same (ie, it should magnify the input object X times). We can think of Foldscope as a compound microscope with above given description. So there was nothing wrong with you dreaming that magnification could be increased 140 times. The fact that you didn’t get the expected magnification implies that there is some problem with the definition of magnification. I have just studied a bit of geometrical optics so I don’t know much. One more thign that is worth trying is that , you can replace eyepiece of the compound microscope with foldscope and see what you get. The problem that could occur is , foldscope requires the target object to be extremely near to its lense and that might not be possible with taking the output of objective as the target object. I can try all these experiments myself once fall semester starts.
    Do let me know your views about this.

  6. laksiyer says:

    @Niramay. Nice hearing from you. Yes it has to do with the concept of magnification. I think you are getting there. I think you should draw those light paths and see if you can arrive at a solution. I think you can get an exact solution.

  7. Niramay Gogate says:

    @Laksiyer , after searching on net , I found one site saying that magnification of microscope is defined as just multiplication of power of objective and the power of the eyepiece. In school , we were taught that magnification for a single lense is just the ratio of image length upon object lenght. So basically , you will get different number for magnification for different definitions. The second definition makes more sense to me . If you see , foldscope is nothing but a single lens and so I think its magnification would have been defined in similar way. I now plan to find relation between the given two definitions. If I get it , your problem would be solved. I will do this by both experimental and theorotical methods. For this , I will have to wait for few more days.

    By the way , On similar lines , I tried to attach foldscope to my telescope to see the effect. I did get bigger image of moon but I couldn’t go further because the light intensity was too low. There too I thought that I would get 140x image of moon but in fact I didn’t : )

  8. Guru & Shree says:

    @Laks: Wow! That’s a real fusion of physics and a human brain there. Your research helped us understand the gap between magnification and foldscope.
    It also opened our minds to as in that there are no bounds in the use of foldscopes. We would love to take it more deeper. We would try on this today in our lab.

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