An eye based on a spherical metal-free reflector

Eye of a scallop     Scallop with many blue eyes,

Left: eye of a scallop, about a millimeter in diameter. Right: picture of a scallop, showing the many blue eyes in context. The right picture was taken by David Moynahan.

Evolution has discovered how to produce a remarkable diversity of eyes, which 162 students should think of as optical devices whose purpose is to distinguish light coming from different directions. A less familiar kind of eye is that of a scallop, as shown in the above images. Instead of using a lens like a mammalian or octopus eye, or a grid of lenses like the ommatidia of a fly's eye, scallops use a silvery spherical mirror to focus incoming light onto a retina. Physics 162 will discuss how light is focused by lenses and mirrors and what are the strengths and weaknesses of optical devices based on lenses and mirrors.

This picture raises a further interesting question that is answered in Physics 162: how does a scallop (or any silvery fish) produce a shiny reflecting surface without using any metal? The answer has to do with the interference of light, based on its wave-like properties. You will learn in 162 that, if one can create alternating layers of different transparent media (think of successive layers of clear jello), and the alternating layers have different indices of refraction, then by carefully choosing the thickness of each layer, one can arrange for light that reflects at each interface to interfere constructively such that all incoming light (within a certain range of wavelengths) is reflected back toward its source, that is the layers act as a high quality mirror. It is fairly easy for biological creatures to secrete such alternating clear layers with slightly different properties, rather harder for humans to do so using chemical and mechanical engineering.

Once light reaches the scallop's retina, many ideas of Physics 162 such as charge conservation, electric potential, capacitance, resistance, and dielectrics become important for understanding how the light initiates the formation of an electrical current, and how the current propagates down the non-metallic "wires" (called axons) that connect one nerve to another, eventually leading to behavior. The so-called Hodgkin-Huxley equations, a mathematical expression for the conservation of charge as charge flows across and along a neuron's membrane in response to various voltage-sensitive membrane proteins that can open and close holes in the membrane, gives a valuable quantitatively accurate description of signal propagation in nervous systems. The Hodgkin-Huxley equations were, in turn, motivated by earlier theoretical work of Lord Kelvin around 1850 to understand how signals propagated in leaky underwater electrical cables that linked Europe to the United States. The importance of this early communication network led to the creation of the first electrical engineering departments, whose initial goal was to train people to solve the Maxwell equations of electrodynamics in the context of cables and so improve the engineering of such cables. Similarly, electrodynamics is profoundly important in the operation of the Internet, in which huge amounts of information are transmitted in the form of solitonic light pulses through ultraclear glass fibers.

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