Class content >Three models of light

Prerequisites:

• Three models of light
• The wave model
• The ray model      
• The photon model

Interesting biological implications of light

There are many practical applications of light refraction, reflection, transmission and absorption.  The ability to see depends on each of these aspects of light.  As a photon of light travels from its source to be detected in the retina, it passes through the cornea, the aqueous humour, the lens, the vitreous humour and then arrives at the retina.  Interestingly, because of the way the vertebrate retina develops, the retinal photoreceptors point towards the back of the eye.  As a result, the photons must traverse the other neural layers such as the bipolar cells and ganglion cells (which project to the brain) before reaching the photoreceptors.  All of these different elements from the cornea to the neural layers of the retina must be highly transmissive to so that light is neither absorbed nor scattered before it reaches the photoreceptors.  However, the photoreceptors themselves are packed full of visual pigment to maximize the probability of absorbing photons when they reach the photoreceptive layer.

The path of the photons from the viewed object is shaped by refraction so that the light is focused onto the retina.  Focusing occurs primarily at the curved air – cornea interface where there is the largest difference in index of refraction.  Air has n_air =1 while the cornea is primarily water and so has n_cornea = 1.376, just slightly higher than that of water (n_water =1.33).  The lens also does some of the focusing, approximately 20% of it, and primarily fine tunes the focus particularly when viewing objects up close.  This occurs at the curved surface of the lens, with the lens changing shape by its ciliary body.

The absorption of light in the photoreceptors depends on how dense the visual pigment is packed into the retina.  Vertebrate photoreceptors, such as the rods, have extremely high densities of the visual pigment rhodopsin.  A typical frog rod is 86 μm long and its visual pigment has an attenuation coefficient of 0.015 μm^-1.  We can calculate the probability of a photon being absorbed by first using Beer's law to calculate the light that is transmitted all the way through the rod:

     T = exp (-αL) = exp (-0.015 μm^-1 * 86 μm) = 0.28

Since the light which is not transmitted is absorbed (we will neglect reflection here), the fraction of light absorbed is 1 – T = 0.72.  So 72% of the light is absorbed by a frog rod, making it a highly efficient photon detector.

Karen Carleton