# How to measure the speed of light with this setup

• chipotleaway
In summary, the conversation discusses an experiment involving measuring the speed of light using a setup that includes a laser, lenses and mirrors, a function generator, high-speed photo detectors, and an oscilloscope. The function generator is used to turn the laser on and off in pulses, and the oscilloscope is used to measure the phase difference between the laser input and the detector signal. Materials from a similar experiment and suggestions from Imperial College are also mentioned.
chipotleaway
This is an experiment I'll be undertaking for labs.

Given the following equipment:
- laser with modulation input
- lenses and mirrors
- function generator
- high-speed photo detectors
- oscilloscope

and the setup as shown in the picture (the function generator is connected to the laser power, the laser power's connected to the laser, the laser sends a beam of light towards a mirror, the mirror reflects the beam of light, the beam of light then goes through a lens and into the photodiode, which is hooked up to the oscilloscope via a variable resistor), how would you measure the speed of light?

From this source* (looks like a similar experiment, though I'm struggling to understand it), the function generator would be turning the laser on and off, so it'll be arriving at the detector in pulses.
I'm not sure how the oscilloscope is used in this - if each pulse creates a peak then the wavelength would just be related to the frequency at of the function generator, though I'm not sure how that comes in either.

http://iopscience.iop.org/0031-9120/35/2/303/pdf/pe0203.pdf

#### Attachments

• setup.png
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Hi Chip,
Good thing you are trying to prepare as much as you can before sitting down in front of the thing. And yes, Mak and Yip's abstract gives away what is being measured, right? Namely the phase difference between input to the laser and photodiode signal. They even give a hint at what to do exactly (i.e. move the mirror).
I didn't buy the article, but I can imagine several ways to go about. Imperial college mentions a few.
Good luck and have fun!

1 person
Thanks BvU - the stuff from Imperial College helped a lot. got the gist of it enough to know what were supposed to be doing.

## 1. How does this setup measure the speed of light?

This setup uses a method called the "interferometer" to measure the speed of light. It involves splitting a beam of light into two paths, bouncing them off mirrors, and then recombining them. By measuring the interference pattern created when the two beams combine, we can calculate the speed of light.

## 2. What materials are needed for this setup?

This setup requires a laser, a beam splitter, mirrors, and a detector. The laser emits a beam of light, the beam splitter splits the beam into two paths, the mirrors reflect the beams, and the detector measures the interference pattern.

## 3. How accurate is this method of measuring the speed of light?

This method is highly accurate, with an error margin of less than 1%. It has been used by scientists for over a century and is considered the most accurate method for measuring the speed of light.

## 4. Can this setup be used to measure the speed of light in different mediums?

Yes, this setup can be used to measure the speed of light in different mediums by changing the path length of one of the beams. This allows for the calculation of the speed of light in different mediums, such as air, water, or glass.

## 5. How long does it take to measure the speed of light with this setup?

The actual measurement process only takes a fraction of a second. However, the setup and calibration process can take several hours. This includes aligning the mirrors, adjusting the beam splitter, and ensuring the laser is at the correct intensity.

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