# Measuring the Speed of Light

• hy23
In summary, the conversation discusses the possibility of measuring the speed of light using simple apparatuses like mirrors and low powered motors and lasers. One method proposed by Fizeau involves using a rotating toothed wheel and a mirror placed 8km away to test how fast the wheel needs to spin for the returning beam to pass through the next two teeth. However, this experiment has complications such as the need for perfect alignment and detecting the returning beam. An alternative method suggested is to align the light source at an angle, which would result in distinct entry and exit points for the beams.

#### hy23

If we were to measure the speed of light as the scientists in the 1800s did (such as Foucault and Fizeau), without knowing the relationship between speed of light and its wavelength and frequency, would it be possible to do so using only simple apparatuses like mirrors and low powered motors and lasers that one can find around the house or buy at a nominal price?

By the way, Fizeau's method involved having a light beam from a light source go between the teeth of a rotating toothed wheel, the beam hits a mirror some 8km away, comes back and goes between the same teeth of the rotating toothed wheel. Then he can test how much he has to speed up the wheel so that the returning beam gets blocked by the teeth, and how much more he has to speed up the wheel so that the returning beam goes between the next two teeth (the slit beside the slit it passed thru initially).

The problem with this experiment is the alignment of the mirror has to be perfectly perpendicular to the light beam, the motor has to spin really fast, possibly at 10000 rpm, and how can one detect the returning beam (it seems to me that the returning beam will just collide with the incident beam).

Is there a possible alternative to this type of experiment that has less complications?

Well so far as that experiment goes, you could align the light source at an angle. The beam would go through the first slit and travel to a mirror which it would hit at just off normal and return to the device.

This angle would mean that both beams are separate.

If the wheel wasn't spinning, you would observe the light leave one tooth and it's return path would mean it comes back through the tooth next to it. Giving you entry and exit points that are distinct.

## 1. How is the speed of light measured?

The speed of light is measured using a variety of methods, including the use of specialized instruments such as lasers and mirrors. One common method is the "time of flight" technique, where the time it takes for a beam of light to travel a known distance is measured and used to calculate the speed of light.

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

The currently accepted value for the speed of light is 299,792,458 meters per second. This value is based on extensive research and experimentation by scientists and is considered to be a fundamental constant in physics.

## 3. How was the speed of light first measured?

The speed of light was first measured in the late 17th century by Danish astronomer Ole Rømer. Rømer observed discrepancies in the timing of eclipses of Jupiter's moons and realized that these discrepancies were due to the varying speed of light as Earth moved closer to or further from Jupiter in its orbit.

## 4. Does the speed of light ever change?

According to our current understanding of physics, the speed of light is a constant and does not change. It is considered to be one of the fundamental constants in the universe and is a key component of Einstein's theory of relativity.

## 5. Why is measuring the speed of light important?

The speed of light plays a crucial role in our understanding of the universe and is used in a wide range of scientific fields, including astronomy, physics, and telecommunications. It is also important for developing technologies and has practical applications in industries such as telecommunications and navigation.