# Different ways to measure the speed of light experimentally ?

• ATH500
In summary, In this conversation, the expert summarizer discusses ways to measure the speed of light. They discuss using a microwave, a home-made tunable dye laser, and an oscilloscope. The expert also suggests using a laptop to measure the speed of light.
ATH500
Hello,
I would like to know different ways to measure the speed of light. It's for one of my project. I would like to know if there are simple ways to do it. I already know that we can measure the speed of light using a microwave: http://orbitingfrog.com/blog/2008/05/13/measure-the-speed-of-light-using-your-microwave/

This seems like the easiest way to do it for me right now but I need to use more than one way to measure it in order to compare different data experimentally and find the best results. I would need easy ways to calculate the speed of light. Do you know any other methods ?

Thank you,
ATH500

I first measured the speed of light in a 4th year lab - using the spinning mirror and a continuous laser. It was NOT easy!

The second time, I was building lasers at York U in Toronto about 1975. My home made tunable dye laser happened to have a pulse duration of about 1 nanosecond. I also had an expensive HP oscilloscope that could show that pulse cleanly from a photocell. Measuring the speed of light was as simple as splitting the beam with a glass slide so part of it traveled a bit further, say a meter extra, and two pulses appeared on the scope separated by 3 nanoseconds. So the speed of light was 1 m divided by 3 nanoseconds.

I have access to all the equipment I need to measure the speed of light with an oscilloscope. I'll use two LEDS connected to fiber optics cable of different length and generate two squared signal using a photodiode connected to a circuit and the oscilloscope.

Thank you !

If anybody has any other suggestions to measure the speed of light, please tell me !

ATH500

I am doing a public event soon involving taking night images from elevations to celebrate equinox. As part of making the event more fun, I wanted to include a simple physics experiment to measure the speed of light. I have lasers, and mirrors and want to use them over distance. I don't have an oscilloscope but I have a lap-top if there's a cheap way to turn one into one. I also have various kinds of photo detectors I can hookup. Is there a way to make the laptop a cheap DIY oscilloscope or a simple electronic circuit I can design to measure the speed with and output some useful readable info with LEDS or some other way??

ATH500 said:
Hello,
I would like to know different ways to measure the speed of light. It's for one of my project. I would like to know if there are simple ways to do it. I already know that we can measure the speed of light using a microwave: http://orbitingfrog.com/blog/2008/05/13/measure-the-speed-of-light-using-your-microwave/

This seems like the easiest way to do it for me right now but I need to use more than one way to measure it in order to compare different data experimentally and find the best results. I would need easy ways to calculate the speed of light. Do you know any other methods ?

Thank you,
ATH500

I saw this or similar from an enthusiastic Science presenter who is having some success for this and similar it on British TV recently.

Shouldn't one feel uneasy until one knows how the frequency of the microwave is known or can be measured? I do.

## 1. How was the speed of light first measured experimentally?

The speed of light was first measured experimentally by Danish astronomer Ole Rømer in the late 17th century. He observed the eclipses of Jupiter's moon, Io, and noticed that the timing of the eclipses varied depending on the position of Earth in its orbit. He calculated that this difference was due to the finite speed of light, and his estimated value was close to the currently accepted value of 299,792,458 meters per second.

## 2. What are some other methods used to measure the speed of light experimentally?

In addition to observing astronomical events, other methods used to measure the speed of light include using mirrors and rotating wheels, using interference patterns in light waves, and measuring the time it takes for light to travel a known distance. These methods have become increasingly accurate and sophisticated over time.

## 3. How do these experiments account for the refraction of light?

When measuring the speed of light experimentally, the refraction of light is taken into account by using materials with known indices of refraction, such as glass or water. By measuring the time it takes for light to travel through these materials, the speed of light can be calculated.

## 4. Has the measured speed of light changed over time?

The measured speed of light has remained relatively consistent over time, with minor variations due to improvements in experimental methods. However, some early measurements had significant errors, leading to slightly different values. The currently accepted value is based on a collaborative effort by various scientists using advanced technology and techniques.

## 5. How does the speed of light compare to other fundamental constants?

The speed of light is considered to be one of the most fundamental constants in the universe, along with other physical constants such as the Planck constant and the gravitational constant. It plays a crucial role in various scientific theories and is a fundamental limit in the universe.

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