How can you measure the speed of light?

THERE IS AN EXPERIMENT THAT uses a disc that looks like a bicycle rear grear. then by rotating this disc having a beam of light strategically positioned and by knowin the frequence of the disc and the numbers of the grear you can derive the speed of light if i recall correctly.

this is how they measured it in the past as well. i am sure you will find something relevant in the net.

such as this that is fun :

http://www.physics.umd.edu/ripe/icpe/newsletters/n...

since electromagnetic energy travels at the speed of light!!!!!

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 ´ Wavelength

Velocity = 2450 MHz ´ 0.122 m

Velocity = 2.99 ´ 108 m/s

Build a telescope - an ordinary one will do.

Observe the satellites of Jupiter. Take one set of reading in January and other set in June-July. Now you should get an idea about the position of Jupiter and it's satellites at these times. This you can easily get from the free astronomy softwares available. Calculating the time difference and knowing the distance, you can calculate the speed of light. The time delay is only due to the time needed for light to reach earth.

It was originally done with mirrors and lasers.

I would set up the mirrors with two optical paths. One path would bounce the laser light back and forth until it has gone several miles in distance. I would set up a second set of mirrors with an optical path of only a few feet. I would set it up so that the two paths start and end adjacently. I would set up two lasers, one in front of each path and I would place a slotted spinning disk in front of the lasers so that as the disk spins, it allows an identically timed pulse of light from each laser to enter the two light paths.

I would set up two identical spinning disks at the exit of each light path, that spins at exactly the same rate as the initial disk. By tuning the spinning of the exit disks, I would find a setting that allows the full pulse of light to exit through the secondary disk. There should be no light reflected off of the back of the disk.

I would then measure the phase shift of the two exit disks. The speed of the spinning disks over the distance of the phase shift of the exit disks should give you the time delta of the exiting pulses. The distance delta of the light traveling different paths divided by the time delta should give you a fairly accurate calculation of the speed of light.

Radio waves travel at the speed of light. Consult with a ham radio operator who may be able to use his equipment to transmit a signal (Morse code key?) and receive an echo on an oscilloscope (with time line). The distance traveled will be the circumference of the earth's atmosphere (or less) and may be smeared with a peak. Perhaps he (she) could demonstrate this in class except transmission often depends on time of day, atmospheric conditions and use of an antenna that may not be readily portable. Also, ham operators are concerned with antennas being full or partial (half-wave?) lengths of the transmitted signal and they often know quite accurately the frequency of the signal using special tuned chips. The wavelength and frequency can be use to calculate the speed of the radio signal (speed of light!).

Google it

using a stop watch ofcourse.