Low cost experimental proposals to bridge from classical to modern physics

Simple experiments with diffraction gratings allow us to build a bridge between classical and modern physics. A LED, a diffraction grating and a screen are sufficient to measure the wavelength of the light emitted by the LED. The energy E of the emitted photon is inversely proportional to wavelength of the light λ, E = hc/λ, where h is Planck's constant. By a simple multimeter and a volt-ampere connection we can obtain the characteristics voltage-current curves of the LED. The threshold voltages of LEDs of different colours are easily measurable. When the LED emits light, we can assume that the electron of the current circulating in the LED when the voltage U transfers its energy eU to the photon and we can derive the Planck's constant by the relationship h = λeU/c. The students of the Quadri Lyceum of Vicenza have successfully tested such an activity within a course on electrical conduction. With the data for the red LED the students obtain h = 6*10⁻³⁴ Js. Now, by means of a CD-ROM we build a simple device to carry out the same measurement in reflection modality. The use of a simple and economic spectroscope enriches the experimental proposal in the perspective to build a path bridging from classical to modern physics. The intervention module based on this kind of lab in modern physics summer school for secondary students allows us to analyze its role in learning. (paper)