We describe how to modify an existing speed-of-light apparatus to measure the speed of light in water. We derive the necessary formulas to account for refraction and light’s passage through both water and air. We obtain for the speed of light in air and for the speed of the light in water. Both results are within 7% of established values.
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The PASCO fixed mirror is a concave spherical mirror with a 13.5 m radius of curvature; it is optimized for a 13.5 m distance between the two mirrors. In actual operation, the 13.5 m separation is not critical, and accurate results are obtained even with much smaller distances between the mirrors. Since the fixed mirror’s radius of curvature is, in practice, not required to be the distance between the two mirrors, the successful use of a plane mirror (infinite radius of curvature) is unsurprising.
© 2010 American Association of Physics Teachers.
2010
American Association of Physics Teachers
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