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Inquiry: Photometry

Purpose: To measure the intensity of sources of light by photometric means and determine the efficiency of several light sources


The laboratory should be darkened and the instruments positioned in such a way as to keep the errors due to extraneous light as low as possible. Place the two light sources (25 and 40 Watt bulbs) at opposite ends of the meter stick and adjust the position of the photometer until it is equally illuminated from both sides. With a Bunsen photometer, this condition is reached when the grease spot is made as nearly invisible as possible. With a Joly photometer, the translucent blocks must appear equally illuminated to the same depth. Record all positions to at least three significant figures. For each trial, determine the distances of the unknown source and standard source from the photometer. Assuming that your photometer functions according to the inverse square law, calculate the intensity of the unknown 25 Watt source for each trial and the average intensity for the three trials. Compute the efficiency of the 25 Watt lamp in cd/watt assuming our 40 watt bulb produces 37.2 candelas.

Part 1: Using all three photometers determine the intensity of the 25 Watt bulb and then the efficiency of the bulb in cd/Watt.

Part 2: If the 40 Watt source has the intensity of 37.2 candles, calculate the illumination on the photocell at the distances of 25, 30, 35, 40, and 50 cm. Record the galvanometer reading for each of these distances.

A.) With a graph, prove that the galvanometer reading is an inverse squared relationship with the distance.

B.) Make a calibration graph for the UHS-Physics Illumination device by graphing the galvanometer reading on the y-axis and Illumination (E) on the x-axis. You will use this graph to answer the following questions in your notebook.

Inquiry Questions:

  1. If a lightbulb has a galvanometer reading of 11 on the UHS-Physics Illumination device what is the illumination on the cell.

  2. If the cell area is 24.0 cm2, how many lumens would be hitting the surface of the cell?

  3. What is the intensity of this particular lightbulb in candles if the photocell was 35.0 cm away?

  4. How many total lumens are produced by this lightbulb?

  5. If the photocell is placed 70.0 cm from the lightbulb, what will be the new illumination?