# An experiment to measure the Speed of Light

• SapientiaPT
In summary,The speaker wants to make an experiment to measure the speed of light. They plan to have a light source in the dark and two sensors that will detect when light is detected. The experiment will stop when the time it takes for the two sensors to detect the light pulse is recorded. They have no idea if this is a good idea, or if it would work, but they want to get opinions.

#### SapientiaPT

Gold Member
So, pretty much I want to make an experiment in order to get the speed of light.
What I plan to do is to have a lantern in the dark(initially off) perpendicular to a wall, two sensors(one closest to the lantern and the other closest to the wall), then turn on the light making sensor 1 go off as soon as it detects light and starting a stopwatch and sensor 2 stopping the same stopwatch, therefore getting the time it took from sensor 1 to sensor 2, and finally dividing distance betweens sensor 1 and 2 by the time the stopwatch registered. I have NO idea if this is even a good idea, or if it would work. Would like to get some opinions on this, and if you think this is possible, let me know if you know of a way to make the sensor/stopwatch system work.

If you can get your fingers and the stopwatch mechanism to both operate at several tens of thousands of times faster than is physically possible, then it might work.

EDIT: actually, thinking about it just a bit more, I think my estimate was probably considerably too optimistic.

SapientiaPT
phinds said:
If you can get your fingers and the stopwatch mechanism to both operate at several tens of thousands of times faster than is physically possible, then it might work.
Yeah, that was what I was afraid of... that they wouldn't be as fast as needed... So I guess that's that... Thank you anyway.

Welcome to the PF.

SapientiaPT said:
So I guess that's that...

Except, you can think about versions of your experiment that you may be able to do with some not-too-expensive equipment someday soon.

First, Have you tried measuring the speed of sound with a similar technique? stand a few hundred feet from a large flat surface (like the side of a warehouse, clap you hands together hard once, and notice the delay of the echo. You can experiment with timing that delay (have somebody else do it for you if you do the clap), and changing your distance to the wall. Take about 10 measurements at each distance, and average them to get a better value. You can look at the distribution of values you get at each distance to see what the "error" is in your experimental setup, and you can think of ways to improve the accuracy. (Hint, there may be a way to use a cellphone to improve on this basic echo setup...)

And for starting to measure the speed of light, you just need light sensors that respond quickly, and a way to measure the difference in time for the light pulse reaching the two sensors. Light travels about 1 foot every nanosecond, so if you can separate your two sensors by several hundred feet, you can start to measure the time delay with a pretty basic oscilloscope (you can see if your local school electronics lab has such an oscilloscope that you can use, for example). The light sensors have to have a fast response time, but you could use pre-packaged fiberoptic receiver modules, for example.

Stay curious and keep asking questions. That's a great way to learn!

EDIT/ADD -- And keep in mind that the cables that you connect to your two sensors need to be the same length (from your two sensors back to your oscilloscope or other time measuring device). Can you say why that is important?

SapientiaPT and phinds
berkeman said:
Welcome to the PF.

Except, you can think about versions of your experiment that you may be able to do with some not-too-expensive equipment someday soon.

First, Have you tried measuring the speed of sound with a similar technique? stand a few hundred feet from a large flat surface (like the side of a warehouse, clap you hands together hard once, and notice the delay of the echo. You can experiment with timing that delay (have somebody else do it for you if you do the clap), and changing your distance to the wall. Take about 10 measurements at each distance, and average them to get a better value. You can look at the distribution of values you get at each distance to see what the "error" is in your experimental setup, and you can think of ways to improve the accuracy. (Hint, there may be a way to use a cellphone to improve on this basic echo setup...)

And for starting to measure the speed of light, you just need light sensors that respond quickly, and a way to measure the difference in time for the light pulse reaching the two sensors. Light travels about 1 foot every nanosecond, so if you can separate your two sensors by several hundred feet, you can start to measure the time delay with a pretty basic oscilloscope (you can see if your local school electronics lab has such an oscilloscope that you can use, for example). The light sensors have to have a fast response time, but you could use pre-packaged fiberoptic receiver modules, for example.

Stay curious and keep asking questions. That's a great way to learn!

EDIT/ADD -- And keep in mind that the cables that you connect to your two sensors need to be the same length (from your two sensors back to your oscilloscope or other time measuring device). Can you say why that is important?

Yeah, those are interesting ways. Thanks

berkeman
SapientiaPT said:
Yeah, that was what I was afraid of... that they wouldn't be as fast as needed... So I guess that's that... Thank you anyway.
Why don't you actually try to calculate the time interval you'd be measuring? It's easy to do and will tell you immediately if a particular stopwatch is precise enough (though there are additional practical concerns...).

PeroK, SapientiaPT and berkeman
You might want to google for “cogged wheel light speed” to see how Fizeau did these measurements in the middle of the 19th century. It’s an an ingenious technique for getting good speed measurements without hyper-accurate clocks.

SapientiaPT, Ibix, Delta2 and 1 other person
russ_watters said:
Why don't you actually try to calculate the time interval you'd be measuring? It's easy to do and will tell you immediately if a particular stopwatch is precise enough (though there are additional practical concerns...).
Yeah, that's also an interesting idea. Might consider it.

Unless you do not believe in the relationship v = fλ, why not try to measure the wavelength of several different frequencies of light? Then plot f versus 1/λ, and the slope of your straight-line fit is the speed of light!

Zz.

SapientiaPT, sandy stone and Nugatory
ZapperZ said:
Unless you do not believe in the relationship v = fλ, why not try to measure the wavelength of several different frequencies of light?
According to Google, this general approach has been implemented for a specific frequency using microwave ovens and chocolate bars.

SapientiaPT
jbriggs444 said:
According to Google, this general approach has been implemented for a specific frequency using microwave ovens and chocolate bars.

Except that in that case, it was done only for ONE particular frequency.

Zz.

jbriggs444 and SapientiaPT
Cheese slices are more effective than chocolate wrt area coverage.
But not as fun in the after-party unless nachos.

jedishrfu, berkeman, russ_watters and 1 other person
Maybe @DaveC426913 suffering from Broken Heart Syndrome:

LOL, okay folks, back on topic please...

SapientiaPT

## 1. What is the purpose of measuring the speed of light in an experiment?

The purpose of measuring the speed of light is to determine the exact speed at which electromagnetic radiation, in the form of light, travels through a vacuum. This has been a topic of scientific interest for centuries and has important implications in fields such as astronomy and physics.

## 2. How is the speed of light typically measured in an experiment?

In an experiment to measure the speed of light, a common method is to use a laser and a set of mirrors to bounce the light back and forth between two points. By measuring the time it takes the light to travel between the two points and using the distance between them, the speed of light can be calculated using the formula speed = distance/time.

## 3. Why is it important to use a vacuum in the experiment to measure the speed of light?

A vacuum is necessary in the experiment because light travels at different speeds through different mediums. In a vacuum, there is no air or other particles to slow down the light, allowing for a more accurate measurement of its speed.

## 4. What is the current accepted value for the speed of light?

The current accepted value for the speed of light in a vacuum is 299,792,458 meters per second (m/s). This value is based on the most recent scientific measurements and is considered a fundamental constant in physics.

## 5. How has the measurement of the speed of light changed over time?

The measurement of the speed of light has become increasingly accurate over time. In the 17th century, Galileo attempted to measure the speed of light by timing the delay between lantern flashes, but his results were inaccurate. In the 19th century, experiments using rotating mirrors and prisms led to more precise measurements. Today, modern technology has allowed for even more accurate measurements, leading to the current accepted value of the speed of light.