Relative Intensities of Light at Various Horizontal Distances from the Lamp

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Lab # 1

Inverse Square
Law Lab

Lab Executed: September 13, 2012
By: Arfan Rehab
Colleagues: Edwin Chen,
Tamzid Hassan, and Tony Zhao


The objective of this experiment was to measure the relative intensities of light at various horizontal distances from the lamp. The light detector (photocell) will enable us to make light intensity measurements that are more precise than those made with the human eye. The photocell converts light intensity to an electrical potential difference, which can be measured with a voltmeter. Using our measurements made from numerous elevations above the table, we assumed voltmeters were proportional to the intensity of light and observed a relationship between position and light intensity.


In technical terms, some physical quantity or strength is inversely proportional to the square of the distance from the source of that physical quantity. The physical law that describes this relationship is called the “inverse-square law”: the apparent intensity of a light source “I” is proportional to 1 divided by its distance squared. Imagine a light bulb placed at the center of three concentric spheres. The bulb puts out a given amount of light, but as it travels it spreads out, so that it reaches spheres of greater radius (first d = 1, the d = 2, and d = 3) it becomes diluted in intensity, because the light spreads out over spheres with larger and larger surface area. The surface area of a sphere is proportional to the d2 (equal to 4 * pi * d2 to be exact). Therefore, the intensity of light is lowered by 1/d2 for the same reason, as does the force of attraction between electrically charged particles.

Experimental Procedure:

Solar cells- An electrical device used to convert light into electricity with the photovoltaic effect. Voltmeters and Wires- Instrument used for measuring electric potential difference or voltage. Light Bulb and Fixture on a ring stand- Used as the primary...
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