Diffraction 1 (7-9) Flashcards
Diffraction is a change in ________NOT due to _________or________.
direction of propagation of light
reflection or refraction.
Snells law?
reflection or refraction.
Can we use Snell’s law to predict diffraction?
If rays travel in straight lines, then the shadow pattern should have sharp edges,
but they don’t! Blurring of the edges is a sign of diffraction at work.
Phenomenon of diffraction -
The deviation from rectilinear propagation
that occurs when light advances beyond an
obstruction
THE HUYGENS PRINCIPLE.
All points on a wavefront can be considered as point sources for the production of spherical secondary
wavelets, and at any later time the new wavefront position is the envelope (or surface of tangency) to these
secondary wavelets.”
Single-Slit Diffraction
Wavelength of source equal to or larger than width of the slit (λ > a), forms diffraction pattern.
Fresnel (Near field) Diffraction
In the Fresnel diffraction, the source of light and the
screen are at a finite distance from the diffraction
aperture.
Fraunhofer (Far-field) Diffraction
In the Frauhofer diffraction, the source of light and the
screen are at a infinite distance from the diffraction
aperture.
Fresnel Diffraction or Fraunhofer Diffraction?
‘Near-field diffraction’
Fresnel
Fresnel Diffraction or Fraunhofer Diffraction?
Finite distance (slit to screen)
Fresnel
Fresnel Diffraction or Fraunhofer Diffraction?
Plane wave
Fraunhofer
Fresnel Diffraction or Fraunhofer Diffraction?
‘Far-field diffraction’
Fraunhofer
Fresnel Diffraction or Fraunhofer Diffraction?
Difficult to observe & calculate
Fresnel
Fresnel Diffraction or Fraunhofer Diffraction?
Infinite distance (slit to screen)
Fraunhofer
Fresnel Diffraction or Fraunhofer Diffraction?
Converging or Diverging wave
Fresnel
Fresnel Diffraction or Fraunhofer Diffraction?
Diffraction patterns vary with distance
Slit to screen
Fresnel
Fresnel Diffraction or Fraunhofer Diffraction?
Easy to observe & calculate
Fraunhofer
Fresnel Diffraction or Fraunhofer Diffraction?
Insensitive to distance changes
(maintains same angular relationship
Fraunhofer
Fresnel Diffraction or Fraunhofer Diffraction?
Poisson spot – Bright in the center of a
shadow (Argo spot or Fresnel’s bright spot)
Ex. Floaters in eye
Fresnel
Combined effect of 2-slits and single-slit interference
With 2-slits, not only you will see diffraction patterns due to individual single-slits
but also interference patterns due to the waves coming from different slits.
Rayleigh’s criterion states :
that the two point resolution limit occurs when the first minimum (the boundary of
Airy’s disk) of one diffraction pattern coincides with the center maximum of the other diffraction limited pattern.
The diffraction pattern formed on a distant screen resulting from a uniformlyilluminated
circular aperture has a bright region in the center, known as _____________
Airy’s disk.
The central disk plus the surrounding series of concentric bright rings is called the _________
Airy
pattern.
Diffraction places a limit on the ability of the system to form ________
good quality images
Diffraction is limited by
2 things:
a. Diffraction effects (small aperture)
b. All aberrations (large aperture)
Optical systems that are “perfect” (correct focus, aberrations
sufficiently corrected) are referred to as :
diffraction limited systems
Snellen acuity is equal to the reciprocal of the. (Keating 506)
minimum
resolvable angular detail (expressed in minutes)
Resolution Efficiency is:
a subjective method of
determining image quality of an optics (IOL,
contact lens, spectacles, or any single optics).
Still considered as the industry ‘gold standard’
for image quality
To remove the rings around the central maximum and concentrate energy in the central maximum is
‘Apodization.’
At the center of the
apodized circular pupil,
the intensity
transmission is :.
100%
At a distance (h = wo
)
from the center, the
transmission drops by:
13.5%.
For a clear circular aperture, Airy’s
disk contains ____ of the intensity
(light) in the diffraction pattern.
84%
In the Gaussian diffraction pattern,
_____of the intensity is inside the
circle described by h = w(z).
86%
Stile Crawford Effect
The Luminous efficiency of rays entering at the center of
the eye pupil is greater than the luminous efficiency of the
rays entering through the edge of the pupil.t
Stiles – Crawford apodization influences:
the visual acuity*
and visual performances+ of the human eye
Transmission grating
\:Acts as a \_\_\_\_ splitter - Less sensitive to \_\_\_\_\_ - Less sensitive to \_\_\_\_\_\_\_\_\_\_\_ - Efficiency reduces with higher \_\_\_\_\_\_\_\_\_\_\_
beam
polarization
alignment
wavelength
A diffraction grating consists of many slits with a very
____ slit separations.
small
Blazed Grating
A diffraction grating that is designed to channel light
away from the central maximum (0th order) into 1st or
2
nd order maximums.
Blazing angle:
Angle at which the efficiency is maximum.
