Light travels in a straight line and at a given speed through any medium, be it in a vacuum, air, water or glass. When light passes from one medium to another, however, light waves are deflected if they hit the boundary between the two media at an oblique angle. This occurs because the speed of light in one medium is different than its speed in the other. As a result, a spoon sticking out of a glass of water appears bent to the human eye.
Not only does the speed of light change when it goes through different media, it changes when the physical properties of a medium change. When the change in the medium is gradual, occurring over a long distance, the refraction occurs little by little. Light rays do not bend abruptly at one point but end up describing a curved path. This explains the formation of mirages where the air is very hot near the ground, as in deserts or on asphalt. The bending of light rays will cause objects to look farther away or closer than they really are.
In the science of optics, a mirage is defined as “an optical illusion caused by atmospheric conditions.” The most frequent mirage effect seen by most people is in the image of the sun when it is low in the sky at sunrise and sunset. For rays from the sun to reach the surface of the earth they must pass through the atmosphere, where pressure and temperature conditions change gradually from the uppermost layers to the ground level. This changes the speed with which light from the sun passes through the layers of air and bends the rays of sunlight. When the path through the atmosphere is longer, the bending of the rays is larger and the displacement of the image of the sun from its true position caused by the mirage effect is greater. Due to this effect, the sun can often still be seen when its true position is below the horizon. See The Formation of a Mirage and “Atmospheric Refraction” in Our View of the Sky. An interactive application showing the true and apparent position of the sun over the horizon depending upon altitude, atmospheric pressure, and temperature can be accessed at Atmospheric Refraction Applet — Apparent Flattening of the Sun.
Refraction within a medium not only depends upon pressure and temperature, but it also depends on the wavelength (color) of the light. This occurs because different colors are refracted differently. Click here for a graphic representation of the refraction of white light into its various colors.
The different refractions of the various colors within sunlight results in the appearance of different “solar discs”, one for each color of the rainbow. The blue, violet and green light from the sun refracts so that the disc-images of these colors are slightly higher than the yellow disc, which is the brightest of them all. The blue, violet and green discs are normally not distinguishable for several reasons. During most of the day, the yellow disc outshines the others. However, at sunrise or sunset, when light from the sun must pass through the maximum amount of atmosphere to reach the viewer, that is, when refraction is at its greatest and the strength of the yellow disc has dimmed, the opportunity to see the different colored disc-images is at its height. But still, the discs are not clearly apparent. The violet and blue discs are those refracted most, and therefore the last to set on the horizon. However, blue light is dispersed away in the atmosphere (this is why the sky is blue in daylight) and leaves the blue image of the solar disc very weak, if visible at all. The violet disc is usually very weak as well and it is, therefore, difficult to distinguish. This leaves the green disc being the last to set but the power of the yellow disk is still very strong and the green disc can be seen only in the fleeting instant in which the yellow disc has already set and the last part of the green disc appears above the horizon. This is what is called the green flash. The term “flash” is not very appropriate, as it raises expectations for a bright flash of light. It also probably influenced the filmmakers of Pirates of the Caribbean into depicting it as such.
A great collection of images of the green flash in some of its manifestations can be seen at Green Flash. Note that the subsequent pages of this site can be accessed by clicking on the arrows in the upper right corner of the website. The gallery includes photographs taken with exceptional atmospheric and visibility conditions that favor the viewing of even the blue and violet discs. For additional websites showing visuals of the green flash, see Wikipedia; Scientific Explorations and Earth Science: Picture of the Day.
The displacement of the green (and blue and violet solar discs) is, of course, present well before the instant the sun disappears behind the horizon. That instant is a good moment to see them without the brighter yellow disc outshining them. However, a photographic or video recording of the setting sun can show, when viewed close-up and slowed down, that the green disc becomes visible amidst the turbulent image of the setting sun even while the sun is above the horizon. Watch it in seconds 21 to 30 of a beautiful video on the sky of the island of Tenerife, Spain: Canary Sky – Tenerife.
Be sure to caution students: Take care of your eyes. Never stare at the sun and never look at it through binoculars, a telescope or the optical viewfinder of an SLR camera – your eyesight could be permanently damaged.