Photoelectric Effect

Procedure provided by Xiaolin Xu.

  1. Before starting doing any measurement, check the wiring (usually wires won’t be disconnected except for the batteries/voltage divider). The fuse in the voltage divider box might be loose - if so, secure it with some tape or some kind of clamp.
  2. There are two ways to measure the dark current - with the mercury light on and with it being off. According to Roger, it’s better to measure the dark current in both ways. Compare the dark current reading to photocurrent - if it is more than 10 percent of the photocurrent, it’s wrong and we should check the setup of light source, the grating, and the photocathode tube. Theoretically, the I-V curve of dark current should be roughly linear.
  3. There are three components in the direct measurement of photocurrent: the dark current, the current from the anode to the cathode, and the current from the cathode to anode, among which we want to measure the last one. We cannot really measure the current from anode to cathode, so we just subtract the dark current from the current from the cathode to the anode, which is what we measure with our ammeter.After subtracting the dark current, remember to normalize the different I-V curves corresponding to different wavelengths according to the saturation intensities.
  4. Because the stopping voltage is what we are looking for from the I-V curves, the students should take much more intensive data around the stopping voltage (arould -1V to 0V usually). There are two ways to decide stopping voltages from I-V curves, as described in the manual. Either way should work, giving similar values of Planck’s constant. Method (a) and (b) will lead to really different readings of the work function. That’s why we don’t really care about work function readings so much.
  5. But don’t be confused about the work function. The one we get will be the work function of the ANODE, not cathode!
  6. In order to get the best reading of current, students should make sure that the image is focused in front of the cathode tube. As for the distances between the mercury light and the grating, and between grating and the cathode tube, they should first try to determine the focal length of the concave grating by using the slit opening (fixed the slit shined by light and find out the focused image; with distance of object and image, focal length can be easily calculated). With focal length known, we can locate light, grating, and cathode tube as we want and imaged focused.
  7. Remember to disconnect batteries at the end of your lab session, otherwise they will be fully drained !!!