3.2 - Refraction, Diffraction and Interference

• Define coherence


  Coherent waves have a fixed phase difference and the same frequency and wavelength
• Why is a laser useful in showing interference and diffraction?

  It produces monochromatic (same wavelength/colour) light so diffraction and interference patterns are more defined
• What was Young's Double Slit experiment?

  A single light source is directed towards two slits, which each act as a coherent light source, the light interferes constructively and destructively to create an interference pattern.
• Describe the interference pattern created using white light


  A bright white central maximum flanked by alternating spectral fringes of decreasing intensity with violet closest to zero and red the furthest

  
• Why does an interference pattern form when light is passed through a single slit?

  The light diffracts as it passes through the slit, where the waves are in phase constructive interference occurs making bright fringes and where the waves are completely
• Increasing the slit width increases the width of the central diffraction maximum. True or False?

  False - The slit is not so close to the wavelength in size s less diffraction occurs - the central maximum becomes narrower and more intense
• What is the approximate refractive index of air?

  1
• When light enters a more optically dense medium, does it bend towards or away from the normal?

  Towards the normal
• When does total internal reflection occur?


  When light is at a boundary to a less optically dense medium and then angle of incidence is greater than the critical angle.
• What is the purpose of the cladding in a step index optical fibre?

  - Protects core from scratches which would allow light to escape and degrade the signal

  - Allows Total Internal Reflection as it has a lower refractive index than the core
• How does signal degradation by absorption in an optical fibre affect the received signal?

  Part of the signal's energy is absorbed by the fibre so its amplitude is reduced
• What is pulse broadening?

  When the received signal is wider than the original, this can cause overlap of signals leading to information loss.
• How does modal dispersion cause pulse broadening?

  Light rays enter the fibre at different angles so they take different paths along it, some may travel down the middle while others are reflected repeatedly, so the rays take different times to travel along the fibre, causing pulse broadening.
• What is material dispersion?

  When light with different wavelengths is used some wavelengths slow down more than others in the fibre so they arrive at different times causing pulse broadening
• How can modal dispersion be reduced?

  Use a single mode fibre (very narrow fibre) so the possible difference in path lengths is smaller.
• How can material dispersion be reduced?

  Use monochromatic light
• How can both absorption and dispersion be reduced?

  Use an optical fibre repeater to regenerate the signal now and then
• State the advantages of optical fibres over traditional copper wires

  - Signal can carry more information as the light has a higher frequency

  - No energy lost as heat

  - No electrical interference

  - Cheaper

  - Very fast
• What path does a light ray take when the angle of incidence is equal to the critical angle?

  It goes along the boundary
• What formula is used to determine the refractive index of a material?


  n = c / v

  n = refractive index

  c = speed of light in a vacuum

  v = speed of light in the material
• State 2 applications of diffraction gratings

  Splitting up light from stars to make line absorption spectra- used to identify elements present in the star.

  X-ray crystallography, a crystal sheet acts as the diffraction grating the X-rays pass through, used to find the spacing between atoms.
• What is diffraction?


  The spreading out of waves when they pass through or around a gap
• How did Young's double slit experiment provide evidence for the wave nature of light?

  Diffraction and interference are wave properties hence the interference pattern of light shows light has wave properties
• What are 4 safety precautions that must be followed when using a laser?

  - Wear laser safety goggles

  - Don't shine the laser at reflective surfaces

  - Display a warning sign

  - Never shine the laser at a person
• What formula is associated with Young's Double Slit experiment?

  w = (lambda * D) / s

  w = fringe spacing

  lambda = wavelength of light used

  D = Distance from screen to slits

  s = slit separation
• What is path difference?


  The difference in distance travelled by the two waves
• How could you investigate stationary sound waves?

  Place a speaker at one end of a closed glass tube, lay powder across the bottom of the tube, it will be shaken from the antinodes and settle at the nodes. The distance between each node is half a wavelength.
• What is phase?


  The position of a certain point on a wave cycle, (units are radians, degrees or fractions of a cycle)