Accurately measuring interference patterns from projections on a screen is rather tedious. However, you can build a simple device that makes it possible to display interference patterns on an oscilloscope, making it easy to measure not only the distance between fringes, but also their amplitude.
As shown in Figure 5, the idea is to use a rotating mirror and a fast-light sensor to convert the interference pattern into an equivalent time-domain signal that can be displayed by a conventional oscilloscope. For the light sensor, we used a TAOS TSL254R-LF light-to-voltage converter. This device is an inexpensive component that incorporates a light-sensitive diode and amplifier on a single chip. It is very easy to use. It requires a supply voltage in the range of 2.7 to 5.5 V (we use two 1.5-V AA batteries in series), and produces an output voltage that is directly proportional to the light intensity. We placed the light sensor behind a narrow slit built from two single-edge razor blades.
Figure 5 A simple scanner makes it easy to measure interference and diffraction patterns with an oscilloscope. (a) Simplified diagram of the basic concept. A small DC motor spins a mirror to scan the pattern onto a narrow-view light sensor, transforming the pattern’s distribution along space into a signal that varies with time. An oscilloscope synchronized to the motor displays the pattern. (b) For the light sensor we used a TAOS TSL254R-LF light-to-voltage converter placed behind a narrow slit made from two razor blades. (c) We used a motor and polygon mirror from a broken bar-code scanner to build our setup.
As shown in Figure 5c, we built the optical stand from 1-in. × 1-in. cross-section, T-slotted aluminum extrusions made by 80/20, Inc. These are meant for building office cubicles and machine frames, so they are widely available (e.g., from McMaster-Carr) and inexpensive. In spite of this, they are very rugged and sufficiently straight to perform optical experiments. Our motor and mirror came from a discarded supermarket bar-code scanner. However, you could rig a small front-surface mirror to the shaft of a small 2,000 to 4,000 rpm DC motor. The TSL254R-LF’s response time (2 μs rise/fall time) is appropriate for these speeds. The advantage of a bar-code scanner motor is that it usually comes installed with a polygonal mirror and speed controller. Having more mirror surfaces per revolution reduces flicker if you are using an analog oscilloscope. The integrated controller maintains a constant rotation speed, which allows you to calibrate the system to produce a constant space-to-time relationship. Figure 6 shows a typical oscilloscope trace obtained with our system for a 10-μm slit spacing with a 630-nm red laser.
Figure 6 Interference pattern obtained with our scanner (Figure 5) for a double slit of s = 10-μm illuminated by a red (630-nm) laser pointer.
Leave a Reply