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The Nature of Light

The Nature of Light. Overview 1. Historical Background 2. Light & Spectroscopy. Feb. 13, 2006. 課程討論過程所產生的一些新觀念或重要觀念,我將以中文註解之!. 1. Historical Background. ~. Early studies of optics came from the Greeks: Middle ages: Light as an emanation from the eye  external entity 17 th century:

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The Nature of Light

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  1. The Nature of Light Overview 1. Historical Background 2. Light & Spectroscopy Feb. 13, 2006 課程討論過程所產生的一些新觀念或重要觀念,我將以中文註解之!

  2. 1. Historical Background ~ • Early studies of optics came from the Greeks: • Middle ages: • Light as an emanation from the eye  external entity • 17th century: • Q: The nature of light? A: Something that moves. ● Emitted from the eye Return to the eye

  3. 1687: • The corpuscular (微粒子的) theory of light became widely accepted • Newton published his Principia • He talked of “rays of light” rather than “particles of light” • Mid 1700s: • The corpuscular theory of light was well-entrenched (根深蒂固的) • Also attacked by people like Euler & Young • Why two beams intersected do not affect each other? • Why the wastage of light was not observed? • Early 19th century: • The double-slit exps of Young • 19th century: • The principle of interference (wave theory) was placed on a more mathematical footing by Fresnel (菲涅耳) • Increasing doubt about the corpuscular theory of light

  4. 補充:這些實驗證明『光在液體中的速度較在空氣或真空中來得小』,補充:這些實驗證明『光在液體中的速度較在空氣或真空中來得小』, 然而,這些結果卻正好和粒子學說的預測相反! • Exps performed in 1849 by Fizeau & in 1862 by Foucault • Measuring the velocity of light in different media • The support of the corpuscular theory was ended • 1873: • Maxwell advanced the theory of light as electromagnetic waves • 1888: • Hertzconfirmed that electromagnetic waves exhibited (1) reflection, (2) refraction, (3) diffraction, (4) interference, and (5) polarization • The wave theory of light was on solid footing 好奇:為何當初 Maxwell 會認為光同時帶有電場和磁場?

  5. Early 20th century: • Some exps (e.g., photoelectric effect) could not be explained in terms of light as a continuous wave • A dilemma (進退兩難的局面) for the theory of light • A • Wave-particle duality Wave-like Particle-like characteristics depend on “the nature of exps”

  6. 2. Light & Spectroscopy Wave-like Particle-like characteristics depend on “the nature of exps” • A E.g. • The corpuscular model: Shadows cast by macroscopic objects • The wave theory of light: Interaction of light with small objects such as slits 類比: 光學 光在寬廣的空間行進,僅有少部分光線與樹幹表皮發生作用,因而所形成的樹影無明顯繞射現象  粒子行為重要 光通過一極窄小的狹縫,因通過的光線極少且其電場和狹縫物質發生作用,所以電場明顯被改變,進而形生清析的 繞射紋路  波動行為重要 流變學 高分子溶液於一般大小管徑中進行輸送,除非在極高 shear rate 下,否則速度梯度難以影響個別高分子鏈行為  連續體力學即可 數條高分子鏈於一奈米大小流道中輸送,在此情況下極易產生高 shear rate,因此,流場可輕易影響高分子的動態, 甚至引發高分子的非線性黏彈性行為  需要分子動態學 ● ●

  7. A wave model of light would work well in describing the spectroscopic interactions of light with matter even though the states of atoms and molecules themselves must be described quantum-mechanically • For the absorber • Electric field is constant in spatial distribution ( wave curvature is negligible)

  8. Polarization Properties of Light Overview 1. The Wave Model & Nomenclature 2. Linearly, Circularly, or Elliptically Polarized Light 3. A Pictorial Presentation of Polarization Feb. 13, 2006

  9. 1. The Wave Model & Nomenclature • The amplitude of the electric field Max. amplitude A constant specifying its absolute phase Electric field Magnetic field Fig. The spatial dependence of waves at some point in time

  10. 2. Linearly, Circularly, or Elliptically Polarized Light • (a) Linearly Polarized Light Oriented along the x axis Oriented along the y axis Orientation of 45o from the x axis Orientation of 30ofrom the x axis Fig. Presentation of linearlypolarized with various orientations

  11. (right) • (b) Circularly Polarized Light Orthogonal components with equal amplitudes & 90o relative phase shift Clockwise Fig. Presentation of right circularlypolarized with various orientations

  12. (left) • (c) Elliptically Polarized Light Fig. Presentation of left ellipticallypolarized with various orientations

  13. 3. A Pictorial Representation of Polarization Linearly polarized light Circularly Elliptically Fig. Pictorial representations of polarized light

  14. The polarization ellipse (橢圓) linearly (measured counterclockwise from the x axis) circularly

  15. Orthogonal & equal phase Equal amplitudes & phase Orthogonal & out of phase determining ellipticity Unequal amplitude & phase determining azimuth Fig. Interconversion of different polarization forms using the pictorial representation of light

  16. Fig. Retarding the phase of one of the two circular components results in rotation of the axis of the ellipse

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