Interferensi Dede Djuhana Kuliah Fisika Dasar 2 Fakultas Teknik – Kelas FD2_06 Universitas Indonesia 2011 Department of Physics University of Indonesia
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Pendahuluan • Interferensi adalah superposisi dua gelombang at au lebih bertemu pada satu titik di dalam ruang d an menimbulkan fenomena fisis yang dapat diam ati. • Untuk menghasilkan interferensi dibutuhkan sum ber gelombang koheren dan beda fase tetap. • Koheren : berhubungan dengan dua gelombang ya ng mempunyai frekuensi sama – Koheren waktu (temporal coherence) – Koheren jarak (spatial coherence)
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Superposisi 2 gelombang frekuensi sama • Interferensi adalah gejala fisis dari perpaduan 2 gelombang
y1 = y01 sin (ωt − kx1 + ϕ1 )
y2 = y02 sin (ωt − kx2 + ϕ 2 ) y = y1 + y2 2 2 y02 = y01 + y02 + 2 y01 y02 cos δ
Amplitudo2 intensitas
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• menguatkan
• melemahkan
cos δ = −1 ⇒ δ = ±(2m + 1)π
cos δ = 1 ⇒ δ = ±2mπ
δ=
2π
λ
(x1 − x2 ) + (ϕ1 − ϕ 2 )
Beda lintasan Department of Physics University of Indonesia
Beda fase awal
Young’s Double-Slit Experiment
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Department of Physics University of Indonesia
Beda fase
• Syarat terang
• Syarat gelap
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• Posisi pita terang
• Posisi pita gelap
ybright =
ydark =
• Jarak 2 gelap/ 2 terang be rhimpitan
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λL
λL d
d
m
(m = 0, ± 1, ± 2,…)
(m + 12 ) (m = 0, ± 1, ± 2,…)
y=
λL d
Conditions for Interference • The sources must be coherent, that is, they must maintain a constant phase with respect to each other. • If two lightbulbs are placed side by side, no interference e ffects are observed.
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Change of Phase Due to Reflection • An electromagnetic wave undergoes a phase change of 1 80° upon reflection from a medium that has a higher inde x of refraction than the one in which the wave is traveling .
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Interference in Thin Films • Interference effects are commonly observed in thin films, such as thin layers of oil on water or the thin surface of a soap bubble. The varied colors observed when white light is incident on such films result from the interference of waves reflected from the two surfaces of the film.
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• Syarat gelap
2nt = mλ
(m = 0, 1, 2,…)
• Syarat terang
2nt = (m + 12 )λ
(m = 0, 1, 2,…)
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Nonreflective Coatings for Solar Cells • Syarat gelap
2nt = (m + 12 λ )
(m = 0, 1, 2,…)
• Tebal minimum film (m = 0)
t =
λ 4n
• n SiO Department of Physics University of Indonesia
baji • Posisi terang
( m + 12 )λ t= 2n
• Posisi gelap
mλ t= 2n
• Bagaimana dengan titik O (g elap/terang?)
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Newton’s Rings • Syarat terang
2nt = (m + 12 )λ
• Syarat gelap
2nt = mλ • Jari2 cincin newton
r=
(m + 12 )λR
r2 t= 2R Department of Physics University of Indonesia
2nt = (m + 12 )λ
The Michelson Interferometer • The interferometer, invented by the American physicist A. A. Miche lson (1852–1931), splits a light beam into two parts and then recombi nes the parts to form an interference pattern. • The device can be used to measure wavelengths or other lengths wit h great precision because a large and precisely measurable displace ment of one of the mirrors is related to an exactly countable number of wavelengths of light.
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holografi • Film 3 dimensi
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soal 1.
Pada percobaan young 2 celah berjarak 2 mm disinari campuran 2 gelombang cahaya 750 nm dan 900 nm. Ja rak layar ke celah 2 m. Pada jarak minimum dari pusat berapa akan didapat pertemuan kedua sinar.
2.
Sebuah antena radar diletakkan di atas tebing pada tep i sebuah pantai. Antena ini beroperasi pada panjang ge lombang 400 nm. Venus naik dari horison dan diamati antena. Jika sinyal minimum pertama dari venus dida pat ketika venus berada 35o dari horison, berapa tinggi tebing
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1.
2.
An air wedge is formed between two glass plates separated at one edge by a very fine wire, as shown in figure. When the wed ge is illuminated from above by 600-nm light and viewed from a bove, 30 dark fringes are observed. Calculate the radius of the wire. Two glass plates 10.0 cm long are in contact at one end and se parated at the other end by a thread 0.050 0 mm in diameter as shown in figure. Light containing the two wavelengths 400 nm a nd 600 nm is incident perpendicularly and viewed by reflection. At what distance from the contact point is the next dark fringe?
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