Part B :
The intensity of the incident light (from the mercury vapor lamp) can be altered by putting an aperture in front of the photodiode.
Apertures available : diameter d = 2, 4, 8 mm
Different aperture means different light intensity (different number of photons of the same energy). The number of photons should be proportional to the area of the aperture (not the diameter).
When the anode is positive relative to the cathode, electrons from the cathode (emitted because of the photons shined on the cathode) will collect on the anode (collector plate).
When the stopping potential is zero (U = 0V) then the anode is positive relative to the cathode (because the cathode is negative).
In this part of the experiment you are to investigate if the following statement is true:
Higher intensity of the light means MORE photons (not photons with more energy).
The theory says :
When higher intensity of the light means more photons then higher intensity of the light must also mean that more electrons will collect on the anode.
The photocurrent measured is directly proportional to the number of electrons hitting the anode. Current is defined as charge per time :
t t s
Z = number of electrons ; |e| = 1.602176
C Note: I > 0 so you need to use |e| > 0
Using two different lines of the mercury spectrum (435.8 nm and 546.1 nm) and three different apertures
(2 mm, 4 mm, 8 mm) the photocurrent will be measured for the anode potential U = 0V. For each of the 6 experiments the number of electrons collected on the anode per second will be calculated. The area of each aperture will be calculated.
Part A :
Refer to page 9, 10 and 11 of the Pasco Instruction manual ‘PHOTOELECTRIC EFFECT APPARATUS’ which is available in the Laboratory . Make a table like Table 1 (Page 10) in your Labnotebook. Record the absolute value of V, (|V|). Do not write in the Pasco Instruction manual.
Disregard the Calculating and Questions part, but do the Extension part using the other two apertures
(2mm , 8mm). You should now have 15 stopping potential values in 3 Tables.
Part B :
Refer to page 13, 14 and 15 of the Pasco Instruction manual ‘ PHOTOELECTRIC EFFECT APPARATUS’ .
Disregard the Analysis and Questions part.
Do experiment 2 with the following changes :
Measuring Current–Voltage Characteristics of Spectral Lines - Constant Frequency, Different Intensity
Preparation for Measurement
2. and 3. Have already been done if you did Part A of the experiment.
Measurement – Constant Frequency, Different Intensities
2 mm Aperture
3. Adjust the -2 to +30V VOLTAGE ADJUST knob so that the current display is zero.