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Variation of Light Intensity

Equipment

1 High Sensitivity Light Sensor PS-2176 1 Sensor Extension Cable 1 Large Rod base ME-8735 1 Rod, Stainless Steel, 45 cm ME-8736 1 Multi Clamp ME-9507

Required but not included:1 Flashlight (or other DC light source) 1 Incandescent Light Source (AC) 1 Fluorescent Light Source (AC)

Introduction

The purpose of this activity is to compare the variation in intensity of light from different light sources. Students will use a light sensor to record and compare incandescent versus fluorescent light sources, as well as light from AC versus DC sources.

Background

Electric light bulbs are powered by a voltage at a frequency of 60 Hz (or 50 Hz in some countries). The maximum amplitude of the voltage, and thus a maximum brightness from the bulb, occurs twice per cycle because an electric bulb is energized when the voltage increases, regardless of the polarity of the voltage. An electric bulb energized by alternating current will have maximum intensity 120 (or 100) times per second, as well as an minimum intensity (zero) 120 (or 100) times per second.

Fluorescent lights blink on and off at a particular frequency. Light from incandescent bulbs powered by alternating current fluctuates at well. The light intensity from a light bulb powered by direct current should not vary.

In this experiment you will use a High Sensitivity Light Sensor to measure the intensity of light from bulbs powered by AC and DC sources, monitoring the light intensity in an oscilloscope display and noting any variations in the intensity.

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Using the Light Sensor

The High Sensitivity Light Sensor has three range settings. To select a range, press one of the buttons on the sensor. The button of the selected range is illuminated. Select a lower range to measure lower light levels with higher resolution. Select a higher range to measure brighter light.

Try the following to become more familiar with the Light Sensor:

1. Connect the light sensor via the sensor extension cable to the 850 interface.

2. Position the Light Sensor on the table so the aperture on the sensor is below an overhead light. Press the button on the sensor for the lowest range (candle icon).

3. Create a Digits Display and select Light Intensity.

4. Change the Sampling Mode to Fast Monitor. Set the sample rate to 1 kHz. Start monitoring data.

5. Switch the Light Sensor to the middle range setting and observe the value of Light Intensity in the Digits display.

6. Switch to the highest range setting and observe the value of Light Intensity in the Digits display.

7. Cover the Light Sensor aperture and observe the value of the value of Light Intensity.

8. Return the range to the middle setting. Pick up the sensor. Observe the value of Light Intensity as you move the Light Sensor closer to the overhead light source. Observe what happens when you move the Light Sensor farther from the light source.

9. Click "Stop" to stop monitoring data.

Figure 1: Aperture of Light Sensor Pointed at Overhead Light

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Procedure: Incandescent AC

1. Change the Sampling Mode to Continuous. Create an FFT Display with Light Intensity on the vertical axis. Set the Sample Rate to 500 Hz.

2. Open the Recording Conditions and set a Start Condition on the measurement Light Intensity when it rises above 5. Then set a Stop Condition for Time-based so recording will stop after 3 seconds.

3. Place the Light Sensor on the table so the aperture faces the AC incandescent lightsource, as shown in Figure 2.

4. Position the Light Sensor approximately 0.5 m form the light source.

5. Turn off the room lights.

6. Begin recording data. Turn on the light source. Data will not appear in the FFT Display until you turn on the light source, and will automatically stop recording after 3 seconds. Turn off the light source.

7. Use the Scale Axis tool in the FFT Display to rescale the axes as necessary so you can see the frequency of the peak light intensity. You can also adjust the scale of each axis by dragging the numbers on the axis up, down, left, or right.

8. Use the Coordinates tool to measure the frequency of the peak light intensity. Record this value.

Figure 2: Setup of Incandescent AC Light Source

9. Create a graph of Light Intensity vs. Time. Set the sample rate to 1 kHz.

10. Turn on the light source.

11. Begin recording data. Data recording will automatically stop recording after 3 seconds. Turn off the light source.

12. Determine the number of times per second the light intensity fluctuates. One way to do this is to count the number of complete cycles that occur within a 0.1-second time period, and multiply by 10. Record this value.

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Procedure: Fluorescent AC

1. Replace the incandescent light source with the fluorescent light source.

2. Begin recording data. Turn on the light source. Data will not appear in the FFT Display until you turn on the light source, and will automatically stop recording after 3 seconds. Turn off the light source.

3. Use the Scale Axis tool in the FFT Display to rescale the axes as necessary so you can see the frequency of the peak light intensity. You can also adjust the scale of each axis by dragging the numbers on the axis up, down, left, or right.

4. Use the Show Coordinates tool to measure the frequency of the peak light intensity. Record this value.

5. Turn on the light source.

6. Using the Light Intensity vs. Time graph, begin recording data. Data recording will automatically stop recording after 3 seconds. Turn off the light source.

7. Determine the number of times per second the light intensity fluctuates. Record this value.

Procedure: Incandescent DC

1. Replace the fluorescent light source with the incandescent DC light source.

2. Turn on the light source.

3. Begin recording data. Data will automatically stop recording after 3 seconds. Turn off the light source.

4. Use the Scale Axis tool in the FFT Display to rescale the axes as necessary so you can see the frequency of the peak light intensity. You can also adjust the scale of each axis by dragging the numbers on the axis up, down, left, or right.

5. Use the Show Coordinates tool to measure the frequency of the peak light intensity. Record this value.

6. Turn on the light source.

7. Using the Light Intensity vs. Time graph, begin recording data. Data recording will automatically stop recording after 3 seconds. Turn off the light source.

8. Determine the number of times per second the light intensity fluctuates. Record this value.

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Analysis & Conclusions

1. How do the frequency of light intensity variation for the fluorescent AC bulb and the incandescent AC bulb compare to the accepted value for AC frequency?

2. How does the fluctuation of an incandescent bulb run on 60 Hz (or 50 Hz) AC differ from the fluorescent bulb?

3. How does the fluctuation of a light bulb run on 60 Hz (or 50 Hz) AC differ from the incandescent bulb powered by DC?