Light: The EM Spectrum

http://www.antonine-education.co.uk/physics_gcse/Unit_1/Topic_5/em_spectrum.jpg

Light: Solar Radiation Spectrum

http://upload.wikimedia.org/wikipedia/commons/4/4c/Solar_Spectrum.png

Light Perception:The Chromophore

11-cis-retinalall-trans-retinal

Diagram modified from Terakita(2005)

The Chromophore:

Light Perception:Opsins I

Diagram modified from Terakita(2005)

Light Perception:Opsins II

Diagram modified from Terakita(2005)

Light Perception: Photoreceptors I

Diagram modified from Nilsson and Arendt(2008)

Light Perception: Photoreceptors II

Diagram modified from Nilsson and Arendt(2008)

RhabdomericPhotoreceptor(depolarizing/“on” receptor)

CiliaryPhotoreceptor

(hyperpolarizing/ “off” receptor)

= Dark-to-light detector

= Light-to-dark detector

Light Perception: Signal Transduction

From Marlow and Speiser et al(in prep).

Opsins in a selection of metazoansSpecies Phylum # of opsins Eyes?

Nematostella Cnidarian 14 No

Hydra Cnidarian 2 – 63 (?) No

Cladonema Cnidarian 18 (?) Yes

Capitella Polychaete 3 Yes

Lottia Mollusk 5 Yes

Drosophila Arthropod 7 Yes

Apis Arthropod 5 Yes

Papilio Arthropod 5 Yes

Stomatopods Arthropod 6 - 15 Yes

Strongylocentrotus Echinoderm 6 No

Amphioxus Chordate 6 No

Homo (human) Chordate 7 Yes

Danio (zebrafish) Chordate 6 Yes

Gallus (chick) Chordate 6 Yes

Mus (mouse) Chordate 6 Yes

Oral disk flexion Tentacle flexionTentacle retraction

From Clark and Kimmeldorf (1977).

Wavelength (nm)

Resp

onse

Building an Eye

Depolarizing photoreceptor("on" receptor)

Hyperpolarizing photoreceptor("off" receptor)

Lens

Pigment layer

Mirror

Light path

COMPONENTS:

Diagrams by Dan Speiser

Optics concept 1:Refraction

Refraction is the deflection from a straight path undergone by a wave (such as light) when it passes obliquely from one medium (such as air) into another medium (such as water) in which its velocity is different.

Camera eye w/ depolarizing photoreceptorsand a lens (ex. squid and octopi)

Camera eye w/ hyperpolarizing photoreceptors and a lens (ex. fish)

Diagrams by Dan Speiser

Camera Eyes: Lens optics

Camera eye w/ depolarizing photoreceptorsand corneal optics (ex. land spiders)

Diagrams by Dan Speiser

Camera eye w/ hyperpolarizing photoreceptors and corneal optics (ex. land vertebrates)

Camera Eyes: Corneal optics

Big Concept 2:Trade-offs (part 1)

f

s

Optical resolution ≈ Inter-receptor angle (ΔΦ) = s/f

f

d

Optical sensitivity (S) ∝ D2Δρ2 (where Δρ = d/f)

D

Diagrams by Dan Speiser

Basic compound eye w/ depolarizing photoreceptors at the base of pigment tubes (ex. many inverts)

Compound Eyes

Diagrams by Dan Speiser

Apposition compound eye w/ depolarizing photoreceptors and lenses (ex. diurnal insects)

Compound Eyes II:Trade-offs (Part II)

Diagrams by Dan Speiser

Compound Eyes III

Reflecting superposition eye with depolarizing photoreceptors (ex. decapod shrimp and lobsters)

A naked photoreceptor gathers light from an entire hemisphere

Ɵi

f

d

An eye gathers light from an area with an angular size of, say, 10°

All else being equal, a naked photoreceptor will be 130x more sensitive than an eye with an angular resolution of 10°.

Big Concept 2:Trade-offs (part 2)

= 10 μm

Lens

Big concept 1:Convergence (part 2)

• Eyes allowed chitons to distinguish 10° objects from shadows.

• However, eyes decreased optical sensitivity: we found that chitons without eyes responded to changes in illumination of 1%, while chitons with eyes only responded to changes in illumination of 5% or greater.

• Eyeless chitons also responded to faster-moving objects.

Big Concept 2:Trade-offs (part 2)

“Bivalve lineages may be aptly described as evolutionary eye factories, in the sense that they have developed eyes of many

different types, often at unusual positions of the body”

- Dan-E. Nilsson

Back to eye diversity . . .

Scallop (Aequipecten)

File shell (Lima scabra)

?

+

Turkey wing (Arca zebra)

Giant Clam (Tridacna)

?

Lantern Shell

?

?

Cockle (Dinocardium)

= 100 μm

Lens

Distal retina

Proximal retina

DAPIAnti-tubulinAutoflourescence

Mirror

The optical resolution of a selection of animal eyesName Optical resolution (degrees)

Eagle 0.004

Human 0.007

Octopus 0.01 Cephalopod mollusk

Human (legally blind) 0.07

Rat 0.5

Honey bee 1.0

Scallop 1.6 Bivalve mollusk

Wolf spider 1.8

Fruit fly 5

Nautilus 8 Cephalopod mollusk

Giant clam 16.5 Bivalve mollusk

Ark clam 20 – 40 Bivalve mollusk

Table modified from Land and Nilsson (2002).

= 100 μmDAPIAnti-tubulinAutoflourescence

Mirror

Receptor spacing (s) = the distance between adjacent receptors

Focal length (f) of a concave spherical mirror = 0.5 x the radius of the mirror

Optical resolution ≈ Inter-receptor angle (ΔΦ) = s/f

Optics concept 2:Spherical aberration

A camera eye with a lens thatdoes not cause spherical aberration

(due to, for instance, having a graded refractive index)

An camera eye with a lens thatcauses spherical aberration

A spherical mirror w/ correcting lens = less spherical aberration

A spherical mirror w/ no lens= more spherical aberration

Optics concept 2:Spherical aberration (in the scallop eye)

Optics concept 3:Chromatic aberration (prism)

Chromatic aberration in a camera eye Chromatic aberration in a scallop eye

Optics concept 3:Chromatic aberration