Lecture Physics A2: Diffraction - PhD. Pham Tan Thi

Lecture Physics A2: Diffraction - PhD. Pham Tan Thi present the content properties of light, effects of materials on light, definition of diffraction, diffraction of light, diffraction and Hyugen’s principle, Fresnel’s diffraction,...
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Lecture Physics A2: Diffraction - PhD. Pham Tan Thi Diffraction Pham Tan Thi, Ph.D.Department of Biomedical Engineering Faculty of Applied ScienceHo Chi Minh University of Technology Properties of LightEffects of Materials on Light• Transmission• Reflection• Refraction• Absorption• Total Internal Reflection• Interference• Diffraction• Scattering of Light• Polarization Effects of Materials on LightMaterials can be classified based on how it responds to light incidenton them:1. Opaque materials - absorb light; do not let light to pass through2. Transparent materials - allow light to easily pass through them3. Translucent materials - allow light to pass through but distort the light during the passage Definition of DiffractionDiffraction is a bending of light around the edges/corners of anobstacle and subsequently spreading out in the region of geometricalshadow of an obstacle. Diffraction of LightWhen a narrow opaque (aperture) is placed between a source oflight and a screen, light bends around the corners of the aperture.This encroachment of light is called “diffraction”.For diffraction, the size of the aperture is small (comparable to thewavelength).As a result of diffraction, the edges of the shadow (or illuminatedregion) are not sharp, but the intensity is distributed in a certainway depending on the nature of the aperture. Difference between Interference and DiffractionInterference: occurs between waves starting from two (or more) butfinite numbers of coherent sources.Diffraction: occurs between secondary wavelets starting from thedifferent points (infinite numbers) of the same waves.Both are superposition effects and often both are presentsimultaneously (e.g. Young’s double slit experiment).Comparison:(a) In an interference pattern, the minima are usually almost perfectly dark while in a diffraction pattern they are not so.(b) In an interference pattern, all the maxima are of same intensity but not in the diffraction pattern.(c) The interference fringes are usually equally spaced. The diffraction fringes are never equally spaced. Diffraction and Hyugen’s PrincipleHyugen’s principle can be used to analyze the diffraction Diffraction pattern of a razor blade What is Huygens’ PrincipleHyugens’ (or Huygens-Fresnel) principle states that every point on awavefront is a source of wavelet. These wavelets spread out in theforward direction, at the same speed as the source wave. The newwaveforms is in line tangential to all the wavelets.Diffraction of Light No diffraction; No spreading after passing through slit Weak diffraction; Weak spreading after passing through slit Diffraction• In Figure 36.3 below, the prediction of geometric optics in (a) does not occur. Instead, a diffraction pattern is produced, as in (b).• The narrower the slit, the broader the diffraction pattern. Types of DiffractionDiffraction phenomena can be classified either as Fresneldiffraction or Fraunhofer diffractionThe observable difference:Fresnel diffractionThe viewing screen and the aperture are located close together, theimage of the aperture is clearly recognizable despite slight fringingaround its periphery.As the separation between the screen and the aperture increases,the image of the aperture becomes increasingly more structured;fringes become more prominent.Fraunhofer diffractionThe viewing screen and the aperture separated by a large distance,the projected pattern bears little or no resemblance to the aperture.As the separation increases, the size of the pattern changes but notits shape.Types of Diffraction Fresnel’s DiffractionIn the case of Fresnel’s diffraction, the source of light or screen orusually both are at finite distance from the diffracting aperture(obstacle)No lenses are usedThe incident wavefront is either spherical or cylindrical Fraunhofer’s DiffractionIn the case of Fraunhofer’s diffraction, the source of light or screenare effectively at infinite distance from the diffracting aperture(obstacle).This is achieved by placing the source and screen in the focalplanes of two lenses (require lenses).The incident wavefront is plane.Difference between Fraunhofer and Fresnel Diffraction No Fraunhofer Diffraction Fresnel Diffraction Source and screen are at infinite Source and screen are at finite 1 distances from slits distances from slits Incident wavefront on the aperture is Incident wavefront on the aperture is 2 plane either spherical or cylindrical The diffracted wavefront is ei ...