In an experiment with Foucault’s apparatus, the various distance used are as follows: Distance between the rotating and the fixed mirror = 16 Distance between the lens and the rotating mirror = 6 m, Distance between the source and the lens = 2 m. When the mirror is rotated at a speed of 356 revolutions per second, the image shifts by 0.7 mm. Calculate the speed of light from these data.

Sol. In the given Focault experiment, R = Distance between fixed and rotating mirror = 16m ω = Angular speed = 356 rev/’ = 356 × 2π rad/sec b = Distance between lens and rotating mirror = 6m a = Distance between source and lens = 2m s = shift in image = 0.7 cm = 0.7 × 10^–3 m So, speed of light is given by, C = 4R^2ωa/s(R + b) = 4 * 16^2 * 356 * 2π * 2/0.7 * 10^-3(16 + 6) = 2.975 * 10^8 m/s