A disco ball is a spherical object covered with small plane mirrors. When light reflects on these mirrors, it is scattered in many directions, producing a novel effect. The mirror globe is usually set to rotate, creating a profusion of moving spots (Fig. 1). In this article, we present a geometrical description of the movement of these spots and an experimental activity to test the model.
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The angular frequency depends on the sphere weight and the motor that spins it.
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A mirrored cylinder would not have this problem. In a cylindrical ring of mirrors, normal lines belong to a single plane P. If a projection plane P′ is perpendicular to P, all spots’ trajectories are straight lines in P′. In a sphere, these normal lines form a conical surface, thus trajectory analysis is more complicated.
6.
These initial angles were estimated considering that a 12-mirrors ring corresponds to an angle π/2, so the first mirror center corresponds to an angle π/48. Each mirror of a second ring was advanced in a quarter of its length in relation to the corresponding mirror of the first ring, so its initial angle was π/48 + π/96 = π/32.
© 2016 American Association of Physics Teachers.
2016
American Association of Physics Teachers
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