GELOMBANG ELEKTROMAGNETIK

Definisi Gelombang Elektromagnetik

Gelombang elektromagnetik adalah gelombang yang dapat merambat tanpa memerlukan medium danmerupakan gelombang transversal. Namun gelombang elektromagnetik merupakan gelombang medan, bukan gelombang mekanik (materi). Pada gelombang elektromagnetik, medan listrik E selalu tegak lurus arah medan magnetik B dan keduanya tegak lurus arah rambat gelombang. Gangguan gelombang elektromagnetik terjadi karena medan listik dan medan magnet, oleh karena itu gelombang elektromagnetik dapat merambat dalam ruang vakum. Gelombang elektromagnetik berasal dari matahari dan angkasa; peralatan elektronik, pemancar radio/TV, satelit, monitor TV, komputer, kilat, bahan radioaktif, alat Rontgen, bara api dan blok mesin yang panas.

Secara umum dapat dikatakan gelombang elektromagnetik muncul dari partikel bermuatan yang dipercepat (bergetar, perputar, diperlambat dan dipercepat).  Energi elektromagnetik merambat dalam gelombang dengan beberapa karakter yang bisa diukur, yaitu:panjang gelombang/wavelength, frekuensi, amplitude/amplitude dan kecepatan. Amplitudo adalah tinggi gelombang, sedangkan panjang gelombang adalah jarak antara dua puncak. Frekuensi adalah jumlah gelombang yang melalui suatu titik dalam satu satuan waktu. Frekuensi tergantung dari kecepatan merambatnya gelombang. Karena kecepatan energi elektromagnetik adalah konstan (kecepatan cahaya), panjang gelombang dan frekuensi berbanding terbalik. Semakin panjang suatu gelombang, semakin rendah frekuensinya, dan semakin pendek suatu gelombang semakin tinggi frekuensinya. Energi elektromagnetik dipancarkan, atau dilepaskan, oleh semua masa di alam semesta pada level yang berbeda-beda. Semakin tinggi level energi dalam suatu sumber energi, semakin rendah panjang gelombang dari energi yang dihasilkan, dan semakin tinggi frekuensinya. Perbedaan karakteristik energi gelombang digunakan untuk mengelompokkan energi elektromagnetik.

Teori Tentang Gelombang Elektromagnetik

Sebelum mempelajari lebih lanjut tentang gelombang elektromagnetik, kita perlu terlebih dahulu mengetahui apakah cahaya itu? Ini menjadi suatu pertanyaan yang besar bagi manusia beberapa abad yang lalu, sebelum teori persamaan Maxwel dikemukakan.  Prediksi awal tentang gelombang elektromagnetik dilakukan oleh James Clerk Maxwell (1831-1879). Maxwell membuat suatu teori yang menyatukan fenomena kelistrikan dan kemagnetan. Dalam mengungkapkan hipotesisnya Maxwell menggunakan kaidah tentang kemagnetan dan kelistrikan yang sebelumnya telah ada, kaidah-kaidah tersebut antara lain:

  1. Hukum Gauss yang menyatakan bahwa muatan listrik statis dapat menghasilkan medan listrik. 
  2. Hukum Bio-savart yang menyatakan bahwa aliran muatan listrik (arus listrik) dapat menghasilkan medan  magnet.
  3. Hukum Faraday yang menyatakan bahwa perubahan medan magnet dapat menghasilkan medan listrik.

Berdasarkan Hukum Faraday, Maxwell mengemukakan hipotesa yaitu perubahan medan listrik dapat menimbulkan medan magnet, selain itu Maxwell juga mengemukakan kesimpulan lain yaitu jika perubahan-perubahan medan magnet dapat menghasilkan medan listrik, maka medan listrik yang dihasilkan itu juga berubah-ubah. Perubahan medan listrik ini juga juga menghasilkan medan magnet yang berubah-ubah dan seterusnya. Dari pernyataan-pernyataan inilah Maxwell membuktikan hipotesisnya dengan menggunakan persamaan-persamaan yang disebut persamaan Maxwell. Dari hasil persamaannya, ia menemukan bahwa perubahan medan-medan yang saling berinteraksi dapat menghasilkan gelombang medan listrik dan medan magnet yang dapat merambat melalui ruang.  

Spektrum Gelombang Elektromagnetik

Susunan semua bentuk gelombang elektromagnetik berdasarkan panjang gelombang dan frekuensinya disebut spektrum elektromagnetik. Gambar spektrum elektromagnetik disusun berdasarkan panjang gelombang (diukur dalam satuan meter) yaitu mencakup kisaran:

  1. Energi yang sangat rendah, dengan panjang gelombang tinggi dan frekuensi rendah, seperti gelombang radio.
  2. Energi yang sangat tinggi, dengan panjang gelombang rendah dan frekuensi tinggi seperti sinar X dan sinar gamma

Spektrum gelombang elektromagnetik terdiri atas tujuh macam gelombang yang dibedakan berdasarkan frekuensi serta panjang gelombang tetapi cepat rambat di ruang hampa adalah sama, yaitu c =3 x 108 m/s. Seperti yang sudah dibahas dalam teori Maxwell tentang gelombang elektromagnetik. Frekuensi gelombang terkecil adalah gelombang cahaya serta panjang gelombang terbesar sedangkan frekuensi terbesar adalah sinar gamma serta panjang gelombang terpendek. Spektrum gelombang elektromagnetik terdiri dari urutan:

  • Gelombang radio dan televisi
  • Gelombang mikro
  • Infra merah
  • Cahaya tampak
  • Ultraviolet
  • Sinar X
  • Sinar gamma

Urutan dari atas ke bawah adalah frekuensi makin besar serta panjang gelombang makin pendek karena frekuensi dan panjang gelombang berbanding terbalik.

Demikianlah pembahasan tentang materi Gelombang Elektromagnetik.

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