# Speed of light

1. Nov 5, 2004

### heman

how to measure speed of light??

2. Nov 5, 2004

### Kurdt

Staff Emeritus
The first accurate measurement involved a rotating hexagonal disc and a beam of light was shone on the hexagonal mirror which reflected the beam to another mirror a long distance away. When the beam struck the second mirror it was reflected to an observer. The speed was then determined by knowing how far the light travelled in total and how fast the hexagonal mirror was rotating to make the light beam constant to the observer.

3. Nov 5, 2004

### bballgirlweez

http://www.what-is-the-speed-of-light.com/

Last edited by a moderator: Nov 5, 2004
4. Nov 6, 2004

### dyknowsore

My favorite speed of light measurment was earlier though. Consisted of two guys with lanterns on different hill tops. One of them would uncover his lantern and the other would uncover his as soon as he saw the first one. Their value for the speed of light? "Extremely fast"

(Wish I could remember who they were.)

5. Nov 6, 2004

### boaz

holy cow, i heard this story too. my physics teacher mentioned it, when i started to learn modern physics. it was a blast, half of the class couldn't stop laughing! :rofl:

6. Nov 6, 2004

### OldTee

Think it's the great Galileo! http://www.colorado.edu/physics/2000/waves_particles/lightspeed_evidence.html [Broken]

Last edited by a moderator: May 1, 2017
7. Nov 6, 2004

### cepheid

Staff Emeritus
Yeah, lol. Their intentions were good, but no matter how far away the hilltop on which Galileo's assitant was standing, the measured value for the speed of light did not change significantly, suggesting that the value was mostly error due to human reaction time. So IIRC their result was that the speed of light was either infinite or too fast for human reaction time to be measured. Either way, "extremely fast".

Some Danish physicist, Ole Roemer, first proved it was finite when he noticed that the perceived time for a moon of Jupiter to pass completely behind the disk of the planet would change depending on the time of year it was measured (i.e. at what point the Earth was in its orbit, and therefore how far it was from Jupiter).

8. Nov 7, 2004

### CeeAnne

Here's my favourite method:
Finding the Speed of Light with Marshmallows-A Take-Home Lab

Robert H. Stauffer, Jr., Cimarron-Memorial High School, Las Vegas, Nevada, USA

I have heard that at 16 years old, Albert Einstein constantly wondered what it would be like to ride on a beam of light. Students in physics always seem to be fascinated by the properties of light. However, speed-of-light demonstrations often require extensive preparation or expensive equipment. I have prepared a simple classroom demonstration that the students can also use as a take-home lab.

The activity requires a microwave oven, a microwave-safe casserole dish, a bag of marshmallows, and a ruler. (The oven must be of the type that has no mechanical motion-no turntable or rotating mirror. If there is a turn-table, remove it first.) First, open the marshmallows and place them in the casserole dish, completely covering it with a layer one marshmallow thick. Next, put the dish of marshmallows in the microwave and cook on low heat. Microwaves do not cook evenly and the marshmallows will begin to melt at the hottest spots in the microwave. (I leaned this from our Food Science teacher Anita Cornwall.) Heat the marshmallows until they begin to melt in four or five different spots. Remove the dish from the microwave and observe the melted spots. Take the ruler and measure the distance between the melted spots. You will find that one distance repeats over and over. This distance will correspond to half the wavelength of the microwave, about 6 cm. Now turn the oven around and look for a small sign that gives you the frequency of the microwave. Most commercial microwaves operate at 2450 MHz.

All you do now is multiply the frequency by the wavelength. The product is the speed of light.

Example:

Velocity = Frequency x Wavelength

Velocity = 2450 MHz x 0.122 m

Velocity = 2.99 x 10^8 m/s

This works in my physics class, often with less than 5% error. Then the students can eat the marshmallows.

(Reprinted with permission from The Physics Teacher, vol. 35, April 1997, p. 231. Copyright 1997 American Association of Physics Teachers )

9. Nov 8, 2004

### OldTee

That's really cool CeeAnne! It's amazing that we can measure the speed of light based on what we all have in our kitchen.

10. Nov 8, 2004

### DaveC426913

I have heard this technique, although it was done with processed cheese slices, which probably lay out better in a sheet than do marshmallows.

The only complaint I have about the experiment is that fact that the required piece of data - the frequency of the microwaves - has to be given. We have to take someone else's word for it. And how they know it, is because they already know speed of light!

11. Nov 8, 2004

### CeeAnne

It's a recursive universe, Dave ... d a r n .