Diffraction

Question 1

What is diffraction

Answer 1

Ability of wave to propagate around corners

Question 2

When is diffraction more pronounced

Answer 2

When the obstacle or aperture size is similar to that of the wavelength

Question 3

What happens when aperture size is small

Answer 3

Diffraction is more while aberration is less

Question 4

When happens when apreture size is big

Answer 4

Diffraction is less while aberration is more

Question 5

What is Huygen’s principle (not that impt)

Answer 5

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

Question 6

What happens when there is a object

Answer 6

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

Question 7

What will the diffraction of a small aperture be

Answer 7

results in curve shape of wavelet

A large amount of diffraction

Question 8

What will the difraction of a large aperture be like

Answer 8

Waves going throuhg the middle remain as plane waves. Little to no diffraction

Question 9

What happens the a wave when there is a obstacle

Answer 9

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

Question 10

What is a single slit diffraction pattern

Answer 10

Where secondary wavelets pass through a slit which gives alternating maximum and minimum illumination

Question 11

What is the central maximum of the single slit diffratcion pattern

Answer 11

Resulting constructive interference

Question 12

How does the pattern change in a single slit diffraction pattern

Answer 12

As aperture size gets closer to wavelength, diffraction pattern spreads out more

Question 13

What would the single slit diffraction pattern of a aperture size equal to wavelength look like

Answer 13

When aperture size equals to wavelength, first minimum falls at 90 degrees where no ripples are formed

Question 14

How is the diffraction of the different wavelength of light effect

Answer 14

Longer wavelengths (red) diffracts/ spreads more than nshort wavelengths (blue)

Question 15

What happens when white light illuminates a single slit

Answer 15

Each wavelength is diffracted independently

Results in irradiance distribution with white center surrounded by colour fringes

Question 16

What is the diffraction by sqaure aperture like

Answer 16

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

Question 17

What is the diffraction by circular aperture like

Answer 17

wave diffract equally in all directions, results in central circular maximum surrounded by first minimum and series of progressively fainter light rings

Question 18

how does wavelength affect diffraction

Answer 18

longer the wavelength, larger the diffraction

Question 19

State Rayleigh’s Criterion

Answer 19

Two point resolution limit occurs when the first minimum of one diffraction pattern conincides with center maximum of other diffraction pattern

Question 20

What is Fraunhofer diffraction

Answer 20

Far field diffraction
- plane wave incidence on aperture
- diffracted wave considered as plane wave
- viewed on distant screen

Question 21

What is Fresnel diffraction

Answer 21

Near field diffraction
- converging or divergning wave incident on aperture
- distance from aperture to obsevervation screen is small

Question 22

What is the difference between Fraunhofer and Fresnel diffraction

Answer 22

Fresnel patterns change significantly as distance from aperture varied

Fraunhofer patterns are not sensitive to distance chagne

Question 23

What is the perfect optical system

Answer 23

Detrimental effect of diffraction, that demonstrates by ability of optical system to resolve two points

Question 24

What is the effect when aperture is large vs small on the optical system

Answer 24

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)

Question 25

What is the resolvable angle

Answer 25

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

Question 26

How are pinholes used clinically

Answer 26

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

Question 27

What are directional haloes

Answer 27

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