Friday, June 27, 2014

Microscopes

The microscope is one of the most widely used instruments in science. Microscopy is a huge subject, and I am definitely not an expert. Russ Hobbie and I talk about the microscope only briefly in the 4th edition of Intermediate Physics for Medicine and Biology. In Chapter 14 (Atoms and Light) we give a series of homework problems about lenses. Problem 43 considers the case of an object placed just outside the focal point of a converging lens. The resulting image is real, inverted and magnified (a slide projector, for those of you old enough to remember such things). In Problem 44, the object is just inside the focal point of the lens. The image is virtual, upright, and magnified (a magnifying glass). Then in Problem 45 we put these two lenses together, first a slide projector casting an intermediate image, then a magnifying glass to view that image; a compound microscope. Our discussion is useful, but very simple.

Nowadays, microscopes are extremely complicated, and can do all sorts of wonderful things. Our simple example is nearly obsolete, because almost no one looks through the second lens (the eyepiece) to view the image anymore. Rather, the image produced by the first lens (the objective) is recorded digitally, and one looks at it on a computer screen. I could spend the rest of this blog entry describing the complexities of microscopes, but I want to go in another direction. Can a student build a simple yet modern microscope?

They can, and it makes a marvelous upper-level physics laboratory project. The proof is given by Jennifer Ross of the University of Massachusetts Amherst. In a preprint at her website, Ross describes a microscope project for undergraduates. The abstract reads:
Optics is an important subfield of physics required for instrument design and used in a variety of other disciplines, including materials science, physics, and life sciences such as developmental biology and cell biology. It is important to educate students from a variety of disciplines and backgrounds in the basics of optics in order to train the next generation of interdisciplinary researchers and instrumentalists who will push the boundaries of discovery. In this paper, we present an experimental system developed to teach students in the basics of geometric optics, including ray and wave optics. The students learn these concepts through designing, building, and testing a home-built light microscope made from component parts. We describe the experimental equipment and basic measurements students can perform to learn principles, technique, accuracy, and resolution of measurement. Students find the magnification and test the resolution of the microscope system they build. The system is open and versatile to allow advanced building projects, such as epi-fluorescence, total internal reflection fluorescence, and optical trapping. We have used this equipment in an optics course, an advanced laboratory course, and graduate-level training modules.” 
This fascinating paper then goes on to describe many aspects of microscope design.
“The light source was a white light emitting diode (LED)… We chose inexpensive but small and powerful CMOS cameras to capture images with a USB link to a student’s laptop….The condenser designs of students are the most variable and interesting part of the microscope design. Students in prior years have used one, two, or three lenses to create evenly illuminated light on the sample plane…After creating the condenser, students next have to use an objective to create an image onto the CMOS camera chip.”
The equipment is not terribly expensive comparared to buying a microscope, but it is not cheap: each microscope costs about $3000 to build, which means for a team of three students the cost is $1000 per person. But the leaning is tremendous, and Ross suggests that you can scavenge used parts to reduce the cost.

But perhaps even this student-built $3000 microscope is too complicated and expensive for you. Can we go simpler and cheaper? Yes! Consider “foldscope.” The website of foldscope's inventors says (my italics)
“We are a research team at PrakashLab at Stanford University, focused on democratizing science by developing scientific tools that can scale up to match problems in global health and science education. Here we describe Foldscope, a new approach for mass manufacturing of optical microscopes that are printed-and-folded from a single flat sheet of paper, akin to Origami….Although it costs less than a dollar in parts, it can provide over 2,000X magnification with sub-micron resolution (800 nm), weighs less than two nickels (8.8 g), is small enough to fit in a pocket (70 × 20 × 2 mm3), requires no external power, and can survive being dropped from a 3-story building or stepped on by a person. Its minimalistic, scalable design is inherently application-specific instead of general-purpose gearing towards applications in global health, field based citizen science and K12-science education.”
Details are described in a preprint available at http://arxiv.org/abs/1403.1211. Also, listen to Manu Prakash give a TED talk about foldscope. The goal is to provide “a microscope for every child.” I think Prakash and his team means EVERY child (as in every single child in the whole wide world).

1 comment:

  1. Incredible and beautiful microtubule movies at the Jennifer Ross lab website, wow! Thanks Brad.

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