2 The First Steps in Vision: Seeing Stars

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1 2 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 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 Additional reading 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 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. 1

Types of waves Wavelength, frequency, speed

frequency f [Hz] (1/s, number of waves per

300,000,000 m/s speed of sound: 340 m/s Big

electromagnetic radiation Light: A wave; a stream

one quantum of energy pico- 10-12 nano- 10-9

m milli- 10-3 -meter 10 0 kilo- 10 3 mega- 10 6

2 Types of waves Wavelength, frequency, speed Longitudinal Transverse wavelength λ [m] λ frequency f [Hz] (1/s, number of waves per second) speed c [m/s] λ f = c speed of light: 300,000,000 m/s speed of sound: 340 m/s Big numbers and small numbers The spectrum of electromagnetic radiation Light: A wave; a stream of photons, tiny particles that each consist of one quantum of energy pico nano micro wavelength of green light: 500 x 10-9 m milli meter 10 0 kilo mega giga tera distance earth - moon distance earth - sun 380 x 10 6 m 150 x 10 9 m 2

3 Light intensities Luminance [ cd m -2 ] photons m -2 sr -1 s -1 paper in starlight paper in moon light photons per receptor Interactions between light and matter Intensity Wavelength Polarisation Direction computer monitor room light ,000 blue sky 2, ,000 absorption & reflection scattering (defraction) refraction paper in sun light 40, ,000 our visual system has to cope with a HUGE range of intensities absorption & transmission bright sunlight is about 10,000,000 times more intense than starlight Seeing direction Cell response Principle of Univariance (William Rushton, 1972) Sensitivity Number of photons absorbed A photoreceptor's response corresponds to just a single variable: the number of photons absorbed Sensitivity Direction Direction 3

to direction Eye cups An eye cup cannot generate an

4 Seeing direction Eye cup Sensitivity Direction The eye cup makes an array of photoreceptors sensitive to direction Eye cups An eye cup cannot generate an image bivalve mollusc flatworm polychaet worm limpet 4

Pinhole eye Pinhole eyes The pinhole eye

5 Pinhole eye Pinhole eyes The pinhole eye can generate an image on the retina marine snail bivalve mollusc Nautilus abalone Nautilus Pinhole eye The pinhole eye can generate an image on the retina... however, lots of valuable light is wasted 5

6 Lens eye Compound eye The solution: a collecting lens Cross section of the vertebrate eye: the lens Zonules 6

7 Analogies between eye and camera Accommodation in 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 Film: Retina records the image Pin hole eye Lens eye The pinhole eye can generate an image on the retina The solution: a collecting lens... however, lots of valuable light is wasted 7

8 Accommodation Accommodation focal length: the distance between lens and image plane for a distant stimulus f focal length: the distance between lens and image plane for a distant stimulus f Cross section of the vertebrate eye: the zonules Accommodation Zonules Lens, zonules and ciliary muscle Ciliary muscle Distant focus relaxed Close focus contracted Zonules tense relaxed Lens flat spherical 8

$Accommodation Accommodation Accommodation Refractive errors of the eye Emmetropia Myopia Short-sightedness Near-point: the closest distance at$

9 Accommodation Accommodation Accommodation Refractive errors of the eye Emmetropia Myopia Short-sightedness Near-point: the closest distance at which accommodation is still possible Presbyopia: far-sightedness in elderly people due to reduced flexibility of the lens Hyperopia Far-sightedness 9

$Refractive errors of the eye Role of the pupil Smaller aperture sharpens the$ aperture sharpens the image and thus increases depth of field The pinhole eye

aperture sharpens the image and thus increases depth of field The pinhole eye

10 Refractive errors of the eye Role of the pupil Smaller aperture sharpens the image and thus increases depth of field Role of the pupil Pin hole eye Smaller aperture sharpens the image and thus increases depth of field The pinhole eye can generate an image on the retina... however, lots of valuable light is wasted 10

11 What is the pupil for? Reduce amount of incident light (but only by a factor 10) Reduces refractional problems Increases depth of field Fechner s law: S = c * log I Brightness: Perceived intensity (sensation magnitude) Brightness Intensity log(int) Fechner s law: S = c * log I Brightness: Perceived intensity (sensation magnitude) Brightness Cell response Log (Number of photons absorbed) Intensity log(int) 11

12 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 12

Vision 1. Physical Properties of Light. Overview of Topics. Light, Optics, & The Eye Chaudhuri, Chapter 8

Vision 1. Physical Properties of Light. Overview of Topics. Light, Optics, & The Eye Chaudhuri, Chapter 8 Vision 1 Light, Optics, & The Eye Chaudhuri, Chapter 8 1 1 Overview of Topics Physical Properties of Light Physical properties of light Interaction of light with objects Anatomy of the eye 2 3 Light A