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Bagaimana Para Ahli Dapat Menghitung Kecepatan Cahaya

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Im:
Berapa kecepatan cahaya? Cahaya melaju dengan kecepatan kurang lebih 300.000 km/detik dalam vakum, tepatnya 299.792.458 meter/detik. Sebagai gambaran, cahaya membutuhkan waktu hanya 0,14 detik untuk mengitari Bumi. Cepat sekali bukan!

Namun, dari mana angka ini didapatkan? Apakah kecepatan cahaya yang super cepat ini diukur seperti kita mengukur kecepatan orang berlari (mengukur jarak dengan meteran dan waktu tempuh dengan stopwatch)? Jika kita ingin mengukur kecepatan cahaya demikian, cahaya terlampau cepat. Stopwatch kita kurang presisi dan Bumi tempat tinggal kita tidak cukup luas.

Salah satu solusinya adalah mengamati bintang yang berada di luar angkasa. Posisi bintang berubah-ubah dalam satu tahunnya. Hal ini dikarenakan cahaya yang merambat dari bintang ke mata sedikit bergeser akibat revolusi Bumi terhadap Matahari. Dengan mengukur sudut pergeseran ini dan mengetahui kecepatan revolusi Bumi, kita dapat menghitung kecepatan cahaya.

Selain pengamatan benda angkasa, kecepatan cahaya dapat diukur dengan pengamatan di Bumi. Pengukuran cahaya ini pertama kali dilakukan oleh seorang Fisikawan Prancis, Fizeau. Ia mengukurnya dengan cara melewatkan cahaya melalui roda gigi yang berputar. Cahaya ini kemudian dipantulkan balik oleh cermin dan diamati.


Fizeau kemudian mempercepat putaran roda gigi. Suatu ketika perputaran roda begitu cepat, membuat cahaya yang kembali terhalangi oleh gigi roda. Cahaya harus melewati celah berikutnya agar dapat kembali dan teramati. Dengan mengetahui kecepatan roda gigi saat ini, jumlah gigi, serta jarak antara roda gigi dengan cermin, kecepatan cahaya dapat dihitung.

Kemajuan teknologi membuat kecepatan cahaya dapat diukur lebih akurat, misalnya dengan menggunakan laser dan prinsip interferensi.

Bahwa cahaya adalah gelombang? Saat dua atau lebih gelombang bertemu, gelombang akan berpadu. Perpaduan ini dapat memperbesar atau memperkecil gelombang. Pada cahaya, perpaduan dapat menghasilkan cahaya terang bahkan kegelapan! Terdengar aneh bukan? Cahaya dipadukan dengan cahaya, hasilnya malah kegelapan. Perpaduan ini disebut interferensi. Alat yang menggunakan prinsip ini untuk mengukur sesuatu disebut interferometer.

sloth:
Dulu saya pernah menghitung kecepatan cahaya, waktu praktikum dulu. metodenya beda dg disebutkan di atas.
Err, sebenernya bukan cahaya sih, tapi gelombang mikro. Tapi sama aja lah.
Jadi ada generator gelombang mikro (saya lupa frekuensinya, seingat saya sekitar 10 GHz). Dilewatkan ke waveguide. Di ujung waveguide itu ditempatkan pemantul. Kita ubah frekuensinya sedikit2, sampe terbentuk standing wave. Pake osiloskop kita ukur jarak dari puncak ke puncak gelombang. Kita dapat panjang gelombang. Kita tahu frekuensi, kita tahu panjang gelombang, kita bisa hitung kecepatannya.
Waktu itu saya excited sekali setelah mencocokan hasilnya dg kecepatan yg di textbook, hasilnya cukup mendekati (kalau tidak salah beda sekian kilometer per detik)

Muhammad Habib Annajar:
dari penurunan persamaan maxwell juga bisa didapatkan secara teoritis,
kalo dapat persamaannya giini
c = (miu0.epsilon0)^ -1/2
mendekati 3 x 10^8 m/s

The Houw Liong:
>   Motta   v



Michelson-Morley Experiment   


          
This entry contributed by Leonardo Motta

After the development of Maxwell's theory of electromagnetism, several experiments were performed to prove the existence of ether and its motion relative to the Earth. The most famous and successful was the one now known as the Michelson-Morley experiment, performed by Albert Michelson Eric Weisstein's World of Biography (1852-1931) and Edward Morley Eric Weisstein's World of Biography (1838-1923) in 1887.


Michelson and Morley built a Michelson interferometer, which essentially consists of a light source, a half-silvered glass plate, two mirrors, and a telescope. Eric Weisstein's World of Astronomy The mirrors are placed at right angles to each other and at equal distance from the glass plate, which is obliquely oriented at an angle of 45° relative to the two mirrors. In the original device, the mirrors were mounted on a rigid base that rotates freely on a basin filled with liquid mercury in order to reduce friction.

Prevailing theories held that ether formed an absolute reference frame with respect to which the rest of the universe Eric Weisstein's World of Astronomy was stationary. It would therefore follow that it should appear to be moving from the perspective of an observer on the sun-orbiting Earth. As a result, light would sometimes travel in the same direction of the ether, and others times in the opposite direction. Thus, the idea was to measure the speed of light in different directions in order to measure speed of the ether relative to Earth, thus establishing its existence.

Michelson and Morley were able to measure the speed of light by looking for interference fringes between the light which had passed through the two perpendicular arms of their apparatus. These would occur since the light would travel faster along an arm if oriented in the "same" direction as the ether was moving, and slower if oriented in the opposite direction. Since the two arms were perpendicular, the only way that light would travel at the same speed in both arms and therefore arrive simultaneous at the telescope would be if the instrument were motionless with respect to the ether. If not, the crests and troughs of the light waves in the two arms would arrive and interfere slightly out of synchronization, producing a diminution of intensity. (Of course, the same effect would be achieved if the arms of the interferometer were not of the same length, but these could be adjusted accurately by looking for the intensity peak as one arm was moved. Changing the orientation of the instrument should then show fringes.)

Although Michelson and Morley were expecting measuring different speeds of light in each direction, they found no discernible fringes indicating a different speed in any orientation or at any position of the Earth in its annual orbit around the Sun.

In 1895, Lorentz Eric Weisstein's World of Biography concluded that the "null" result obtained by Michelson and Morley was caused by a effect of contraction made by the ether on their apparatus and introduced the length contraction equation



where L is the contracted length,  is the rest length, v is the velocity of the frame of reference, and c is the speed of light. Although the main interpretation of Lorentz Eric Weisstein's World of Biography for this equation was rejected later, the equation is still correct and was the first of a sequence of new equations developed by Poincaré, Eric Weisstein's World of Biography Lorentz, Eric Weisstein's World of Biography and others, resulting in a new branch of physics ultimately brought to fruition by Albert Einstein Eric Weisstein's World of Biography in special relativity. Einstein's idea of space-time contraction replaced Lorentz's interpretation of the contraction equation, and once and for all relegated ether to the history books.

Ether, Michelson Interferometer, Speed of Light




References

Feynman, R. P.; Leighton, R. B.; and Sands, M. The Feynman Lectures on Physics, Vol. 1. Redwood City, CA: Addison-Wesley, pp. 15-3-15-4, 1989.

Fowler, M. "The Michelson-Morley Experiment." http://www.phys.virginia.edu/classes/109N/lectures/michelson.html.

Lorentz, H. A. "Michelson's Interference Experiment." In Lorentz, H. A.; Einstein, A.; Minkowski, H.; and Weyl, H. The Principle of Relativity: A Collection of Original Memoirs on the Special and General Theory of Relativity. New York: Dover, pp. 3-7, 1952. Reprinted form Lorentz, H. A. Versuch einer Theorie der elektrischen und optischen Erscheinungen in bewegten Körpern. Leiden, 1895.

Michelson, A. A. "The Relative Motion of the Earth and the Luminiferous Aether." Amer. J. Sci. 22, 120-129, 1881.

Michelson, A. A. and Morley, E. W. "On the Relative Motion of the Earth and the Luminiferous Ether." Amer. J. Sci. 34, 333-345, 1887.

Michelson, A. A. and Morley, E. W. "On the Relative Motion of the Earth and the Luminiferous Aether." Philos. Mag. 24, 449-463, 1887.



© 1996-2007 Eric W. Weisstein

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