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Photometry is the measurement of the properties of light, especially luminous intensity. In this lab we will use photovoltaic and photo-resistive methods to sense and measure light intensity.

As light strikes a photovoltaic (solar) cell a voltage is produced. The more light, the higher the voltage. With this in mind, we can connect a voltmeter to a photovoltaic cell and use the voltage reading as a measure of the amount of light hitting the surface of the cell.

As light strikes a photo-resistor(CdS cell), the resistance changes. By placing an ammeter in series with a voltage source and photo-resistor we can use Ohm’s law to determine that changing resistance will result in changing current flow.

Materials

Photovoltaic cell

CdS cell

Power Supply

Meter Stick

Incandescent lamps

Mirror

Procedure
Part A - One Lamp Photovoltaic Method
1. Set up the apparatus as shown in the diagram.  Connect the lamp to a 3V source. Dim the room lights.
2. Measure the light intensity as a function of distance from the incandescent lamp in 5 cm intervals over a 1 m distance. Record your data in the table below.

Part B - Two Lamps Photovoltaic Method
1. Set up the apparatus, with two lamps hooked up in parallel to the voltage source. 
2. Measure the light intensity, as a function of distance, from the incandescent lamp in 5 cm intervals over a 1 m distance. Record your data in the table below.

Part C
One Lamp with reflector – Photovoltaic method
Place a mirror reflector behind the light source and repeat the procedure in part A.

Part D
One Lamp – Photo resistive method

1. Set up the CdS cell photoresistor sensor.  Make sure the meter is in series with the sensor, and that 3 V are applied to the sensor circuit from the power supply.

2. Measure the light intensity as a function of distance from the 2 incandescent lamps in 5 cm intervals over a 1 m distance, using the photoresistive method. Record your data in the table below.

Analysis
1. Graph the distance vs. luminous intensity data for part A and B on the same graph.  What is the mathematical relationship between luminous intensity and distance from a light source?  Explain.

2. How does increasing the amount of light at the source affect the measured luminous intensity?

3. Graph your data from part C on the same graph using a dashed line.  How does placing a reflector behind the source affect the measured intensity? Explain.

4. Make another graph of distance vs. luminous intensity for the data collected in procedure part D.  How does the data compare to the data collected in Part A?  Explain