Light Intensity Laboratory

This experiment is designed for the beginner with detailed instructions. Performing this lab will allow you to become familiar with the Light Probe, ChemBio and Physics programs with CBL and TI Graphing Calculators so that teachers and students can design their own labs and inquiry activities.

You will notice icons scattered througout the lab instructions. Clicking on the icon will pull up a window with
  • information and helpful hints
  • sample calculator screens
  • pictures of the equipment or experiment
Lab Directions

Sample Data and Graph

National and State Science Standards
Introduction
Relationships and functions are important themes in both math and science. This activity is a quick and practically fail proof way to gather data to study inverse relationships. All students know that the further you move from a light source, the dimmer it appears, but what exactly is the relationship between light intensity and distance?
Materials
CBL Unit meter stick
Light Probe candle
TI Graphing Calculator (73, 81, 82, 83 or 83+)
(with Physics or ChemBio program or application)		 masking tape
small box


Procedure

  1. Tape a meter stick to the table top. Place a candle at the 0 cm mark.

  2. Place your light sensor on a box or other object on the meter stick so that the probe is lined up with the center of the flame. Don't light the candle until you are ready to start collecting data

  3. Plug the light probe into Channel 1 of the CBL unit. Connect the CBL to the TI Graphing Calculator with the black link cable using the port on the bottom of each unit. Make sure the connection is tight by pressing firmly.

  4. Turn on the CBL unit and the calculator.

  5. Press PGRM and select CHEMBIO or PHYSICS. Press ENTER, then ENTER again to get to the MAIN MENU.

  6. Set up the calculator and CBL for one light probe.
    • Select SET UP PROBES from the MAIN MENU
    • Enter 1 as the number of probes
    • Select LIGHT from the SELECT PROBE menu (In CHEMBIO choose MORE PROBES to find LIGHT).
    • Enter 1 as the channel number.

  7. Set up the calculator and CBL for data collection.
    • Select COLLECT DATA from the MAIN MENU
    • Select TRIGGER PROMPT from the DATA COLLECTION menu

  8. Collect data.
    • Position the end of the light sensor at the 10 cm mark and light the candle. For the best results turn out the room lights and make sure that the light from other candles is not directed toward your light probe.
    • Wait 5 seconds, then press TRIGGER on the CBL.
    • Enter "10" (for 10 cm), then press ENTER to store this light intensity - distance data pair.
    • Select MORE DATA on the DATA COLLECTION menu.
    • Move the light sensor to the 15 cm line. Wait 5 seconds, then press TRIGGER on the CBL. Enter "15" (for 15 cm), then press ENTER. Select MORE DATA from the DATA COLLECTION menu.
    • Repeat this process at 5 cm intervals through 40 cm.
    • Blow out the candle.

  9. To view a graph of the light data:
    • Select STOP AND GRAPH from the DATA COLLECTION menu.
    • Press ENTER, then select NO when asked if you want to repeat.
    • Select VIEW DATA from the MAIN MENU or choose QUIT and press GRAPH

  10. To view a list of the light intensity data
      • Select STOP AND GRAPH from the DATA COLLECTION menu.
      • Press ENTER, then select NO when asked if you want to repeat.
      • If you are using ChemBio select VIEW DATA from the MAIN MENU
        If you are using Physics select QUIT
      • Press STAT and choose EDIT

  11. Optional (for a more rigorous study) - Record the distance and light intensity data
    • Measure the background light intensity. Repeat the entire activity without the candle. Make sure the lights are still out when taking data.
    • Calculate the adjusted light intensity at each distance by subtracting the background intensity from the corresponding light intensity and record your results.
    • Press STAT and choose EDIT. Enter your adjusted light intensity in L2, replacing the previous data
Analysis and Questions
Describe the curve of your graph. What type of function (if any) do you think would describe this curve? Use the calculator to try a few.
  • Quit the ChemBio or Physics program
  • Press STAT and choose CALC from the menu across the top of the screen. Choose from the available options such as a linear regression (LinReg), quadradic regression (QuadReg), cubic regression (CubicReg), exponential regression (ExpReg), power regression (PwrReg) and more.
  • Press L1 (2nd-1), comma, L2 (2nd-2), comma.
  • On the TI 83 and 83+ press VARS and choose Y-VARS. On the TI 82 press Y-VARS.
  • Choose Function and Y1.
  • Press ENTER to view the equation and R2 value.
  • Press GRAPH to see a graph of your data with the regression line

Although many different functions may appear to fit your data, the equation with the R2 value closest to 1.00 has the best fit.

  1. From your best fit equation - what is the relationship between light intensity and distance?

  2. Theory tells us the relationship between light and distance is inverse squared. That is if you cut the distance the light probe is from the light in half, the intensity would be 4 times as much. Does your data support this? Why/why not?

  3. If you were to double the distance the light probe is from a light bulb, by how much would the intensity change? If you were to move the light probe 5 times further from the light, by what fraction would the intensity change?

  4. Try graphing light intensity versus distance squared. If your data supports the theory you should see a straight line.

Sample Data and Graphs

Distance (cm)
Intensity
10
0.18072
15

0.08042
20
0.04516
25
0.02889
30
0.02010
35
0.01475
40
0.01130