Methodologies

We want students to be able to solve a specific problem posed by scientific instrumentation, using problem solving methodology. This includes the definition of the work plan, separating the accessory from the fundamental, and, starting from scratch, the development, testing and demonstration of the final solution.

The strategy was planned around the competencies related to knowledge and control of instrumentation that can be acquired in a third year bachelor course with 7.5 ECTS. In the two previous years, the students took the fundamental courses on Mathematics and Physics, as well as two classical laboratory courses.

In planning the course, the instrumentation was chosen in order to have a broad spectrum of instruments, including old GPIB controlled instruments that are still present in many laboratories, as well as new USB controlled instruments. Students were also required to do the drawing of mechanical parts and electronic circuits to be built at the Department’s mechanical and electronic workshops, and to come up with innovative solutions.

The final report allows the evaluation of the student’s capacities of synthesis and analysis. Teachers are aware that a group report may lead to incompatibilities in organization and style; to prevent this, they undertake a close supervision looking for signals of emerging problems, in order to take action before it escalates.

The oral presentation, made by each working group, is an essential evaluation tool. It gives students the understanding that it is very important to learn how to communicate and present, clearly and briefly, the strategy followed and the results obtained, as well as a critical retrospective of the decisions taken. The format adopted is similar to that in scientific conferences: 12 minutes of presentation followed by 8 minutes of questions. The two elements of the group must participate in the presentation and answer the questions, giving this component of evaluation the ability to distinguish between the group elements. All students are required to be present at all presentations and encouraged to ask their colleagues questions. This session is public for the whole staff of the department.

Spaces

Classes take place in a 150 m² laboratory that is separated in two sections. The larger one is dedicated to Electronics, having ten working benches fitted with basic equipment and four tables for support. The section dedicated to instrumentation has ten workplaces equipped with computers, and four for support. The students work in groups of two and classes have a maximum of 16 students. The instrumentation laboratory is equipped with multiple acquisition interfaces (from several manufacturers), digital oscilloscopes, power supplies, signal generators and multimeters, all allowing computer control. Students can use the Electronics laboratory to construct small setups and have the supports of the department workshops to build small components for the project.

Photoelectric effect and measurement of h

It was proposed to the students to fully automate an experiment to measure the stopping potential as a function of wavelength and, from those potentials, to calculate the value of the Planck constant. It was proposed to use several LEDs instead of the traditional Mercury lamp and spectral filtering.

The students studied the problem, with some guidance from teachers, and divided it into several small problems: lighting one LED at a time (interface, software and transistor circuits), measuring the stopping potential (capacitor charged to the potential, communicating with the voltmeter through the GPIB interface), passing light from the LEDs to the cell, reducing ambient light and averaging and storing data in the computer for posterior analysis. After testing each subset, everything was assembled together and tested again before the final measurements. The final assembly is presented in figure 1.

Figure 1.

Student setup for the photoelectric effect. 1- Computer; 2- Interface (USB-6009); 3- Electronic control circuit; 4-Capacitor (100 nF); 5- Voltmeter (not needed); 6- Photoelectric cell (Phywe); 7- lens; 8- LEDs; 9- Cable for electric connection of LEDs; 10 and 11- Mechanical holders.

With this setup, students obtained automatically an averaged value for h=5.1 10^-34 J s and an extraction energy of 1.43 eV. In figure 2 data averaged for each LED is presented with error bars.

Figure 2.

Student averaged results for the photoelectric effect. Horizontal error bars are based on the LEDs’ spectral width and the vertical ones are the variance on the stopping potential.

Students weren’t very satisfied with the large difference to the accepted value, but when they were driven towards a deeper analysis of the LED spectra, they noticed that the low power skirt was very different among the LEDs, particularly on the short wavelength side (this is the trade off to simplify the experiment without the use of spectral lamps). The students got convinced of this fundamental flaw by removing the two worst LEDs, thereby improving the value of h to 6.4 10^-34 J s.

Acquiring diffraction patterns with one photodiode and a rotating mirror

This proposition was to take a stepper motor (from an old photocopier), an interface, a mirror and a photodiode and record the diffraction pattern to the computer. After dealing with the electronics and four digital outputs from the interface for control, the students discovered that the step angle was larger than the angular diffraction pattern. Teachers were about to give then a thinner diffraction object when they came up with the idea of using reducing gears to get a smaller angular step, building it with the set of LEGO Technic available in the laboratory. Figure 3 presents the schematics of the setup and a photo of the gear detail and figure 4 shows the result obtained with a hair from a student.

Figure 3.

Student setup for the measurement of diffraction patterns.

Figure 4.

Diffraction pattern of a hair obtained with the setup.

Acquiring fluorescence spectra and its variation with temperature

The problem was to acquire fluorescence spectra as a function of temperature. Partial problems: controlling a mini USB spectrometer with optical fiber input, turning on and off a violet LED, acquire temperature from a thermistor and storing and processing spectra. In figure 5 we present the students’ solution for the experimental setup. Some experimental results are presented in figure 6.

Figure 5.

Setup for fluorescence measurements.

Figure 6.

Left-Fluorescence spectra as a function of temperature. Right-Energy difference (ΔE) associated with spectrum width as a function of temperature.

Other projects

Some projects are proposed by research groups in activity in the Physics and Astronomy Department. They are presented to the students after an evaluation of the workload and level of knowledge needed is considered acceptable for them. Those projects need a special attention, in order to keep the workload within the limits established for the course. Two examples in recent years have been the control of the translation stages for a fs direct writing setup and the automatic data acquisition and partial control in a magneto-optical setup.

Other projects are proposed that seem easier than average (examples: simulation of traffic lights and speed trap and automatic control of the illumination using a photo resistor as the detector). These projects can be very interesting since students with low self-esteem can easily complete the more basic part of the project and feel able to build other activities from there.