JOEL WILLIAMS [email protected]PITTSFIELD, Mass., Nov. 28, 2019 — The transparent pumpkin pie craze began in 2017 when chefs Simon Davies and Mike Bagale of cutting-edge, three-Michelin-star, Chicago restaurant Alinea created one using “molecular gastronomy,” a modern discipline that challenges the established conventions of cooking. The pie was made via a complex process to nail down the proper flavor. According to an October 2017 article in Vogue, the chefs used a rotary evaporator to distill a fairly traditional pumpkin pie filling and create a concentrated and translucent extract with which to flavor the gelatin filling. The result is a visually stunning and hopefully delicious conversation piece that can serve as a great segue into a discussion about transparency and optics. A transparent pumpkin pie. Courtesy of Michael Wheeler, Photonics Media. Photonics Media spoke with two optical scientists about the optically transparent pie, and while they were wary of the texture, they did note interesting concepts that the pie illustrates. “It looked like it would show off a red laser pointer very nicely,” said Judy Donnelly, a retired professor of physics and technology at Three Rivers Community College. Because it has a similar refractive index to glass or water, gelatin serves as an excellent disposable medium to demonstrate a concept such as Snell’s law. “If you were to put a laser beam into one edge of the pie slice, it would bend toward the curvature and come out the other side and bend like a prism would,” Groot Gregory, another authority on gelatin optics, told Photonics Media. Gregory serves as technical marketing director at Synopsys Inc. and has served in leadership positions at SPIE and The Optical Society (OSA). When a laser is shined into the pie, the beam encounters sugar molecules, spices, and gelatin, which scatter the light, allowing for a visible beam that demonstrates the curvature of the light. The typical slice of pie is shaped somewhat like a prism, so the logical question is: Can you get a rainbow out of it? “Oh man, that would be really tricky,” Donnelly said. “The only way you get a rainbow is if the wavelengths are refracted by a different amount.” Donnelly suspects that it’s possible, but likely difficult. A rainbow requires a great deal of dispersion, meaning it needs a large difference in refractive index while the light travels between media. Gregory shared a similar sentiment. “If you can get a rainbow with a continuous-spectrum light source into water, you’ll be able to get it in Jell-O. The way to see that may be to have the visualization after the light exits the Jell-O very far away," he said. “Because you’ve got the small angles, the longer the distance is from the angles, [and] the greater the spread would be. You probably would not be able to see a rainbow within the pie, just because those angles are very small. Unless you have a very big piece of pie — like several meters.” Ditching the crust allows for even more opportunity. Donnelly has seen nearly all manner of optical elements crafted with gelatin: concave and convex lenses, aspherical lenses, optical fibers, and even an optical switch. She said one of the most impressive gelatinous feats she has seen came from Gregory when he made a gradient index lens at one of her workshops. “When you make the Jell-O, you add as much sugar as possible into the water. And then when the Jell-O solidifies, you put water on one side, and that will start to leech out the sugar,” Gregory said. “And as the concentration of sugar in the solidified Jell-O goes down through diffusion, you’ll change the index of refraction, and light rays will bend along a curve as a regular-graded index material would do.” If there are young ones around, Donnelly and Gregory both recommend keeping a close eye on any carefully crafted gelatin optics, lest they be crushed or eaten. The Experiment Photonics Media's first attempt at a transparent pumpkin pie. Courtesy of Joel Williams, Photonics Media. Photonics Media made two attempts at creating a transparent pumpkin pie. The first followed a simple recipe for the gelatin filling and used a premade crust. However, the crust on sale was graham cracker, which caused issues with clarity. When the hot gelatin liquid was added to the crust, crumbs mixed in with the liquid, a situation that is less than desirable for a transparent pie. The second recipe called for a greater ratio of water to gelatin powder, and much less sugar, which resulted in a higher level of clarity than the first attempt. The recipe also called for the filling to be refrigerated before being added to the crust. When seeking recipes, be mindful of the ratio of water to sugar and gelatin, as well as refrigeration instructions. Happy Thanksgiving!