Ok-5 Speed of Light
AIM
To demonstrate how the speed of light can be determine using the Foucault Method.
APPARATUS
- Pasco Speed of Light Apparatus
- 1mW Mini Laser
- 1m Optics Bench
- 48mm Lens
- 252mm Lens
- High Speed Rotating Mirror
- High Speed Rotating Mirror Controller
- Calibrated Polarizers
- Measuring Microscope
- Microscope Camera
DIAGRAMS
DESCRIPTION
The Foucault method of measuring the Speed of Light consists of a Laser Beam going through a beam splitter, then reflecting off a high speed rotating mirror towards a fixed mirror. It is then reflected back to the high speed rotating mirror, back towards the beam splitter. The position of the reflected beam can be measure using a microscope that is attached the beam splitter.
The speed of is given the equation.
Which can be rewritten as
Where A= the distance between lens L2 and lens L1, minus the focal length L1
B = the distance between lens L2 and the rotating mirror MR
D = the distance the rotating mirror MR and the fixed mirror MF
Scw = position of micrometer in the clockwise direction
Sccw = position of micrometer in the counter clockwise direction
Rev = RPM of the high speed motor
ΔSʹ= the displacement of the image point, as viewed through the microscope.
(ΔS=S1-S; where S is the position of the image point when the rotating mirror MR is stationary, and S1 is the position of the image point when the rotating mirror is rotating with angular ω.)
Initially the bright spot has to be located in the microscope with the rotating mirror being stationary. Which can be done by adjusting the lens, fixed mirror, rotating mirror by hand, beam splitter and micrometer. The position of the bright spot is centralised using the micrometer with the aid of the cross-hairs scale on the microscope. The reading on the micrometer is noted. The high speed motor is turned on at a speed of 750rpm in the clockwise direction, the position of the bright spot will move. It has to be relocated by adjusting the beam splitter and micrometer, and centralised on the cross-hairs. The position of the micrometer is noted. The speed of the high speed motor is increased to 1500rpm in the clockwise direction. The bright spot will move again within the scope of the microscope. The bright spot is centralised again using the micrometer and the reading noted. The whole process is repeated with the high speed rotating mirror, rotating in the counter clockwise direction.