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• The speed of light in vacuum is a fundamental constant.
• The speed of light is the same in all frames of reference.
• In 1983, the speed of light was defined as exactly 299,792,458 m/s.
• Early attempt to measure the speed of light: Galileo (failed due to human reaction time limitations).
• First recorded speed of light in modern era: Olaf Roemer (astronomical observations).
• Bradley measured the speed of light from astronomical observations. The value was quite close to the correct one.
• First measurement of the speed of light from terrestrial experiments: Fizeau.

The experiment involves a concave mirror (M2), a rotating plane mirror (M1), a lens (L), a glass plate (G), and a light source (S). 1. **Setup:** * A beam of light from source S is reflected by the glass plate G and converges due to the lens L onto the rotating mirror M1. * When M1 is in a specific position, the light reflected from it converges at the concave mirror M2. * The light retraces its path back to L, and a portion is reflected by G to form an image at I. * When M1 rotates by a small angle Δθ to M'1, the reflected light now converges at a slightly different point on M2, and after retracing its path, forms a new image at I'. 2. **Measurements:** * `R`: Radius of the concave mirror M2 and distance from B to O (BO = BC = R). * `b`: Distance from M1 to L. * `a`: Distance from L to S. * `ω`: Angular speed of the plane mirror M1. * `s`: Shift in the image position (II'). 3. **Derivation:** * The distance OO' is related to the angular displacement by OO' = R(2Δθ). * The shift in the image is SS' = II' = s. * The magnification of the lens L is given by SS'/OO' = a/(R+b). * Combining these relations, s / (R*(2*delta_theta)) = a / (R+b) => s = a*R*2*delta_theta / (R+b) ..... (iii) * The time taken for light to travel from M1 to M2 and back is Δt = 2R/c. * The angle rotated by M1 during this time is Δθ = ωΔt = ω(2R/c). * Substituting Δθ in the earlier expression, we get s = a*R*2*(omega * 2R/c) / (R+b). * Solving for c, we obtain the speed of light: c = (4R²ωa) / (s(R+b)). 4. **Advantages:** * Small space requirement, allowing the experiment to be performed in a laboratory. * Allows for the insertion of a transparent material between the mirrors to measure the speed of light in that medium. * Demonstrates that light travels slower in a medium compared to vacuum, contradicting Newton's corpuscular theory.