Proofing the DNA Structure With a Bulb

The discovery of the DNA double helix by James Watson (1928–2025) and Francis Crick (1916–2004) in 1953 is inextricably linked to X-ray diffraction (or X-ray crystallography), and in particular to the research of Rosalind Franklin (1920–1958). In 1952, Rosalind Franklin, a chemist at King’s College London, produced high-quality X-ray diffraction images of DNA together with Raymond Gosling. The most famous image, “Photo 51,” showed a characteristic X-shaped diffraction pattern.

Watson and Crick, working at the Cavendish Laboratory in Cambridge, used this X-ray data [which was partially made available to them without Franklin's knowledge by Maurice Wilkins (1916–2004)] to develop their theoretical model of DNA structure. The X-shaped diffraction pattern, in their view, clearly indicated a helical (spiral) structure. Their calculations, based on the physical data of the X-ray diffraction, ultimately led to the discovery of complementary base pairing (A-T, C-G) within the double helix. Rosalind Franklin's experimental work was thus the crucial foundation that enabled Watson and Crick to solve the puzzle of DNA structure.

Their conclusions were:

1. DNA has a helical structure
2. The nitrogenous bases are located on the inside, the sugar-phosphate backbone on the outside
3. The helical structure consists of two strands (double helix).

In this project, I want to investigate the structure of DNA using the diffraction of a laser beam by a helical spring. The helical spring closely resembles the double helix of DNA, so the results obtained with the helical spring can ultimately be compared with the famous photo 51. Let's get to work… For this experiment, we only need:

1. a very small helical spring, such as those found in small light bulbs
2. a laser
3. two diffraction gratings with 300 lines/mm

First, we will examine the diffraction patterns of one and two gratings. Diffraction grating foil can be obtained very cheaply, for example via eBay, but only with 500 lines/mm and 1000 lines/mm. My experiments have shown that 300 lines/mm is ideal. Due to the lower number of lines per millimeter, the diffraction pattern is more compact/narrower than with 500 lines/mm.

What do we expect with one or two diffraction gratings? Well, if we use a single, horizontally oriented grating, we expect a bright, zero-order diffraction maximum in the center, flanked on the right and left by progressively fainter, higher-order diffraction maxima. If we tilt the single diffraction grating, the axis of the maxima also tilts. But what happens if we place another grating behind the slightly tilted one, tilted slightly in the opposite direction? Each individual maxima ray from the first grating is then refracted into further rays by the second grating. The complex diffraction pattern shown on the right of the figure is then expected. The brightest maxima should be located along a central X.

The attached illustrations show the diffraction patterns for a.) horizontally oriented diffraction gratings, b.) obliquely oriented gratings, and c.) two diffraction gratings inclined to each other and arranged one behind the other.

But let's now turn to our DNA, or rather, our coil spring. What does a coil spring have to do with a diffraction grating? Let's take a closer look at a coil spring from the side.

The individual spirals have a regular arrangement and thus, in a sense, form one or even two diffraction gratings. The first grating is shown above and is tilted to the left. The second grating has the same grating constant but is tilted to the right.

In summary, the helical spring represents a double diffraction grating, with both gratings having opposite inclinations.

The diffraction pattern of the helical spring should therefore resemble that of the two mutually inclined diffraction gratings. However, noticeable diffraction phenomena only occur when the grating spacing/grating constant is on the order of the wavelength of light, specifically around 0.5 µm. Unfortunately, I don't have such a small helical spring. But there is a simple source for a very small helical spring: the filament of a small incandescent bulb (e.g., 24V/2W).

To be able to photograph the very small filament, I quickly built myself a smartphone microscope. All you need is an inexpensive lens for a laser module. These can be bought individually on eBay for a small price. They have an M9 thread on the outside. To make the spiral structure more pronounced, I carefully stretched the filament.

For better visibility, the room must be darkened during the experiment. But you can clearly see a squashed X in the middle, Eureka…

I had a small coil spring in my collection, which I also examined. The X-structure is clearly visible again. However, finer diffraction patterns are lost or not visible due to the much larger grating constant compared to the light bulb filament.

Comparing our result with photo 51, one can see quite a few similarities. So, using an ordinary lightbulb, we have deciphered the structure of DNA, the fundamental hereditary molecule in cells that serves as the blueprint for living organisms. Watson, Crick, and Franklin would be proud of us, or rather, if we had conducted this experiment 75 years ago, we would have won the Nobel Prize 😉

Due to the simplicity of the experiment, the very inexpensive materials, and its historical significance, it is, in my opinion, practically a must-have for the school physics lab. Students should, however, make their own conjectures, such as what the diffraction patterns of a tilted diffraction grating and two mutually tilted diffraction gratings look like. And then they should also find out for themselves what a coil spring might have to do with it. Finally, they should learn that the history of science/physics has often been anything but fair, especially regarding the recognition of women…

If you're interested in more exciting physics projects, here's

my homepage: link

my YouTube channel: link

With that in mind, stay curious and Eureka!