This work demonstrates an experiment on the optoelectronic properties of a p-n junction suitable for students of undergraduate physics. It investigates, from an educational point of view, the origin of the wavelength of radiation emitted by a light emitting diode (LED) and determines the emission wavelength of an infrared LED without using precision instruments. The wavelength ranges were estimated for four LEDs emitting visible radiation using a common laboratory spectrometer and grating arrangement. The turn-on voltages of these LEDs and that of an infrared LED were estimated from their forward current-voltage characteristics. The wavelength of the infrared LED was determined by extrapolating the turn-on voltage versus the corresponding emission frequency plot for the visible LEDs. The variation was found to be nonlinear. The results were compared with standard results obtained from precision measurements, and reasonable accuracy was obtained. (Contains 2 tables and 6 figures.)