Bilingual edition: English is preserved and Chinese follows each unit. Terminology uses the confirmed v20260916 glossary; automated semantic review remains traceable.
中英双语版：英文原文完整保留，中文紧随对应单元；术语采用 v20260916 确认表，自动语义审校结果可追溯。

Speed of Light

光速

Light from a modulated He-Ne laser is detected at the beginning and end of a measured path. The AC signals from photodetectors at these two points are compared on an oscilloscope to measure the phase difference and from that the velocity of the light.

从一个调制的氦-氖激光器发出的光在测量路径的开始和结束处被检测。这两个点的光电探测器产生的交流信号在示波器上进行比较，以测量光的相位差，并由此确定光速。

A beam splitter is used to divide the beam for detection at begining and end of the measured path. Plane, front-surfaced mirrors are used to deflect the beam to get as long a path as possible.

分束器用于将光束分成两部分，以便在测量路径的开始和结束处进行检测。平面、前表面的镜子用于将光束反射，以尽可能延长路径长度。
  1. Equipment needed.
    所需设备。
  2. Basic setup.
    基本设置
  3. Experimental problems
    实验问题
  4. Troubleshooting
    故障排除
  5. Sample data
    示例数据
Example setup
示例设置
Index
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Equipment Needed

所需设备
Modulated Helium-Neon laser, Metrologic Model ML-868
Double photo-detector and amplifier for detecting laser at beginning and end of path.
Beam splitter
Signal generator for modulating the laser.
Oscilloscope
Front-faced plane mirrors and mirror holders
Converging condenser lens for the second photodetector.
9V battery for detector box.
Frequency counter
Several BNC cables and about 2 ea red and black banana leads abt 18"
Four BNC tees
调制的氦-氖激光器，计量型号ML-868双光电探测器和放大器，用于检测路径两端的激光。光束分割器信号发生器用于调制激光。示波器 面对的平面镜和镜架 收敛的聚光器透镜用于第二个光电探测器。9V电池用于探测器箱。频率计数器 几根BNC电缆和约1对红色和黑色香蕉插头，约18英寸 四个BNC三通。
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Basic Setup

基本设置
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Experimental Problems: Speed of Light

实验问题：光速

1. Frequency must be high to give a measurable separation. 1.4 MHz was used by W95 group. A triangle wave made reading the wave position easier.

频率必须足够高，以产生可测量的分离。W95小组使用了1.4 MHz。三角波使读取波形位置更方便。

2. Mirror shake was a problem. Hopefully the use of optical tables and mounts will help with that.

镜面振动是一个问题。希望使用光学平台和支架能有所帮助。

3. The signal from one photocell would bleed into the other, making the signal indistinct. The W95 group used the signal generator as a standard and measured the two photocells against that standard and took the difference.

一个光电池的信号会泄漏到另一个光电池中，使信号变得模糊。W95组将信号发生器作为标准，用该标准测量两个光电池，并取其差值。

4. The scope is difficult to read because of vibrations and a fuzzy signal.

由于振动和模糊的信号，范围难以读取。

5. Internal delays of the triggering of the oscilloscope? caused the line of best fit of the data to not cross zero. To overcome this problem, the slope of the data at different distances with respect to time was taken to get the speed of light.

示波器触发的内部延迟导致数据的最佳拟合线不通过原点。为了解决这个问题，取了数据在不同时间位置处的斜率以得到光速。
 Index

Trouble- shooting
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Troubleshooting: Speed of Light

光速

There are a number of experimental problems inherent in the speed of light experiment. Here are some further comments about problems encountered.

光速实验中存在一些实验问题。以下是对所遇到问题的进一步评论。

1. The high gain amplifier used to amplify the signal from the photodetectors tends to oscillate.

用于放大光电探测器信号的高增益放大器倾向于产生振荡。
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Sample Data

示例数据

Modulation: 600 kHz triangle wave

调制：600 kHz三角波

Path length Time Delay (microsec)
75.0 m ..................... 0.2
36.5 m ..................... 0.1
24.5 m ..................... 0.05

路径长度时间延迟（微秒） 75.0米 ..................... 0.2 36.5米 ..................... 0.1 24.5米 ..................... 0.05


c = 3.4 x 10^8 m/s

Modulation: 1.4 MHz triangle wave

调制：1.4 MHz三角波

Path length .......... Time Delay (microsec)
...........................front of wave..... back of wave
60.36 m ..................... 1.9 ..................... 1.1
40.12 m ..................... 1.8 ..................... 1.8
20.06 m ..................... 1.7 ..................... 0.85

路径长度 .......... 时间延迟（微秒） ...........................波前..... 波后 60.36 米 ..................... 1.9 ..................... 1.1 40.12 米 ..................... 1.8 ..................... 1.8 20.06 米 ..................... 1.7 ..................... 0.85

Using the slope of the line gives c = 2.0 x 10^8 m/s

利用直线的斜率可得光速 c = 2.0 × 10⁸ m/s
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