Investigating the Photoelectric Effect Using LEDs and a Modular Spectroscope

Elementary spectroscopic investigations often utilize a commercial spectroscope. To maintain calibration these devices have fixed positions for the slits and dispersive element. Due to the rigidity and non-modular nature of these devices, they may seem like a "black box" to students since students cannot investigate the role of each component or the effect of position and orientation of each component on calibration or resolution of the instrument. With a modular spectroscope, students can monitor the effect of adjusting individually the position and orientation of the light source, grating, and slits. Investigating the position of the individual components dramatically illustrates the advantages of using long monochromators in conjunction with narrow slit widths in maximum-resolution spectroscopy. Once students fully understand the operation of the modular spectroscope, they calibrate it using a light source with a known spectrum. A circuit board containing colored light-emitting diodes (i.e., LEDs) is then used to measure a value for Planck’s constant through a photoelectric-effect-type equation. The exercise is suitable to both college and high school students. All components may be found in a chemistry laboratory in combination with a local electronics store.