DifraksiCahaya. Dede Djuhana Kuliah Fisika Dasar 2 Fakultas Teknik Kelas FD2_06 Universitas Indonesia 2011

Transkripsi

1 DifraksiCahaya Dede Djuhana Kuliah Fisika Dasar Fakultas Teknik Kelas FD_06 Universitas Indonesia 011 Department of Physics University of Indonesia 1

2 Pendahuluan Difraksi adalah peristiwa pembelokan arah ram bat gelombang yang melalui suatu celah sempit. Timbul pola terang dan gelap dimana intensitas polaterangtidaksamaataumakinjauhmakinke cil intensitasnya Department of Physics University of Indonesia

3 Macam-macamDifraksi Difraksi Franhofer Cahaya jauh dari celah sehingga cahaya yang masuk sejajar Celahsempit lebarcelah<< jarak celah-layar Difraksi Fresnel Jarak sumber cahaya ke celah dekat Berkas cahaya tidak perlu sejajar Lebar celah besar Celah tunggal Lubang bulat(circular aperture) Dua celah Kisi (celah banyak) berbentuk lingkaran Lubang berbentuk persegi Penghalang berbentuk piringan Penghalang berbentuk tajam(lancip Department of Physics University of Indonesia 3

4 DifraksiFraunhoferCelahTunggal θ = sin gelap m a Department of Physics University of Indonesia 4

5 IntensitasCelahTunggal Fraunhofer Celah tunggal diasumsikan terdir i dari daerah kecil dengan lebar Δy β Perubahan medan (Listrik) yang berhubungan y π β = ysinθ a = N y π π π β = N β = N ysinθ = asinθ π asinθ = gelap Department of Physics University of Indonesia 5

6 E R β sin = = ER / R β E0 β β = = 0 n β ( / ) Rsin sin E si / sin( β / ) ( π asin θ / ) I = Imax = I max sin = β / π asin θ / E R =Amplitudogelomb ang(medanlistrik) π asin θ sin gelap I ( θ 0 ) max π = m θ = m a = Terang pusat minimum Department of Physics University of Indonesia 6

7 KurvaIntensitasDifraksi sin( β / ) ( π asin θ / ) I = Imax = I max sin = β / π asin θ / Department of Physics University of Indonesia 7

8 Difraksicelahganda(double slit) I π d sin θ ( π asin θ / ) = Imax cos sin π asin θ / d sinθ m d asinθ = a = d=jarak antar celah a=lebar celah m Department of Physics University of Indonesia 8

9 Resolusicelahtunggal Kriteria Rayleigh Department of Physics University of Indonesia 9

10 Resolusi slit berbentuk lingkaran(circular) θ =1, a Department of Physics University of Indonesia 10

11 Resolusimata(eye resolution) Let us choose a wavelength of 500 nm, near the center of the visible spe ctrum. Although pupil diameter varies from person to person, we esti mate a daytime diameter of mm. We use Department of Physics University of Indonesia

12 Kisi Difraksi Side view of a diffraction grating. The slit separation is d, and the path difference be tween adjacent slits is d sin θ. Department of Physics University of Indonesia 1

13 Diagram of a diffraction grating spectrometer. The collimated beam inci dent on the grating is spread into its various wavelength components w ith constructive interference for a particular wavelength occurring at th e angles! bright that satisfy the equation d sin θ bright = m, where m = 0, 1,,.... Department of Physics University of Indonesia

14 Dayapisahpadakisi(Resolving power of t he diffraction grating) The diffraction grating is useful for measuring wavelengths accuratel y. Like the prism, the diffraction grating can be used to separate white light into its wavelength components. Of the two devices, a grating wi th very small slit separation is more precise if one wants to distinguis h two closely spaced wavelengths. For two nearly equal wavelengths 1 and between which a diffracti on grating can just barely distinguish, the resolving power Rof the gr ating is defined as = = + R = 1, = 1 1 Thus, a grating that has a high resolving power can distinguish small differences in wavelength. If N slits of the grating are illuminated, it c an be shown that the resolving power in the mth-orderdiffraction is R = Nm Department of Physics University of Indonesia

15 DifraksipadakristaldenganX-ray Tungsten (bcc) d sin θ = m Hukum Bragg Department of Physics University of Indonesia

16 a (a) A Laue pattern of a single crystal of the mineral beryl (berylliu m aluminum silicate). Each dot represents a point of constructive i nterference. (b) A Laue pattern of the enzyme Rubisco, produced with a wide-band x-ray spectrum. This enzyme is present in plant s and takes part in the process of photosynthesis. The Laue patter n is used to determine the crystal structure of Rubisco. b Department of Physics University of Indonesia