Feb 1 2002 physics of light, image formation

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ScienceB-44

Lecture 2Light, Optics, & Eyes

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Course Details• Basic Vision is in bookstore• Sections Thursday & Friday this week• Sectioning starting after class on our web

site: www.fas.courses.harvard.edu/~scib44• Go to the Administrivia menu item on the

left, then the Gradebook/ Sections optionand follow instructions

• If you can’t get a section on the web site,just show up, Rm 765, Thurs 2 or 3, Friday1 or 2.

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LightImage formationAnimal eyesHuman eyeBlood vesselsBlind SpotCone & RodsAfterimage

Outline

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Light travels in a straight line

Light is an electromagnetic wave

Wavelength400 to 700 nm

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Part of electromagnetic spectrum

Sunlight

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We see objects because theyreflect light

But light reflects in all directions from each point of theobjectArrives at another surface from all points, destroyingthe pattern of light and dark reflection.

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How to recover the pattern?

Select only one direction per point

TubesPinholeLens

8Tubes

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Ibn Al-Haitham 965 - 1040

Discovers Pinhole Camera 10

Pinhole should make perfect imagesof the world, so what’s wrong with it?

Not enough light

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prism

Speed of light is different indifferent materials

Light bends when crossingboundary between twomaterials, refractionproportional to ratio of speeds

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no lens, 1 receptor: single cell creatures

Photoreceptors

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no lens, a few hundred receptors: flatworms, limpits

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Pinhole “camera” in naturechambered nautilus

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The unusual scanning eye of thecopepod Copilia

2 lenses, 2 receptors, copilia

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500 million year old eyes

compound eye: 1 lens per receptor20

As many as 125,000receptors per “eye”

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Compound eyeswith resolutionequal to ours

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Some invertebrateshave simple eyes

Simple eyes:one lens manyreceptors

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Sea scallop:100 eyes

Squid eyes: like oursbut bigger, better

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Human eyes2 lenses, 260,000,000 receptors

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25Iris Bar Code 26

eye maintains it shape by intraocular pressure (like a balloon)

No blood vessels on cornea or lens but plentyof blood vessels right on retina

Cornea has the most focusing powerLens is mainly used for fine tuning

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retinalvesselsplainlyvisible

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If blood vessels are in front of theretina, why don’t we see them?

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Fovea densely packed receptors, cones onlyblood vessels and nerves pushed away

Macula shields fovea from blue light30

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Distribution of rods andcones

Cones are mostconcentrated inthe fovea.

Rods are absentfrom the fovea.

No receptors at allexist in the blindspot.

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Negative cone afterimage: from depletionand/or adaptation of cones. Lowerresponsiveness in the regions exposed tobrighter light then creates a negative versionof the image when a blank field is viewed.

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Positive afterimage, from an overload of input. Lightactivates molecules in receptor and these are thendeactivated to generate an electrical signal. When alot of light arrives all at once, the backlog ofactivated molecules takes some time to deactivateand extends the duration of the response wellbeyond the duration of the light flash.

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Summary

OpticsBiological image formationHuman eye defects

Receptors backwardsBlood vesselsBlind Spot

SpecializationsHigh resolution, low sensitivity at

center: conesLow resolution, high sensitivity in

periphery: rods