1 Light and Eyes

Organism’s adaptation
- plants: grow so that leaves face the strongest available light
- animals: movement of the body by contraction of muscles
Perception
- ability to detect structures and events in the surroundings, so that the movement is regulated by the environment
- sensitive to energy that can provide information about the environment
  - chemical substances diffusing through air or water
  - mechanical energy, such as pressure on body surface, forces on the limbs and muscles, or waves of sound pressure in air or water
  - electric and magnetic fields
  - light provides information about inanimate world or silent animals at a distance ➡️ vision

1.1 Light and the information it carries

The nature of light

Light
- one form of electromagnetic radiation (电磁辐射), a mode of propagation of energy through space
- light includes radio waves, radiant heat, gamma rays, and X-rays
Monochromatic
- light that contains only a single wavelength
Photon
- it travels in a straight line at the speed of light
- each photon consists of a quantum of energy. The shorter the wavelength of the light, the larger the energy quantum.

The processes governing travel of light through space

When light passes through a medium, it undergoes absorption.
- absorption in water in stronger than in air
- longer wavelengths are absorbed more strongly
- reason: photons collide with particles of matter, give up their energy and disappear
When light passes through a medium, it is diffracted
- example: diffraction of runlight by the atmosphere make the daytime sky bright, and the blue color is because shorter wavelengths would be scattered more
- reason: rays are scattered on striking small particles of matter
The velocity of light is lower when it passes through a transparent medium than a vacuum
- the grater the optical density of the medium, the lower the velocity of light
- the change of velocity causes the rays of light to be bent or refracted
When light strikes an opaque surface, some of its energy is absorbed and some of it is reflected
- the texture of a surface determines how coherently it reflects light (polished surface and textured surface)
- a surface may reflect some wavelengths more strongly than others

How does light carry information for animal about their environments?

Ambient optic array
- the array is divided into many segments, containing light reflected from different surfaces, and differing in average intensity and spectral composition
- any movement in the environment will cause change in the spatial pattern of the optic array
- the spatiotemporal pattern in the optic array can carry information about the direction, speed, and form of the movement involved
the spatial and temporal pattern of light converging on a point provides information about the structure of the environment and events occurring in it
- exception: deep oceans and completely dark caves
phenomenon of bioluminescence
- the emission of light by organisms

How do different kinds of light-sensitive structures allow light energy to influence the activity of animals’ nervous systems?
What scope do these structures have for detecting the fundamental information-carrying features of optic arrays?
Spatial pattern and changes in spatial patterns

1.2 The evolution of light-sensitive structures

Various biochemical mechanisms that absorb electromagnetic radiation then change in chemical structure
- photosynthesis: plants absorb the light then powers the biochemical synthesis of sugars
- light-sensitive molecules: harnesses the absorption of light and make animals move
amoeba
- it can avoid bright light though having no specialized light-sensitive structures
- it moves by a streaming motion of the cytoplasm to form extension of the cell (pseudopods). As those pesudopods extend into bright light, the streaming would stop and divert in different direction
Ciliate Stentor coeruleus
- propel it through the water by reversing the waves of beating go its cilia
- a blue pigment used to capture light would cause a change in the membrane potential of the cell, and that affects the movement of the cilia
the basic principles of transduction of light energy that operate in more complex animals
- a pigment molecule absorbs light
- chemical structure changes
- alteration in the structure of the cell membrane
- membrane’s permeability to ions is modified, which leads to a change in the electrical potential across the membrane
photoreceptor cells
- an animal with single receptor cells or patches of cells in eyespots cannot detect the spatial pattern of light in the optic array. However, it can detect changes in light intensity over time.
- an animal’s photoreceptors are have some directional sensitivity, and further directional sensitivity can be achieved in simple eyespots by screen the receptor cells with a layer of dark pigment.
eye-cups
- vary in the detail of the structures: open to the water, filled with gelatinous materials, contain a crystalline lens
a true eye = the concentration of photoreceptor cells into one part go the body + some apparatus for forming an image on them

The Compound eye 复眼

the compound eye
- making eye-cups more directionally sensitive + making them smaller + grouping them all together into a single structure
- a compound eye is made up of a number of ommatidia
variability
- degree of optical isolation between adjacent ommatidia:layer of cones and layer of thabdoms
superposition eye
apposition eye