Silicone-replicated materials are easy to make and provide an inexpensive—and unbreakable—alternative to glass optics for classroom experiments.
White light dispersion on the surface of a grating replica fabricated from clear silicone.
In a typical grade-school lecture about optics, a teacher might use a prism to illustrate the dispersion of white light or a mirror to show reflection. While such glass optics make for effective demonstrations, they are often kept out of young hands because they are easily breakable.
Where we teach, in the Philippines, most schools are crowded and have limited funds to spend on laboratory equipment—providing all the more reason for teachers to keep the science materials they have under lock and key. Classrooms with 60 pupils are not uncommon, and some schools operate in shifts that divide students between morning and afternoon sessions.
We saw a potential opportunity to give students a more hands-on learning experience by introducing silicone optics into the classroom. We molded replicas of lenses and diffraction gratings from a silicone elastomer, and then designed basic experiments for grade-school students that used these tools. These materials can be made easily and cheaply, and they are far more durable than glass. In fact, the lens replicas are virtually unbreakable and can even be washed.
One of us (Reguya) volunteered to use elastomeric lenses and diffraction gratings as demonstration tools for her sixth-grade science class. We thought it was important to provide students with simple yet meaningful experiments in which they could fully participate. Initial activities for the students included observing white light dispersion with a diffraction grating and magnifying text with a lens.
The optics were fabricated from Sylgard 184, a silicone elastomer from Dow Corning. Sylgard 184 is an affordable and readily available sealant that is packaged as an elastomer base and curing agent.
Each lens replica was made from less than 10 g of silicone, prepared by combining the base and curing agent at a prescribed ratio. To create the curvature of the lens, we used a small paint mixing tray as a mold. We poured silicone in its original liquid state into a hemispherical cavity and allowed it to cure for one week at room temperature.
Once it was fully cured, the silicone lens could be easily lifted from the plastic mold. The resulting solidified elastomer was transparent and took on exactly the same shape as its container. Although the lens surface was far from optically smooth, due to the quality of the mold, the replica still functioned quite well as an imaging element.
We measured the focal length of the lens to be approximately 4 cm. Lenses of various diameters and focal lengths could also be fabricated simply by selecting a different mold.
We prepared an activity sheet for the students to fill out as they performed their experiments. It included the objectives and procedures for the experiment as well as guide questions to aid students in their observations. Students were asked to draw letters of a word as seen through a typical magnifying glass and imaged through the silicone lens.
We also fabricated a silicone diffraction grating, using a discarded metal diffraction grating as a template. The grating measured 1.25 × 2.5 cm with a groove density of 1,200 lines/mm. We mounted it on a special stage with the grooves facing up and then applied a layer of silicone to the surface of the grating. After curing, we carefully peeled off the elastomer layer, leaving a copy of the grating grooves imprinted on the silicone surface.
White light dispersion creates a brilliant display on the replicated grating area, as our students were quick to note! All the students were able to illustrate a rainbow spectrum on their activity sheet. The primary goal of this exercise was for students to see how light behaved on the grooved surface and, from there, to determine the function of a diffraction grating.
In answering the guide questions, students observed that they see “many colors when the diffraction grating is placed near a light source” or observe “a rainbow in the diffraction grating.” The appearance of bands ranging from red to violet as a result of slightly tilting the silicone sample clearly captured the interest of
One student concluded that gratings are used “to change white light into many colors.” She also wrote that “white light is not totally white but has many colors,” a surprisingly insightful comment. We note that the replica functions as a true diffraction grating in either reflection or transmission mode, although transmission experiments would provide a brighter output due to the low reflectivity of the elastomer. With advanced students, light from a laser pointer may be used to generate symmetric first-order diffracted beams.
The students performed these activities with minimal assistance. They were noticeably less apprehensive during experiments, having been informed that silicone optics are sturdy and inexpensive. Without having to worry about damaging the materials, the class became more focused on making observations. As facilitators of the experiments, we found the classroom experience very rewarding and hope that this hands-on exposure to optics will deepen students’ appreciation for science and learning.
[ Raphael A. Guerrero is with the department of physics, Ateneo de Manila University, Loyola Heights, Quezon City, Philippines. Arlyn S. Reguya is with the H. Bautista Elementary School, Marikina City, Philippines. ]