Purpose:
We've been talking about one of the ways that the intensity of a wave (whether light, sound, or water) dies away as it spreads out from a point.
Our equipment for this lab will simple in the sense that it is familiar. We will use light bulbs of different types and desk lamps. We will personally explore a number of the calculations we made in class in order to bring more clarity to those ideas:
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I) Process Description: Each of the lamps will have one of three bulbs in it. A 40 W incandescent bulb that produces 380 lumens, a 9 W compact flourescent bulb that produces 450 lumens, and a 45 W flood that produces 385 lumens in a conical (not spherical) pattern. Which ever bulb you have your first task is to determine the efficiency of the bulb in lumens/watt. This is a measure of how much light (lumens) we get relative to the power we pay for (watts).
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II) Intensity at 50 cm: Calculate the intensity(in lux = lumens/m2) of the light produced by your bulb 50 cm from the center of the bulb. Be thoughtful about contamination from the reflector behind the bulb. Those of you with the flood will need to design a different process because the light is not radiated spherically. When you finish your calculation swap lamps with someone else and repeat I and II for each type of bulb.
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III) Equivalent Intensity: Now determine a distance from each bulb where the intensity (lux) is the same. This will involve multiple uses of the inverse square law. Check your result by holding a piece of paper at that location and determining if it looks equally bright.
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IV) 1/4 intensity: For the lamp you currently have determine the location where the intensity (in lux) is 1/4 of the intensity at 50 cm. Hold a sheet of paper up at the locations and comment on the apparent difference in brightness (your eyes are not good linear detectors).
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V) Bonus effort: Use on of the bulbs and determine the location at which it feel like looking at the sun (only short looks please!). Using that intensity in lux (not watts!) and the inverse square law calculate the intensity of the sun in lumens. I will share the official answer when you show me your work.
LAB DELIVERABLES (turn in):
An introduction and 5 (or more) paragraphs describing your data and calculations for each of the steps above. In each section present any actual measurements, explanations and descriptions of the task you are working on, calculations that you make, and comments on the results.
