Lecture 5- Aberrations

Question 1

What is aberrations?

Answer 1

are deviation from perfection in an optical system

when light focuses at different points forming a blurred image

Question 2

What is an ideal image formation?

Answer 2

when all rays of light meet at one point producing a sharp image

Question 3

What is a non-ideal image formation?

Answer 3

when rays hit different parts of lens, they refract at different amounts, forming a blurred image point

Question 4

How is a non-ideal image formed?

Answer 4

So lenses tend to be thinner at edges so therefore they will travel less distance then to the thicker part of lens which is in the middle

Question 5

What is the wavefront representation of an ideal lens?

Answer 5

unaberrated
ideal optical system
wavefront is spherical

Question 6

What is the wavefront representation of a real lens?

Answer 6

aberrated
non-spherical wavefront
a slight deviation of the spherical shape of the wavefront

Question 7

Why is the wavefront of a real lens useful?

Answer 7

it allows measuring the aberrations of the optical system

Question 8

What is a spherical wavefront?

Answer 8

parallel rays of light focusing at one point

Question 9

What is a monochromatic aberration?

Answer 9

aberrations that arise due to geometry (shape of lens o

they occur for a single wavelength

Question 10

What do monochromatic aberrations not depend on?

Answer 10

the wavelength of light

Question 11

What are the 5 types of monochromatic aberrations?

Answer 11

Spherical aberration
Coma
Astigmatism
Field curvature
Distortion

Question 12

What are the monochromatic aberrations knows as?

Answer 12

seidel aberrations

Question 13

What are the effects of seidel aberrations?

Answer 13

expanded size for a point image- becomes blurred
curved image plane
extended images are no longer geometrically similar to object

Question 14

What is the spherical aberration?

Answer 14

rays on optical axis where image rays converge causing blurred image point

Question 15

What happens to the 2 extreme rays in the non-ideal image formation?

Answer 15

they converge infront of the second focal point

Question 16

How do we limit spherical aberration?

Answer 16

limit the rays via paraxial region

within the reigion, imaging systems are ideal

Question 17

What can human eye do to limit the extreme rays from coming in?

Answer 17

placing an aperture

Question 18

What does the human eye have that is similar to the aperture?

Answer 18

a pupil as it limits the amount of light getting into the eye

Question 19

What is coma?

Answer 19

when rays come into optical axis at an angle

the bigger the angle the more coma

Question 20

What does coma do?

Answer 20

increases for off-axis rays and has a comet like appearance

Question 21

Why does it appear comet like?

Answer 21

there is a distortion on image plane which looks like a comet
-the off-axis rays are close together in the image plane, then closer to the optical axis, then become further away creating this comet-like shape

Question 22

What is astigmatism?

Answer 22

when 2 planes of best focus, one in horizontal a one in verticle

Question 23

What is field curvature?

Answer 23

it increases for off-axis rays

the rays which are off-axis, will come at an angle and will be slightly focused in front of image plane

Question 24

What structure is the field curvature useful for?

Answer 24

retina (curved at back)
as it minimises field curvature
therefore minimises aberration
as the retina is curved, the curved image plane can get everything in focus.

Question 25

What is distortion?

Answer 25

happens when there is a different magnification with rays coming off-axis

Question 26

What is pincushion distortion?

Answer 26

when there is magnification oustide

Question 27

What is barrel distortion?

Answer 27

when there is magnification in the centre

greater magnification in centre

Question 28

how do we limit chormatic aberration?

Answer 28

by using achromatic doublet lens

Question 29

What happens when you shine white light through a lens made of crown glass?

Answer 29

so depending on refractive index of different wavelenths, you have different focal points for different wavelengths

Question 30

What is chromatic aberration dan effect of?

Answer 30

resulting from dispersion as lenses have different refractive indicies for the different wavelengths of light

Question 31

What does it mean if there is a higher refractive index?

Answer 31

the denser the material, the slower light travels through it (more steep e.g blue light)

Question 32

What can a prism do?

Answer 32

disperse light into different wavelengths

Question 33

What is achromatic doublet lens?

Answer 33

counter-balances the differences in speed of colours/different wavelengths

Question 34

What happens when light goes through an aperture?

Answer 34

it diffracts on the edges

Question 35

What happens if the smaller the aperture?

Answer 35

diffraction increases

Question 36

What is the effect of diffraction?

Answer 36

perfect object point= airy disk has the effect of diffraction

Question 37

What does the aperture do?

Answer 37

minimises aberrations

Question 38

What happens when you have a large aperture?

Answer 38

you have more aberrations

Question 39

So how do you minimise the aberrations?

Answer 39

by making the aperture smaller

Question 40

What happens when you make the aperture smaller?

Answer 40

diffraction increases which means you get an Airy Disk

Question 41

What is an airy disk?

Answer 41

is the image of a point object due to the effect of diffraction

Question 42

What is the airy disk if its a symmetrical image?

Answer 42

bright centre dot

Question 43

What happens if it appears asymmetric?

Answer 43

then you know it has aberrations