Diffraction Flashcards
What is diffraction
Ability of wave to propagate around corners
When is diffraction more pronounced
When the obstacle or aperture size is similar to that of the wavelength
What happens when aperture size is small
Diffraction is more while aberration is less
When happens when apreture size is big
Diffraction is less while aberration is more
What is Huygen’s principle (not that impt)
All points on wavefront can be considered as point sources for the product of spherical secondary wavelets, and at any later time the new wavefront posistion is the envelops to these secondary wavelets
What happens when there is a object
Diffraction will take place, where light in the same bundle, secondary wavelets undergo mututal interference. Ripple effect due to constructive and destructive interference of secondary wavelets
What will the diffraction of a small aperture be
results in curve shape of wavelet
A large amount of diffraction
What will the difraction of a large aperture be like
Waves going throuhg the middle remain as plane waves. Little to no diffraction
What happens the a wave when there is a obstacle
Secondary wavelets will be blocked at the obstacle. The wave next to the obstacles will not be blocked but forms spherical envelops, reseults in diffraction where some light bends around the obstacle and propagates into geometric umbra region
What is a single slit diffraction pattern
Where secondary wavelets pass through a slit which gives alternating maximum and minimum illumination
What is the central maximum of the single slit diffratcion pattern
Resulting constructive interference
How does the pattern change in a single slit diffraction pattern
As aperture size gets closer to wavelength, diffraction pattern spreads out more
What would the single slit diffraction pattern of a aperture size equal to wavelength look like
When aperture size equals to wavelength, first minimum falls at 90 degrees where no ripples are formed
How is the diffraction of the different wavelength of light effect
Longer wavelengths (red) diffracts/ spreads more than nshort wavelengths (blue)
What happens when white light illuminates a single slit
Each wavelength is diffracted independently
Results in irradiance distribution with white center surrounded by colour fringes
What is the diffraction by sqaure aperture like
The diffraction direction is perpendicular to the position of the slit
- horizontal slit gives vertical diffraction etc.
Therefore, the combination of vertical and horizontal slit gives a resultant pattern of dominated maximums along horizontal and vertical axis while other maximums are fainter
What is the diffraction by circular aperture like
wave diffract equally in all directions, results in central circular maximum surrounded by first minimum and series of progressively fainter light rings
how does wavelength affect diffraction
longer the wavelength, larger the diffraction
State Rayleigh’s Criterion
Two point resolution limit occurs when the first minimum of one diffraction pattern conincides with center maximum of other diffraction pattern
What is Fraunhofer diffraction
Far field diffraction
- plane wave incidence on aperture
- diffracted wave considered as plane wave
- viewed on distant screen
What is Fresnel diffraction
Near field diffraction
- converging or divergning wave incident on aperture
- distance from aperture to obsevervation screen is small
What is the difference between Fraunhofer and Fresnel diffraction
Fresnel patterns change significantly as distance from aperture varied
Fraunhofer patterns are not sensitive to distance chagne
What is the perfect optical system
Detrimental effect of diffraction, that demonstrates by ability of optical system to resolve two points
What is the effect when aperture is large vs small on the optical system
Larve aperture:
- little diffraction
- airy disk is small
- two illuminated patterns are close to points (easy to resolve the two img)
Aperture decreases in size
- Increase diffraction
- Airy disk becomes bigger
- two airy disk start to overlap ( two img are not resolvable)
What is the resolvable angle
Anything more/ equal to the resolvable angle, there will be resolution of image
When the angle is less than resolvable angle, there will be no resolution
How are pinholes used clinically
used to asses if poor vision is due to optical causes or pathlogical causes
- poor acuity due to optical cauases, where pinhole effect minimizes blur and increase acuity
- poor acuity not due to optical blur, pihole will no give improvement to acuity
What are directional haloes
When a random array of many circular apertures or obstacles are illuminated by plane waves from a white point source, each aperture generates a airy type diffraction pattern. When apertures are small and close tgt, diffraction patterns are large and overlap and produces a readily visible halo
