(3.2) Refraction, diffraction and interference

• What is the difference in the distance travelled by two waves called?

  Path difference
• What property of a light source makes it coherent?

  Same frequency and wavelength with a fixed phase difference
• What type of light source is usually used in diffraction experiments?

  Lasers
• How do you create an interference pattern in Young's double slit experiment if you don't have a coherent light source?

  Use a single slit before the double slit to make the light have a fixed path difference, and a filter to make the light monochromatic
• Where do light fringes form in Young's double slit experiment?

  Where the path difference between waves is a whole number of wavelengths

  
• Where do dark fringes form in Young's double slit experiment?

  Where the path difference between waves is a whole number and a half wavelengths

  
• What is the formula for the fringe spacing (w) in Young's double slit experiment?

  w = (λ*D)/s
• What is the effect of using white light instead of monochromatic light in Young's double slit experiment?

  Wider maxima, less intense diffraction pattern, and a central white fringe with alternating bright spectra fringes

  
• What safety precautions must be followed when using lasers?

  Wear laser safety goggles, don't shine the laser at reflective surfaces, display a warning sign, and never shine the laser at a person
• How can interference of sound waves be demonstrated?

  Use two speakers connected to the same signal generator instead of a double slit, and measure the intensity with a microphone to find maxima and minima
• What evidence did Young's double slit experiment provide for the wave nature of light?

  It showed that light exhibits diffraction and interference, which are wave properties
• What is diffraction?

  The spreading out of waves when they pass through or around a gap
• When does the greatest diffraction occur?

  When the gap is the same size as the wavelength
• What is the diffraction pattern for monochromatic light through a single slit?

  A bright central fringe double the width of the other fringes, with alternating dark and bright fringes on either side

  
• What is the diffraction pattern for white light through a single slit?

  A central white maximum with alternating bright spectra fringes, with violet closest to the central maximum and red furthest away

  
• How does varying the slit width and wavelength affect the diffraction pattern?

  Increasing slit width decreases diffraction, making the central maximum narrower and more intense. Increasing wavelength increases diffraction, making the central maximum wider and less intense.
• What is a diffraction grating?

  A slide containing many equally spaced slits very close together

  
• How does the interference pattern from a diffraction grating compare to a double slit?

  The interference pattern is much sharper and brighter with a diffraction grating because there are many more rays of light reinforcing the pattern

  
• What is the formula for a diffraction grating?

  $\sin\theta =$$\frac{n\lambda}{d}$
• What is the refractive index of a material?

  The ratio of the speed of light in a vacuum to the speed of light in that material
• What is the refractive index of air approximately?

  1
• What is Snell's law?

  $\frac{\sin\theta_1}{n_1} =$$\frac{\sin\theta_2}{n_2}$
• What happens to the speed and direction of light when it crosses the boundary between two materials with different refractive indices?

  The speed of light changes, causing the direction of the light to bend either towards or away from the normal depending on the relative refractive indices
• What is the critical angle?

  The angle of incidence at which the angle of refraction is exactly 90 degrees and the light is refracted along the boundary
• What is the formula for the critical angle?

  $\sin\theta_c =$$\frac{n_2}{n_1}$
• What is total internal reflection?

  When the angle of incidence is greater than the critical angle and the incident refractive index is greater than the refractive index of the material at the boundary
• What are the main components of an optical fiber?

  An optically dense core surrounded by cladding with a lower optical density
• How do optical fibers use total internal reflection to transmit information?

  The optically dense core allows total internal reflection to occur, trapping the light signals inside and preventing signal degradation from light escaping
• What are the main causes of signal degradation in optical fibers?

  Absorption and dispersion (modal and material)
• How can signal degradation be reduced in optical fibers?

  Absorption can be reduced by using an optical fiber repeater, modal dispersion can be reduced by using a narrow core, and material dispersion can be prevented by using monochromatic light
• What are the key properties of a coherent light source?

  • Same frequency and wavelength
  • Fixed phase difference
• What are the steps of Young's double slit experiment?

  1. Shine a coherent light source through 2 slits about the same size as the wavelength
  2. Each slit acts as a coherent point source making a pattern of light and dark fringes
  3. Light fringes form where the path difference is a whole number of wavelengths
  4. Dark fringes form where the path difference is a whole number and a half wavelengths

  
• How does the diffraction pattern differ when using white light vs. monochromatic light in a single slit experiment?

  With white light:

  • Wider maxima
  • Less intense diffraction pattern
  • Central white fringe with alternating bright spectra fringes (violet closest, red furthest)

  
  

  With monochromatic light:

  • Bright central fringe double the width of other fringes
  • Alternating dark and bright fringes on either side

  
• How does the interference pattern differ between a double slit and a diffraction grating?

  Diffraction grating:

  • Interference pattern is much sharper and brighter
  • More rays of light reinforcing the pattern
  • Allows for more accurate measurements of slit widths

  
• How does varying slit width and wavelength affect the diffraction pattern?

  • Increasing slit width decreases diffraction, making the central maximum narrower and more intense
  • Increasing wavelength increases diffraction, making the central maximum wider and less intense

  
• What are the key components and working principle of an optical fiber?

  Components:

  • Optically dense core
  • Cladding with lower optical density

  
  

  Principle:

  • Total internal reflection traps light signals inside the core, preventing signal degradation