Speed of Light

Table of Contents

Tips provided by Xiaolin Xu

Experiment Setup

  1. Align light path without the lens. For the rotating mirror, the foot ONLY with green paint can be adjusted. After you can see a good spreaded laser spot in the frosted screen (probably you won’t be able to see the full spot, but make sure you see most part of it at least), put the lens in the light path. At this time, student may need to adjust the rotating/fixed mirror a little bit to see a good focused laser spot in screen. For a good alignment, laser beam should pass the center of lens perpendicular and superimposed with the laser spot of coming back to rotating mirror. The correct location of lens should be the center of the distance from laser to fixed mirror (focal length of mirror is 5m and distance from laser to fixed mirror is approximately 10m). Students should know where the lens should be to get the best image by measuring distance carefully. But roughly locating the lens on top of laser black box should be OK to get a image.

  2. We can read position information from the linear caliper or the dial. But the dial is with higher precision, and we usually suggest they read from dial meter. Make sure they know how to read the dial correctly at the beginning since most of them don’t have any idea. Range is 1 inch, so all reading should be “0.abcd inch”. First significant digit “a” should be read from the small dial, followed by “bc” reading from the large dial (there are two scales for the large dial, you can decide which to use by seeing is whether the needle of small dial is closer to previous number or next number; closer to previous number reading large dial with number smaller than 50, otherwise read the numbe lager than 50). The last digit “d” is out of estimation.

  3. Frequency reading is a little bit of tricky. They can read frequency either from digital multimeter or oscilloscopy. From previous experience, oscilloscopy gives better readings. But make sure it’s calibrated before reading of numbers. Both of digital multimeter and oscilloscopy depend on the photediode in front of the rotating mirror. It should be levered with laser beam and pointing toward the rotating mirror. There are tow batteries on the window shelf to drive the photodioge and amplifier. Only one of them is working. If you couldn’t get any significant peaks on the oscilloscopy, you may want to try the other battery.

  4. There is a switch on top of the rotating mirror. If it’s not rotating with motor on, check the switch.

  5. Somethimes the beam splitter inside the black box get dislocated. So if you couldn’t get any laser spot shown on the screen no matter how carefully aligh the light path, check it out.

  6. disconnect the battery and turn of motor (not just at zero voltage) and laser!!!

Equation Derivation

  1. Three factors of two: distance traved is 2D; actual angle change of coming back laser beam is 2 theta; double sided mirror, so angle frequency of the laser beam we measured should be twice of the actual angular frequency of the rotating motor.

  2. Make sure they are conver unit from inch to meter considering that they read position from dial.

  3. Questions listed on the manual are tricky!