The Sunset Effect

Note the fog in the valleys of the Black Forest (©Margit Anhut)

Look at this beautiful afterglow, a photo taken by my good Feierabend friend Margit at the Schauinsland, the famous mountain in the Black Forest.

 
The red colors of sunrise and sunset, and the blue sky, too, are explained by the scattering of sunlight.

The scattering of light by small particles compared to the wavelength is called Rayleigh scattering. Calculations show that light is scattered inversely proportional to the fourth (!) power of its wavelength. Thus, blue light is scattered much more than red light.

When the sun is high, the path of sunlight through the atmosphere is relatively short. Gas, dust, and water particles in the air scatter light mainly at short wavelengths, so the sky appears blue rather than black during the day.

When the sun is low, the path of direct sunlight through the atmosphere is much longer. Due to the scattering of blue light, the latter decreases considerably, so that the red in sunlight becomes dominant.

A cloudless sky is blue during the day and red at dawn and dusk.

The more polluted the air is, the more impressive the sunsets are.

Too much fundamental physics? Let's change the subject. I'll tell you about the two operations on my left eye.

Around 2010, I noticed that my vision in my left eye was deteriorating. The diagnosis was epiretinal gliosis, a tissue layer that forms on the retina. This layer can be removed, but at that time, only the eye clinic in Offenbach, near Frankfurt, dared to operate on the open eye. I did not go to Offenburg because I weighed the question of my ophthalmologist, "Does it bother you much?" against the risk of an operation.

As the layer thickened over time, my eyesight deteriorated further, even though the decline was not too noticeable due to physiological compensation. However, eventually, I could read small print only with the help of a magnifying glass.

In the meantime, the eye clinic at the University of Freiburg has gained experience and successfully performs surgery on epiretinal gliosis. So, in spring 2021, I decided to undergo the operation, entailing a two-night hospital stay.

The surgery first involves removing the vitreous body (vitrectomy) and then carefully ablating the layer with a micro scalpel. The missing vitreous body is then replaced by a saline solution and air. As a result, I had a water surface in my field of vision during the following two weeks. Because the eye's lens reverses the image, the water level protruded into my field of vision from above. After 14 days, the eye's interior was filled with natural body fluid, and I could see clearly.

It is also known to me that, after removing the vitreous body, the lens is no longer nourished, leading to a rapidly developing cataract. Again, the vision in my eye was diminishing over time. An artificial lens can be inserted during an outpatient surgery within a quarter of an hour.

Red Baron got his lens replaced on November 9, Germany's day of fate. After some initial adaptation, I now see better with my left eye than with my right.

Here, the physics explained above comes into play. I see a clear, slightly bluish image with my left eye, while my right, non-operated eye shows a reddish image, i.e., cold light versus warm light.

The explanation is that of the sunset. In my operated left eye, natural daylight hits the retina without scattering through a cloudy lens or an old vitreous body; in the right eye, daylight is scattered by a lens that is no longer young and an old vitreous body. Mostly, the red part of the solar spectrum reaches the retina and is perceived by the brain.

The more the lens and the vitreous body are clouded, the "warmer" the vision becomes.

At the moment, I use my old glasses, as a necessary adjustment can only be made in the new year. According to my ophthalmologist, the operated eye must first get used to the new condition.

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