For some reason that eludes me, I always live directly west of my office. As a result, for much of the year I drive directly into the rising sun in the mornings and into the setting sun in the evenings. Usually my commute provides me with a peaceful and welcome transition between my home and work environments. But on those mornings and afternoons when the sun dips low and traffic on Route 40 slows to a crawl, my frustration builds and I have been known bellow out “encouragements” such as, “Come on people, it’s just the sun!” and “Keep going!” as well as some other more colorful exhortations. It’s hard for me to understand why everyone can’t just keep moving along. But then again, I have brown eyes, so I wouldn’t understand the challenges of my blue-eyed fellow commuters. To understand why the sun doesn’t bother me but stymies them, we need to delve into some details about electromagnetic waves, evolution, physics, and the biology of the human eye.
The energy that the earth receives from the sun comes in the form of electromagnetic (EM) waves. The radiation of EM waves through the universe can be hard to wrap your mind around, but as I will discuss below, we are all quite familiar with them nonetheless. The full spectrum of EM wavelengths is shown in the graphic below. Low energy EM waves have long wavelengths and high energy ones have short wave lengths.
As a convenience, we have given names to different parts of the EM spectrum based on how human beings have learned to utilize particular subsets of wavelengths. For example, we found that it was convenient to encode sounds in longer EM waves, so we called them radio waves. We found that shorter wavelength EM waves could penetrate the human body and provide an image of the bones within, so we called them X-rays. We call electromagnetic waves in the wavelength range of 700 to 400 nanometers light. But bear in mind that radio waves, light, and X-rays are the same thing. They only differ in the same way that the sound waves generated by different keys on a piano are the same but have different pitches.
The EM radiation generated by the sun that reaches the earth is concentrated in the visible light range of the EM spectrum. Therefore, during the last 4.6 billion years, the earth has been bathed in EM waves with wavelengths ranging from 700 to 400 nanometers. It should not surprise us that life on earth evolved to utilize light. Plants use light for photosynthesis and most animals have developed eyes that use light to see.
Let’s consider how the human eye utilizes light. Here is a diagram for reference. In order for us to see something, light has to strike an object within our field of view and some of that light has to reflect off the object and into our eyes. When this light reaches our eyes, it first passes through the cornea before reaching the pupil. The pupil’s main job is to regulate the amount of light that can enter the eye. If too much light is entering your eye, your pupil will contract. If too little light is entering, then your pupil will dilate. Your pupil is surrounded by the iris, the colorful part, about which more below. The light that your pupil allows through encounters the lens, which can change its shape, within limits, to focus the incoming light waves on your retina on the back of your eye. (Note that short- or farsightedness arises from the lens not having sufficient capacity to change its shape to achieve the proper focus.) Your retina is connected to your optical nerve. Based on the wavelengths of the light that strikes the retina, your optical nerve sends differing electrochemical signals to your brain, which then perceives the color. Referring to the EM spectrum above, if the wavelength of the light reaching the retina is around 700 nanometers, the object will look red, and if wavelength is around 400 nanometers, it will look purple. When you sit back to consider it for a moment, sight is rather amazing.
So, back to the iris. The primary job of the iris is to block, or in scientific terms, absorb light. In the ideal case, the iris would absorb all of the light that strikes it, leaving the rest of the job of regulating the amount of light entering the eye entirely to the pupil. The substance within the iris that absorbs light is called melanin. If your iris has plenty of melanin, essentially all light waves from 700 to 400 nanometers are absorbed and, thus, prevented from entering the eye. People with lots of melanin have brown eyes and, unitl about 6,000-10,000 years ago, all human beings had brown eyes. Then one person somewhere in Central Europe had a mutation which resulted in him/her having less melanin in the iris. As I will explain below, he or she is the Adam or Eve of all blue-eyed people.
An iris with a low amount of melanin will absorb longer wavelength light in the red range but shorter wavelength light waves in the blue range, 400-500 nanometers, will be scattered. Therefore, a person with low melanin concentration in their irises will scatter blue light both outwards, making their eyes appear blue, and also back into their eye towards their retina. In a situation with bright light, like driving into the sun on Route 40, a blue-eyed person cannot sufficiently reduce the amount of light reaching his/her retina via pupil constriction, because too much blue light is making its way through the iris. This experience must be rather distressing to blue-eyed drivers.
As an interesting aside, let me address the phenomenon of people sneezing upon exposure to bright light, often called a sun sneeze. When the inside of your nose becomes irritated, your trigeminal nerve sends a signal to the brain, causing you to sneeze in an attempt to alleviate the irritation. On its way from the nose to the brain, the trigeminal nerve passes in very close proximity to the optic nerve. Recall that exposing your eye to bright light, particularly for blue-eyed people, results in increased electrochemical activity along the optic nerve. The prevailing theory on the sun sneeze is that high levels of electrochemical activity along the optic nerve stimulates the adjacent trigeminal nerve and “tricks” the brain into sneezing even when your nose is not irritated.
I always publish my columns on Sundays. Therefore, my Monday morning commute approaches and, during Januarys, the sun shines right down Route 40 East around 8:00 am. So tomorrow, when traffic slows to 5 miles an hour, I will try to be tolerant. Wish me luck.
Jeff Danner discussed this week’s column with Aaron Keck on WCHL.
Have a comment or question? Use the interface below or send me an email to email@example.com. Think that this column includes important points that others should consider? Share a link to this column on Facebook or Twitter. Want more Common Science? Follow me on Twitter on @Commonscience.