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Lab 7 / The Inverse-Square Law 1 Rowan Introduction to Astronomy Lab 8 / The Inverse-Square Law for Light Name: ________________________________________________ Score: __________________________ Objectives Measure the brightness of a light bulb with a photoelectric detector. Measure the intensity change as the light bulb and detector are moved farther apart. Tabulate the intensity change data for two different wattage light bulbs. Graph the data tabulated. Predict the brightness of the light bulb at unmeasured locations. Materials Scientific Calculator (borrow one if necessary) Graph Paper (online see www.mathematicshelpcentral.com/graph_paper.htm ) Optical bench Spherically radiating light source: two light bulbs of different wattages Photoelectric detector with meter (meter is done digitally through the computer workstation) Student Requirements Students may work in pairs or groups during data collection. Graphing and answering questions are to be done individually, using the student’s own words. Background and Theory Almost everyone is aware that as a light source gets closer it appears to become brighter. Conversely, as it moves farther away, the source appears less bright. It may be tempting to believe that when a light source appears to double in brightness, its distance has decreased by one half. In Figure 1, one can see that light radiating from a spherically radiating source spreads out over an area four times larger when the distance from the source is doubled. Thus, the brightness decreases to one fourth of the original intensity. Mathematically, this relation can be expressed as ratios 2 2 1 1 2 d d I I (1) where I 1 and I 2 are the intensities at distances d 1 and d 2 , respectively. If d 1 is one distance unit and the initial value of I 1 at d 1 equals one intensity unit, then equation 1 becomes 2 2 2 1 d I I (2) More generally, equation 2 can be written as 2 1 d I (3) Note: This lab has been modified so that the intensity data for both 60-watt and 100-watt bulbs have already been provided.
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