PASCO scientific Vol. 2 Physics Lab Manual: P55-1 Experiment P55: (Light Sensor, Motion Sensor) Concept Time SW Interface Macintosh file Windows file illuminance 30 m 500/700 P55 Light vs. Position P55_LTVM.SWS EQUIPMENT NEEDED Interface Dynamics Cart Light Sensor Dynamics Track (1.2 m or 2.2 m) Motion Sensor light source Base and Support Rod reflector (e.g., small index card) battery pack (for light source) PURPOSE The purpose of this laboratory activity is to investigate the relationship between light intensity and the distance from a point light source. THEORY The light from a point light source spreads out uniformly in all directions. The intensity at a given distance r from the light will be equal to the power output of the light divided by the surface area of the sphere through which the light has spread. Since the area of the sphere varies as the square of its radius, r, the intensity will vary as 1/r 2. I = 1 r 2 PROCEDURE For this activity, a light source will be mounted on top of a cart. The Light Sensor will measure the intensity of the light from the source as the cart moves slowly away from the Light Sensor. The Motion Sensor will measure the position of the cart as it moves. The program records and displays light intensity and position. You will use the built-in curve fitting to determine the best mathematical fit for the plot of light intensity versus position. PART I: Computer Setup 1. Connect the Interface to the computer, turn on the interface, and turn on the computer. dg 1996, PASCO scientific P55-1
P55-2: Physics Lab Manual Vol. 2 PASCO scientific 2. Connect the Light Sensor DIN plug into Analog Channel A of the interface. 3. Connect the Motion Sensor stereo phone plugs into Digital Channels 1 and 2 on the interface. Connect the yellow plug to Digital Channel 1 and the other plug to Digital Channel 2. 4. Open the file titled as shown: Macintosh P55 Light vs Motion Windows P55_LTVM.SWS The document opens with a Graph display of light intensity (%) vs. position (m). Note: For quick reference, see the Experiment Notes window. To bring a display to the top, click on its window or select the name of the display from the list at the end of the Display menu. Change the Experiment Setup window by clicking on the Zoom box or the Maximize button in the upper right hand corner of that window. 5. The Sampling Options are: Periodic Samples = Fast at 10 Hz (10 measurements every second). PART II: Sensor Calibration and Equipment Setup This experiment uses the Light Sensor without the fiber optic probe. Remove the fiber optic probe if it is attached to the BNC connector on the sensor. (To remove the probe, turn the probe s connector one-quarter turn counter-clockwise.) Sensor Calibration 1. Attach the light source and its power supply to the top of the dynamics cart. P55-2 1996, PASCO scientific dg
PASCO scientific Vol. 2 Physics Lab Manual: P55-3 2. Turn the light source on and the room lights off. 3. Change the Experiment Setup window from REDUCED SIZE to FULL SIZE by clicking on the Zoom box or the Maximize button. FULL SIZE ZOOM BOX MINIMIZE BUTTON RESTORE BUTTON CLOSE BOX CONTROL- MENU BUTTON MAXIMIZE BUTTON MACINTOSH REDUCED SIZE WINDOWS 4. In the Experiment Setup window, doubleclick the Light Sensor icon to open the Sensor Setup window. 5. In the Sensor Setup window, the current value of the voltage from the Light Sensor appears at the bottom of the column labeled Volts. Turn the SENSITIVITY ADJUST knob on the Light Sensor until the current value of the voltage reads a maximum (as close to 9.99 V as possible). dg 1996, PASCO scientific P55-3
P55-4: Physics Lab Manual Vol. 2 PASCO scientific If the voltage value is already at the maximum, turn the SENSITIVITY ADJUST knob toward MIN (counterclockwise) until the voltage drops just below the maximum. Then turn the knob slightly back toward MAX (clockwise). LIGHT SENSOR MAX MIN SENSITIVITY ADJUST 6. Click the top Read button in the Sensor Setup window to record this value of voltage as the maximum value (corresponding to 100% intensity). Click OK to return to the Experiment Setup window. 7. Change the Experiment Setup window to REDUCED SIZE by clicking the Zoom box or the Restore button. NOTE: Keep the SENSTIVITY ADJUST knob in the same calibrated position throughout the rest of the activity. Equipment Setup 1. Place the track on a horizontal surface and level the track. 2. Put a small reflector on the cart so the Motion Sensor s signal is able to reflect from a flat surface. 3. Put the Light Sensor at the same end of the track as the cart so that the port on the Light Sensor is at the same level as the light source and so it is pointed toward the light source. 4. Use the base and support rod to position the Motion Sensor so it can measure the motion of the light cart as it moves away from the sensor down the track. 5. Put the Motion Sensor 0.4 meters from the starting position of the light cart and pointed in the direction that the light cart will move. 6. Make sure that the Motion Sensor can measure the position of the light cart as it moves and is not reflecting off of anything except the cart s reflector. P55-4 1996, PASCO scientific dg
PASCO scientific Vol. 2 Physics Lab Manual: P55-5 LIGHT SOURCE MOTION SENSOR TRACK REFLECTOR CART LIGHT SENSOR To INTERFACE 40 cm MINIMUM DISTANCE BETWEEN TARGET AND SENSOR PART III: Data Recording 1. When you are ready, click the REC button ( ) to begin collecting data and gently push the light cart away from the Light Sensor at the same time. 2. Click the STOP button ( ) to stop collecting data when the light cart reaches the end of the track. Run #1 will appear in the Data list in the Experiment Setup window. 3. Turn off the light source on the cart. ANALYZING THE DATA 1. Click the Graph to make it active. Click the Statistics button ( ) to open the Statistics area of the Graph. Click the Autoscale button ( ) to rescale the graph to fit the data. 2. Use the cursor to click-and-draw a rectangle around the region of the data that is relatively smooth. dg 1996, PASCO scientific P55-5
P55-6: Physics Lab Manual Vol. 2 PASCO scientific 3. Click the Statistics Menu button ( ) in the Statistics area. Select Curve Fit, Power Fit from the Statistics Menu. The program will attempt to fit the data to a mathematical formula based on the variable (distance) raised to a power. NOTE: You may see a message that says there is no valid solution. In other words, the program needs your help to create the mathematical formula that best fits your data. In the Statistics area, click the a4 coefficient to make it active. Type in -2 as the parameter value. Press <enter> or <return> on the keyboard to record your value. 4. Examine the value of chi^2, the measure of closeness of fit. The closer the value is to 0.000, the better the data fit the mathematical formula shown in the Statistics area. 5. Record the value of chi^2 for the Power Fit. QUESTIONS Power Fit chi^2 = 1. How does the intensity change as position increases? What is the position value corresponding to the highest light intensity? What is the position value that corresponds to a light intensity of approximately 25%? 2. Is the graph of light intensity versus position a straight line? What shape does it have? What is the relationship between light intensity and position? 3. The light bulb is not really a point source. How does this affect the experiment and what can be done to minimize this error? P55-6 1996, PASCO scientific dg