Psych 129 - Sensory processes

Light and the eye

The nature of light

• Electromagnetic waves are time-varying electric fields with orthogonal magnetic fields that are 90 deg. out of phase.
  
  
• Electromagnetic waves occur at different frequencies, thus giving rise to different wavelengths (because of finite propagation time). Relation between frequency, wavelength and velocity is , where v=c=186,000 mi/sec.
  
  
• The region of the electromagnetic spectrum termed “visible light” (that is, visible to us humans) is between approximately 400 and 700 nm. Wavelengths below this are generally harmful to humans (ultraviolet, x-rays) and are filtered out by the earth’s atmosphere. Wavelengths above this are the infrared (heat) and frequencies used in communication/detection technology (radar, radio, TV, microwave).
  
  
• UV/polarization sensed by some other animals.
  
  

• Refraction refers to the bending of light waves - i.e. changing their direction.
  
  
• A lens refracts light because it slows down slightly the speed of light. Thus, light entering at an angle to the refracting medium gets bent. Wavefronts parallel to the refracting surface are not bent.
  
  
• The development of a refracting medium - i.e., a cornea and lens - was an important innovation in the evolution of the eye. Without it, the only means of making photoreceptors directionally selective would be to make a small aperature, or pin-hole camera, which loses light.
  

• Major physical structures are the cornea, lens, iris, vitreous, and retina.
  
  
• The cornea provides the lionshare of refraction, with the lens providing mainly an adjustable focus (the focal length of the cornea is fixed).
  
  
• The iris is composed of colored, thin filaments that contract and expand to adjust the aperture of the eye (i.e., the amount of light admitted). A small aperture provides a larger depth of field. Controlled largely by the autonomic nervous system.
  
  
• The vitreous is the gelatin-like substance filling the eyeball. Floaters within the vitreous sometimes obscure vision.
  
  
• The retina contains the photoreceptors, neural circuitry, and blood vessels at the back of the eye.

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• Approximately 120 million photoreceptors sample the retinal image.
  
  
• Photoreceptors contain rhodopsin molecules inserted within the membrane of outer segments. Retinal molecules attached to the opsin chain cause the molecule to change configuration when light is absorbed, allowing ions to flow through the membrane and thus leading to hyperpolarization in response to light.
  
  
• Two main classes of photoreceptors: rods and cones.
  
  
• Approximately 5-8 million are cones; the rest (vast majority) are rods. Cones are concentrated mainly within the fovea.
  
  
• Rods are more sensitive, slower in response, and are selective to only one wavelength regime.
  
  
• Cones are less sensitive, faster in response, and come in three different varieties, selective to three different wavelength regimes: short (S), medium (M), and long (L) wavelengths.
  
  
• Cones are what allow for color vision because they allow for different portions of the electromagnetic spectrum to be differentiated - i.e., light of different wavelengths will cause hyperpolarization in different classes of photoreceptors.
  
  
  

• The dynamic range in natural scenes is enormous: from 0.1 Cd/m on a moonlit night to 100,000 Cd/mon a sunlit day. (By contrast, a typical CRT screen will produce a maximum intensity of 100 Cd/m.)
  
  
• The human visual system copes with this large dyamic range via several different mechanisms. One is the iris of the eye, which provides a variable aperture depending on the ambient light level. Another is that neurons in the retina signal contrast, which is a relative measure of light intensity. Still another is the luminance differences computed by retinal ganglion cells via neural circuitry in the retina.
  
  
• The ability of the human visual system to tell the lightness of an object (or its albedo) despite large fluctuations in the absolute amount of reflected light is termed lightness constancy. Without it, the composition of objects would appear much different to us depending on whether they were viewed indoors or outdoors.