Topic: Diffraction

1. Huygens’ Principle:

• NCERT Reference:

  • Class 11 Physics: Chapter 10, “Wave Optics” (page no. 231-233)
  • Class 12 Physics: Chapter 9, “Wave Optics” (page no. 192-194)

• 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:

• NCERT Reference:

  • Class 11 Physics: Chapter 10, “Wave Optics” (page no. 239-243)
  • Class 12 Physics: Chapter 9, “Wave Optics” (page no. 198-202)

• Notes:

  • 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.

  • 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.

3. Diffraction Grating:

• NCERT Reference:

  • Class 12 Physics: Chapter 9, “Wave Optics” (page no. 205-207)

• Notes:

  • A diffraction grating is a device consisting of a large number of parallel slits or grooves etched on a glass or metal surface.

  • When light passes through a diffraction grating, it produces a series of bright spots on a screen placed behind the grating.

  • 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.

4. Fraunhofer Diffraction:

• NCERT Reference:

  • Class 12 Physics: Chapter 9, “Wave Optics” (page no. 207-210)

• 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:

• NCERT Reference:

  • Class 12 Physics: Chapter 9, “Wave Optics” (page no. 210-212)

• Notes:

  • 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.

  • 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.

6. Diffraction by a Slit:

• NCERT Reference:

  • Class 11 Physics: Chapter 10, “Wave Optics” (page no. 239-243)
  • Class 12 Physics: Chapter 9, “Wave Optics” (page no. 198-202)

• 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:

• NCERT Reference:

  • Class 12 Physics: Chapter 9, “Wave Optics” (page no. 212-214)

• Notes:

  • 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.

8. Fresnel Diffraction:

• NCERT Reference:

  • Class 11 Physics: Chapter 10, “Wave Optics” (page no. 243-247)
  • Class 12 Physics: Chapter 9, “Wave Optics” (page no. 202-205)

• 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:

• NCERT Reference:

  • Class 11 Physics: Chapter 10, “Wave Optics” (page no. 235-238)
  • Class 12 Physics: Chapter 9, “Wave Optics” (page no. 195-198)

• 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.