Lab 6: See Through

The goal: Measure the transmission properties of glass at several wavelengths.

Overview

Glass is known for being transparent to the visible wavelengths of light. In the lab, we will measure the amount of light that passes through a thin pane of glass at several different wavelengths.

Instructions

Hardware

Conceptual Diagram

Begin by constructing laying out your measurement device. You will need two basic parts: a) the LED circuit, and b) the photoresistor circuit. You will want to plan out your placement of these things so that you can easily move the glass plate in and out of the light path between the LED and the photoresistor. Try to keep the LED and Photoresistor as close as possible, while still maintaining a gap that can accommodate the glass plate. The LED can be powered by the +5 Volt output from the arduino, and the photoresistor can be assembled into a standard voltage divider arrangement with a known resistor for R1.

The LEDs you have to work with are as follows:

ColorWavelength
UV/Purple 490 nm
Green 569 nm
Yellow 587 nm
Red 625 nm
InfraRed (IR) 940 nm

Software

In this lab we will implement an averaging routine to the data collection. This will help give us more stable measurements. The following is based on this tutorial here: https://www.arduino.cc/en/Tutorial/BuiltInExamples/Smoothing



    //with help from https://www.arduino.cc/en/Tutorial/BuiltInExamples/Smoothing

    // These constants won't change:
    const int numReadings = 10;          // How many points to average over?
    const int PhotoResAnalogPin = A0;    // pin that the sensor is attached to
    //these do change
    
    int readings[numReadings];      // this makes the array to stare the data
    int readIndex = 0;              // the index of the current reading
    int total = 0;                  // the running total
    int average = 0;                // the average
    
    int inputPin = A0;
    
    void setup() {
    
      // initialize serial communications:
      // this is not very time sensitive so we can be slow
      Serial.begin(9600);
      
      // put zeros in the array called readings
      for (int thisReading = 0; thisReading < numReadings; thisReading++) {
        readings[thisReading] = 0;
      }
    }
    
    void loop() {
    
      // remove the oldest data point from the array
      total = total - readings[readIndex];
        
      // read from the sensor:
      readings[readIndex] = analogRead(PhotoResAnalogPin);
      // add the reading to the total:
      total = total + readings[readIndex];
      // advance to the next position in the array:
      readIndex = readIndex + 1;
    
      // if we're at the end of the array...
      if (readIndex >= numReadings) {
        // ...wrap around to the beginning:
        readIndex = 0;
      }
    
      // calculate the average:
      average = total / numReadings;
      // report the value over Serial
      Serial.println(average);
      delay(100);        // delay in between readings
    }

Conceptual Diagram

In this plot we can see the difference between two signals. One has been averaged during acquisition (dark blue), the other has not (light blue). Since we are not concerned with changes in time, we don't have to worry about loosing temporal resolution when doing the averaging. (Normally, this could be an issue)

Procedure

Record the amount of light that passes through the glass and compare with the amount of light when the glass is not present.For example, if the photoresistor reads 810 when no glass is present, and 793 when there is glass, then the percent of the light that passes through will be 793/810 = 97.9%.

You will likely want to do this with some kind of cover (i.e. a cardboard box ) over the apparatus during each measurement so that you are sure you are only getting the light from the LED and not background illumination.

Record you data in a nice table format and prepare a plot that shows the % transmitted on the vertical axis and wavelength on the horizontal axis.

When you are done, in addition to the standard report, please submit your values here:

(Link to form: https://docs.google.com/forms/d/e/1FAIpQLSfX5N-PVKsDAORq6ZbxGc8mL0O8XfLxPrPn450sntC1INF-aA/viewform?usp=sf_link

To be included: