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The First Steps in Vision: The First Steps in Vision: Seeing Stars
Outline
What is light?
• Types of waves
• Dual nature of light: waves and particles
• Spectrum of electromagnetic radiation
• Intensity, wavelength, polarization, direction
• Range of light intensities
• Interactions between light and matter• Interactions between light and matter
Anatomy and function of the eye
• univariance principle
• eye cups
• pinhole eye
• lens eye
Optics of the eye
• accommodation
• refractive errors
• role of the pupil
To suppose that the eye, with all its inimitable contrivances for adjusting the focus to different distances, for admitting different amounts of light, and for the correction of spherical and chromatic aberration, could have been formed by natural selection, seems, I freely confess, absurd in the highest possible degree.
Charles Darwin
Additional reading
Richard Dawkins (1996). Climbing Mount Improbable. W.W. Norton & Company. A wonderful text about how evolution works. Chapter 5 describes the 40 different ways of designing light sensing organs that nature came up with.with.
Longitudinal
Types of waves
Transverse
Wavelength, frequency, speed
wavelength λ [m]
frequency f [Hz] (1/s, number of waves per second)
speed c [m/s]
λ
λ ∗ f = c
speed of light: 300,000,000 m/s
speed of sound: 340 m/s
Big numbers and small numbers
pico- 10-12
nano- 10-9
micro- 10-6
milli - 10-3
wavelength of green light: 500 x 10-9 m
-meter 100
kilo- 103
mega- 106
giga- 109
tera- 1012
milli - 10-3
distance earth - sun 150 x 109 m
distance earth - moon 380 x 106 m
The spectrum of electromagnetic radiation
Light: A wave; a stream of photons, tiny particles that each consist of one quantum of energy
Light intensities
Luminance[ cd m-2 ]
photonsm-2 sr-1 s-1
photons per receptor
paper in starlight 0.001 1013 0.01
paper in moon light 0.2 1015 1
computer monitor 65 1017 100
room light 350 1018 1,000
blue sky 2,500 1019 10,000
paper in sun light 40,000 1020 100,000
• our visual system has to cope with a HUGE range of intensities
• bright sunlight is about 10,000,000 times more intense than starlight
Intensity
Wavelength
Direction
Interactions between light and matter
Polarisation
absorption &reflection
refraction
scattering(defraction)
absorption &transmission
Number of photons absorbed
Cel
l res
pons
e Principle ofUnivariance
(William Rushton, 1972)
A photoreceptor'sresponse corresponds to
just a single variable:
Direction
Sens
itiv
ity
just a single variable:the number of photons
absorbed
Sens
itiv
ity
Direction
Seeing direction
Sens
itiv
ity
Direction
Seeing direction
The eye cup makes an array of
Eye cup
an array of photoreceptors sensitive to direction
flatworm
bivalve mollusc
Eye cups
polychaet worm
limpet
An eye cup cannot generate an image
The pinhole eye can generate an image on the retina
Pinhole eye
marine snail bivalve mollusc
Pinhole eyes
abalone
Nautilus
Nautilus
The pinhole eye can generate an image on the retina
Pinhole eye
... however, lots of valuable light is wasted
The solution:a collecting lens
Lens eye
Compound eye
Cross section of the vertebrate eye: the lens
Zonules
Analogies between eye and camera
� Aperture: Iris/pupil. Regulates the amount of light coming into the eye and affects depth of field
� Focus: Lens changes shape to adjust focus� Focus: Lens changes shape to adjust focus
� Film: Retina records the image
Accommodation in eye and camera
The pinhole eye can generate an image on the retina
Pin hole eye
... however, lots of valuable light is wasted
The solution:a collecting lens
Lens eye
Accommodation
focal length: the distance between lens and image plane for a distant stimulus
f
Accommodation
f
focal length: the distance between lens and image plane for a distant stimulus
Cross section of the vertebrate eye: the zonules
Zonules
Lens, zonules and ciliary muscle
Distantfocus
Closefocus
Accommodation
Ciliary muscle
relaxed contracted
Zonules tense relaxed
Lens flat spherical
Accommodation
Accommodation
Accommodation
Near-point: the closest distance at which accommodation is still possible
Presbyopia: far-sightedness in elderly peopledue to reduced flexibility of the lens
Refractive errors of the eye
Emmetropia
MyopiaMyopia
“Short-sightedness”
Hyperopia
“Far-sightedness”
Refractive errors of the eye
Smaller aperture sharpens the imageand thus increases depth of field
Role of the pupil
Role of the pupil
Smaller aperture sharpens the imageand thus increases depth of field
The pinhole eye can generate an image on the retina
Pin hole eye
... however, lots of valuable light is wasted
� Reduce amount of incident light(but only by a factor 10)
� Reduces refractional problems
� Increases depth of field
What is the pupil for?
� Increases depth of field
2
3
Fechner‘s law: S = c * log IB
righ
tnes
sBrightness: Perceived intensity (sensation magnitude)
10 1000
1
2
1 Intensity
log(Int)0 1 2
Brig
htn
ess
2
3
Fechner‘s law: S = c * log IB
righ
tnes
sBrightness: Perceived intensity (sensation magnitude)
10 10000
1
2
1 Intensity
log(Int)0 1 3
Brig
htn
ess
2100
Cel
l res
pons
e
Log (Number of photons absorbed)
Cel
l res
pons
e
Outline
What is light?
• Types of waves
• Dual nature of light: waves and particles
• Spectrum of electromagnetic radiation
• Intensity, wavelength, polarization, direction
• Range of light intensities
• Interactions between light and matter
Anatomy and function of the eye
• univariance principle
• eye cups
• pinhole eye
• lens eye
Optics of the eye
• accommodation
• refractive errors
• role of the pupil