Notes from Toppers
Topic: Diffraction
1. Huygens’ Principle:
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NCERT Reference:
- Class 11 Physics: Chapter 10, “Wave Optics” (page no. 231-233)
- Class 12 Physics: Chapter 9, “Wave Optics” (page no. 192-194)
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Notes:
- Huygens’ principle states that every point on a wavefront can be considered as a source of secondary wavelets. These wavelets interfere with each other to form a new wavefront.
- The secondary wavelets spread out in all directions, but the wavefront advances only in the forward direction.
- Huygens’ principle can be used to explain the phenomena of reflection, refraction, and diffraction of waves.
2. Types of Diffraction:
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NCERT Reference:
- Class 11 Physics: Chapter 10, “Wave Optics” (page no. 239-243)
- Class 12 Physics: Chapter 9, “Wave Optics” (page no. 198-202)
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Notes:
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Diffraction due to a single slit:
- When a monochromatic light passes through a narrow slit, it spreads out and produces a diffraction pattern on a screen placed behind the slit.
- The pattern consists of a central bright band flanked by a series of alternating dark and bright bands.
- The width of the central maximum is directly proportional to the wavelength of light and inversely proportional to the slit width.
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Double-slit diffraction:
- When a monochromatic light passes through two narrow slits, it produces a diffraction pattern consisting of a series of alternating bright and dark fringes on a screen placed behind the slits.
- The positions of the bright fringes are determined by the path difference between the light waves from the two slits.
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3. Diffraction Grating:
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NCERT Reference:
- Class 12 Physics: Chapter 9, “Wave Optics” (page no. 205-207)
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Notes:
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A diffraction grating is a device consisting of a large number of parallel slits or grooves etched on a glass or metal surface.
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When light passes through a diffraction grating, it produces a series of bright spots on a screen placed behind the grating.
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The positions of the bright spots are determined by the grating equation:
d sin θ = n λ
where d is the grating spacing, θ is the angle between the incident light and the diffracted light, n is the order of the diffraction, and λ is the wavelength of light.
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4. Fraunhofer Diffraction:
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NCERT Reference:
- Class 12 Physics: Chapter 9, “Wave Optics” (page no. 207-210)
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Notes:
- Fraunhofer diffraction occurs when the light source and the observation screen are placed at a large distance from the diffracting aperture.
- Fraunhofer diffraction patterns are characterized by sharp and well-defined fringes.
- The pattern obtained from a circular aperture is called the Airy disk.
5. Diffraction by a Circular Aperture:
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NCERT Reference:
- Class 12 Physics: Chapter 9, “Wave Optics” (page no. 210-212)
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Notes:
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When a monochromatic light passes through a circular aperture, it produces a diffraction pattern consisting of a central bright spot surrounded by a series of concentric dark and bright rings.
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The radius of the first dark ring is given by:
r = 1.22 λ f/D
where λ is the wavelength of light, f is the focal length of the lens used to focus the light, and D is the diameter of the aperture.
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6. Diffraction by a Slit:
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NCERT Reference:
- Class 11 Physics: Chapter 10, “Wave Optics” (page no. 239-243)
- Class 12 Physics: Chapter 9, “Wave Optics” (page no. 198-202)
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Notes:
- When a monochromatic light passes through a narrow slit, it produces a diffraction pattern consisting of a central bright band flanked by a series of alternating dark and bright bands.
- The width of the central maximum is directly proportional to the wavelength of light and inversely proportional to the slit width.
7. Applications of Diffraction:
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NCERT Reference:
- Class 12 Physics: Chapter 9, “Wave Optics” (page no. 212-214)
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Notes:
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Diffraction has a wide range of applications, including:
- The design of optical instruments such as spectrometers, telescopes, and microscopes.
- The study of crystal structures using X-ray diffraction.
- The production of holograms.
- The measurement of the thickness of thin films.
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8. Fresnel Diffraction:
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NCERT Reference:
- Class 11 Physics: Chapter 10, “Wave Optics” (page no. 243-247)
- Class 12 Physics: Chapter 9, “Wave Optics” (page no. 202-205)
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Notes:
- Fresnel diffraction occurs when the light source and the observation screen are placed at a finite distance from the diffracting aperture.
- Fresnel diffraction patterns are characterized by smooth variations in intensity.
9. Phase Difference and Path Difference:
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NCERT Reference:
- Class 11 Physics: Chapter 10, “Wave Optics” (page no. 235-238)
- Class 12 Physics: Chapter 9, “Wave Optics” (page no. 195-198)
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Notes:
- Phase difference is the difference in the phase of two waves.
- Path difference is the difference in the distance traveled by two waves.
10. Numerical Techniques for Diffraction: - NCERT Reference:
- Class 12 Physics: Chapter 9, "Wave Optics" (page no. 214-216)
- Notes:
- Numerical techniques, such as the Fast Fourier Transform (FFT), are used to simulate and analyze diffraction patterns.