Question

For experimental modern physics, you want to build an optical interference fringe counting circuit. The output from a phototransistor reaches a maximum of 3 V when it is fully illuminated by a bright fringe, and stray light in the room results in about 1 V output when the phototransistor is viewing a dark fringe. Select, and justify, threshold voltages to be used for this circuit. Sketch a schematic for this experiment that includes values for the resistors to be used.

Answer 1

To build an optical interference fringe counting circuit, set a threshold voltage of 2 V to differentiate between bright and dark fringes. Create a schematic that includes a phototransistor, comparator circuit, and appropriately valued resistors for connecting the output to digital circuitry.

Step-by-step explanation:

To build an optical interference fringe counting circuit in experimental modern physics, it is important to set a threshold voltage that discriminates between the bright and dark fringes detected by the phototransistor. Given that a fully illuminated phototransistor outputs 3 V and about 1 V when viewing a dark fringe, a good threshold voltage might be set around 2 V, which is halfway between the maximum and minimum signals. This ensures that the circuit triggers only when the phototransistor detects sufficiently bright light.

The schematic for the experiment would include the phototransistor connected to a comparator circuit. The comparator would compare the phototransistor voltage to the 2 V threshold, outputting a digital signal to count the fringes. Resistor values will depend on the specific comparator used. Typically, a pull-up resistor might be needed on the output, and its value could range from 1 kΩ to 10 kΩ, depending on the current requirements of the subsequent digital circuitry.