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