Nadine and I used to live on the beach here in Mazatlan, and every night without fail we noticed our neighbors sitting on their patios with their glasses of wine, waiting for the sun to set. Several times we joined them, and every time we were there, they made and effort to observe the green flash that is supposed to occur just as the sun sets. At almost every sunset, they got excited about witnessing this green flash, while I'm sorry to say I was never really able to see it myself. This got me interested in the green flash phenomenon, so I decided to look into it on the world wide web. By far the best www site describing the green flash is by Andrew Young at http://mintaka.sdsu.edu/GF/ which includes a much better and more detailed explanation that I am going to give, plus simulations of what is actually going on and a set links to photographs and references.
Now, for my former neighbors and other green flash watchers, I'm afraid there is good news and bad news. The good news is that green flashes do really exist, and have in fact been photographed. See the site above for still and video images of the green flash captured on camera. The bad news is that I doubt that my friends on the beach ever actually saw one. If you doubt me, I challenge you to get out your video cameras and record a sunset where you believe you've seen a green flash, and then play it back on your TV to confirm if there actually was one. After you've done that, reread this article and you'll believe me.
To understand what causes green flashes, you have to understand two properties of light, called refraction and dispersion. Don't give up here, because these things aren't really that difficult. Let's start with refraction. We all know that light travels in straight lines. This is true in empty space, but not completely true when light interacts with matter. Just stick a pencil in a glass of water at an angle, and you'll see for yourself that the pencil appears to be bent at the surface of the water. This is because the light traveling through the water goes slower than the light traveling through the air. Let's not get into the details of why this happens, but remember this important fact about refraction: When light travels through matter it bends towards the denser material.
Okay, now lets take a look at dispersion. The light we get from the sun, which we perceive as white, is really a combination of all the colors of the rainbow. You can confirm this by taking a very close look at your color television set, and noticing that it really consist of a bunch of tiny sets of red, green, and blue dots. Areas on your TV set that appear white have all three dots equally illuminated. Dispersion is the phenomenon that as white light passes through matter, the different colors that make up white light go through the matter at slightly different speeds, and thus separate. This is what a prism does, and also the raindrops that cause a rainbow. The amount of dispersion also depends on the density of the material, and since water is much denser than air, we see rainbows only when it rains. Even ordinary air causes dispersion, but about one thousand times less than water, which is why our eyes don't generally perceive it.
Now the standard explanation of the green flash at sunset is this: As the sun sets, the dispersion in the atmosphere causes the white light from the sun to separate into its constituent colors. Since the sun is very low in the sky, the shorter wavelengths of light (the green and blue) are scattered away. See my article on why the sky is blue for an explanation of this. Thus the setting sun is perceived as red. However, in the instant just as the sun sets on the horizon, the red portion of these dispersed colors sets first, and the blue is scattered more strongly than the green, so just for an instant, the green is visible by itself, hence the green flash phenomenon. Unfortunately, as Prof. Young graphically demonstrates at his www site, this is largely bunk. Well, its not really bunk, it really does happen, but the effect is so small, that it is invisible to the naked eye. In order for us to see the green flash it has to be magnified.
Now there is an easy way of magnifying the sunset, just get out a pair of binoculars or a camera with a zoom lens and watch it though that. Probably better to use a camera, so you can capture the flash as you see it. When you do, and you develop the film, nine times out of ten you will see a nice yellow flash on the film, not green, even though you could swear that you saw a green flash while you were watching. Why? Well, the sun is a very bright object, and looking at it directly tends to bleach out the cones in your retina, especially the ones sensitive to longer wavelengths, such as red. Remember Roy G. Biv? Those are the colors of the rainbow: red, orange, yellow, green, blue, indigo, and violet. As you watch the sun set, the red and orange colors set first, and in the process bleach out the cones in your retina sensitive to these longer wavelengths. Thus as the yellow sets, the eye perceives it as emerald green. You click the shutter and get a great shot of the yellow flash, swearing it was green. I believe that this is what happened to my friends at the beach. After staring at the sun while it was setting, and bleaching their cones, they saw green where there wasn't any.
So finally, if these green flashes really do exist, how do they happen? There isn't really a single simple answer, but rather there are several similar but different mechanisms at work. For the real nitty gritty details, I again refer you to Prof. Young's www site. I will describe here the most common cause of a real green flash sunset, and then explain why I doubt that my friends have ever witnessed one.
Did you remember our main fact about refraction? When light travels through matter it bends towards the denser material. Now let's apply this to our atmosphere. In general, the atmosphere gets denser the closer you are to the surface of the earth. Why? Because the weight of the air on top compresses the air on the bottom. This means that light traveling through the atmosphere is continuously bent downward, in other words it curves down towards the earth. Thus the light we see at the horizon is actually coming from a place slightly below the horizon, because of this downward curvature. Suppose now that the air near the surface of the earth is warmer than the air above. Warmer air is less dense than cooler air, as any hot air balloonist will tell you. Thus light traveling through this warmer air will curve upwards, towards the denser cooler air above it. This is what causes a kind of mirage known as the inferior mirage, so called because the inverted mirage image appears below the erect image. You have seen this a thousand times, when you were driving on a hot day and the air temperature near the pavement was higher than the air above it. Even though refraction is responsible for this, as you were driving it seemed as though a mirror had been placed on the road, and you were seeing a reflection of the sky. The shimmering came about because of fluctuations in the temperature and hence density of the air just above the road surface. If you've ever seen a person walking into this mirage area, you would have also found that at some point he appeared to be stretched in the vertical direction.
Okay, what have we learned in the previous paragraph? Under certain conditions, namely when the air temperature near the surface of the earth is higher than the air above it, the layer of hot air acts like a mirror that stretches images in the vertical direction. (It also compresses them in the horizontal direction, but I haven't explained why, and we'll ignore that.) As the sun nears the horizon, we will see the bottom of the sun be reflected and soon join the top of the sun, making a shape like the Greek letter omega (see fig 1). This is your cue that you will really witness a green flash. Just before the top of the sun dips below the horizon, there will come a few seconds of time when the image of the tip of the sun and its reflection is greatly stretched in the vertical direction. At this point, the different colors caused by dispersion are greatly magnified, and easily visible to the naked eye. Look for the omega, and warn your friends that a green flash is about to take place. There are other mechanisms that can cause a visible green flash, but they are much less common than the inferior mirage.
Here in Mazatlan, the air temperature, both winter and summer, is generally quite a bit warmer than the ocean temperature at sunset, thus the condition required for this mirage will be quite rare. That's not to say it never happens, but I think the wine and the retinal bleaching are much better explanations for my neighbors frequent sightings.
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