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Question
Describe Fizeau’s method to determine the speed of light.
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Solution
Speed of light by Fizeau’s method
Apparatus:
- The light from a source S was first allowed to fall on a partially silvered glass plate G kept at an angle of 45°
- It is then allowed to fall on a rotating tooth wheel with N teeth and N cuts of equal widths.
- Light passing through one cut will be reflected by a mirror M kept at a long distanced (about 8 km)
- If the wheel is not rotating, it gets reflected by a partially silvered glass plate.
Working:
- Angular speed of rotation of wheel was increased from zero to GO Until it blocked by an adjacent tooth.
- It is ensured by the disappearance of light while looking partially silvered glass plate.
Expression:
v = `"2d"/"t"`
where, d = distance, t = time taken
angular speed ω = `theta/"t"`
`theta = (2pi)/(2"N") = pi/"N"`
t = `pi/("N"omega)`
v = `(2"dN"omega)/pi = 2.9972 xx 10^8 "ms"^-1`
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