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Saturday, July 30, 2011

Evolution, the master of optical science!

Do not get me wrong; evolution is an expert of all physical science. But it intimately links nature to optical science without doubt -- from cyanobacteria that have been converting solar energy to chemical energy for 3 billion years to human beings who rely on vision for surviving.

Neuroscience indicates that about 25%~ 50% of the brainpower and as many as 30 different areas of the brain are devoted to vision processing. This simply means that each human being is hard wired as an optical scientist, although we hardly recognize this. Over the past millions of years, evolution has perfected our imaging device in a subtle way. Recently, a report on Biomedical Optics Express shows for the first time the eyes’ imaging sensors -- cones and rods by using adaptive optics to minimize the aberration caused by the eye structure. As shown in the figure 1, cones, the round structures, create red, green, and blue perception of colors. There are about 6-7 millions of them, concentrated at the center of the retina -- forvea. A friendly and easy to digest article about this topic can be found here.

Figure 1. The cones in the fovea region of the eye. The retina is illuminated by 796 nm and 680 nm, respectively. The scale bar is 10 micron. Photo is courtesy of A. Dubra and Y. Sulai in Biomedical Optics Express Vol. 2 No. 6. 1757 (2011).

The other component of eye that always surprises me is the crystalline lens. It is 9 mm in diameter and 4 mm thick – a tiny optics. The structure of it is more or less like a transparent onion, formed by ~ 20000 very fine layers. Each layer is composed of cells of elongated shape (about a few micron thick and ~ 10 mm long). What makes it special is the variation of the index of refraction. In the inner core of the lens, the index of refraction is about 1.406, while it changes to 1.386 at the less dense cortex. This kind of design combined with the change of the lens shape makes us see things clearly whether they are far or close. In fact, evolution designs gradient index optics way before we even learned about it.

Changing the focus to animal kingdom, you can find more examples that not only make your eyes wide open, but also give us ideas to advance our knowledge of optical science. For example, Lobster uses reflective unit in the eye to focus much more light onto retina (figure 2a). This is definitely one of nature’s demonstrations on micro lens system. Mantis shrimp can detect circular polarized light thanks to intrinsic quarter wave plates made of cells in their eyes (figure 2b, for details about this work, here it is). You might wonder why we need to have polarized vision except 3D movie utilizing this principle to create stereo perception. Next time, when you buy sunglasses, get polarized ones. Wear them and observe the world! Asphalt road reflects light differently depending on light’s polarization. You can see that windshields are not that homogeneous in transmitting light any more. Due to its dichroism caused by tension when molding, it transmits one polarization better. Press a piece of thick plastic, and observe its change in transmission of light (you will induce dichroism by stress). LCD screens can only been seen clearly when you tilt your head in one direction. Turn your head in a different way, the screen become completely dark since most screens emit polarized light. After these daily life experiments, you might regret that we lose this feature during evolution.

Figure 2. (a) The reflective unit of a lobster's eye. (b) A cross section of the cells that work as quarter wave plate in the eye of Mantis shrimp. The scale bar is 10 micron. (c) The phase retardation introduced by the cells. They work nicely in the entire visible region of the light spectrum. The photo is courtesy of T.-H Chiou, S. Kleinlogel, T Cronin, and et al. in Current Biology 18 429 (2008).

We actually lose even more. Homo sapiens can only see three different colors, while birds, some mammals, and insects see the forth kind – Ultraviolet. For instance, Reindeer has UV eyesight. This special ability has evolution advantage. Lichen, on which the animal feeds, absorbs UV light, so it would appear black to reindeer eyes. The animal's traditional predator, wolves, would also appear darker against the snow, as their fur absorbs UV light. Apparently, under UV illumination, things are very different in polar area. I will suspect most of the animals living in Arctic or Antarctic area are endowed with this ability. How birds acquire UV vision is another fun story. Thanks to a beautiful article by Scientific American, bird’s UV vision is illustrated in details. A quick summary from this article: I realize our vertebrate ancestors had 4 types of cones in the eyes.  While birds inherit this feature, our mammal ancestors actually lost two of them! Fortunately, nature is kind to us. Through mutation, we regained a third variation of cones when walking down the evolution road. Without that mutation, we will all be color blinded!

Figure 3. How our perception of the colorful world improved, or degraded down the evolution road? Courtesy of Scientific American Magazine.
It is time for some wild experiment and conjecture. An article published on Nature in 2004 seems to tell us that the migratory avian creatures use the interplay of light, electrons, and earth magnetic field to guide their navigation. In a nutshell, it is called “radical-pair mechanism”. The light creates a coupling between an unpaired electron spin and nuclear spin through a light induced electron transfer. The earth’s magnetic field alters the dynamics of transitions between spin states. These transitions in turn affect reaction rates and products. Such effects can be amplified and used by the creatures. What does this tell us? Quantum mechanics and light matter interaction are working in a fine and elegant form. For a quick read, follow this link.

Story like this can be found at every corner on earth. After all, we are all offspring of nature, and nature relies on sun (the ultimate light source) in countless and ingenious ways. 

The opinions expressed herein are those of the author and do not represent the Optical Society of America (OSA) or any OSA affiliate.