In this paper an experiment to study Planck's radiation law is presented. The spectra of a heated furnace and of a halogen lamp under various conditions were measured with a small USB grating spectrometer and fitted using Planck's law. The temperature determined from the fit was then compared with the results of comparative temperature measurement techniques (e.g. with the measurement of filament temperature from the temperature dependence of filament resistance). We have shown that even spectrometers with a limited wavelength range may be successfully used to study the Planck distribution. The temperature obtained from the fit of Planck's law to the measured spectra was, within the uncertainty, close to the results of comparative techniques.