diffraction

Question 1

fraunhofer vs fresnel

Answer 1

fraunhofer

• is known as far field diffraction.
• plane wave incident on aperture.
• distance of aperture to screen is long.
• change in aperture distance to screen is not sensitive. (no major change in diffraction pattern)

fresnel

• is known as near field diffraction
• converging/diverging wave incident on aperture.
• distance of aperture to screen is short.
• change in aperture distance to screen is sensitive. (significant change to diffraction pattern)

Question 2

what is Rayleigh’s criterion?

Answer 2

• two point resolution limit occurs when first minimum (boundary of Airy’s disk) of one diffraction pattern coincides w centre maximum of other diffraction pattern.
• when pattern overlap more, image points are not resolvable
• when pattern overlap less, image points are resolvable

Question 3

what is Huygen’s principle?

Answer 3

all points on a wavefront can be considered as point sources for the production of spherical secondary wavelet, and at any later time the new wavefront position is the new envelope to these secondary wavelets.

Question 4

how does aperture size affect diffraction and aberration?

Answer 4

smaller aperture size
- more diffraction
- less aberration
larger aperture size
- less diffraction 
- more aberration

Question 5

what is the diffraction limit on resolution?

Answer 5

when aperture is large
- little diffraction 
- Airy’s disk is small
- two illuminated pattern are close to points (easy to resolve the two images)
as aperture decreases in size 
- increase diffraction 
- Airy’s disk becomes bigger
- two Airy’s disk starts to overlap (the two images are not resolvable)

Question 6

diffraction haloes

Answer 6

• random array of many circular apertures/obstacles illuminated by plane waves from a white point source
  > each aperture generate a Airy type diffraction pattern
• when aperture are small and close together
  > diffraction patterns are large & overlap & produces a readily visible halo
  *white disk surrounded circular coloured rings (red outermost, blue/violet innermost)
• e.g. fogged up car window & corneal swelling

Question 7

diffraction & acuity

Answer 7

• for large pupil sizes, human eye does not form perfect point image due to aberration
• for smaller pupil sizes(<=2.4mm), human eye appears to be a diffraction-limited system
• pinhole used clinically to assess if poor vision is due to optical causes(blur)/pathological causes(eye disease)
  > poor acuity due to optical causes, pinhole effect can minimise blue and increase acuity
  > poor acuity is not due to optical causes, pinhole will not give improvement in acuity

Question 8

what is diffraction?

Answer 8

• ability of waves to propagate around corners
• diffraction depends on size of obstacle/aperture relative to size of wavelength
• diffraction is more pronounced when the obstacle/aperture size is similar to that of the wavelength

Question 9

what is single slit diffraction?

Answer 9

• several secondary wavelets pass through slit.
• superpose to give rippled irradiance distribution resulting in alternating maximum/minimum illumination due to constructive & destructive interference
• diffraction pattern will be brightest in the middle & the intensity will get lesser towards the periphery.
• for given slit width, spread in pattern depends on wavelength(directly proportional in smaller angles)
• long wavelength(red) diffract more than short wavelength(blue/violet)
• for white light illuminating on a single slit, each wavelength will be diffracted independently, resulting in white center surrounded by colour fringes with red at the outermost ring.

Question 10

what is circular aperture diffraction?

Answer 10

• when circular aperture is illuminated by monochromatic plane wave, it diffracts equally in all directions
• results in central circular maximum surrounded by first minimum & by series of progressively fainter light rings
• Airy’s disk is the bright central circular maximum bounded by the first minimum
• boundary for Airy’s disk is the angular location of the first mimimum
• the smaller the aperture, the larger the diffraction
• the longer the wavelength, the larger the diffraction