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Better One Or Two? Br J Ophthalmol 2004;88:733 733 Br J Ophthalmol: first published as 10.1136/bjo.2004.045500 on 17 May 2004. Downloaded from COVER ILLUSTRATION ......................................................... Better one or two? s the largest terrestrial lens, the sun sets with the proper conditions, is a atmosphere produces myriad red sun, a yellow sun, and eventually a Ainteresting effects, but perhaps green sun or green ray or flash, although none so fabled as the green flash. And occasionally a blue flash will be seen. yet, for all of its stories, its description is Visual physiology may play a subtle but recent. Any society can that can con- meaningful part in explaining why photo- struct structures like the Pyramids or graphs are not as brilliant as perception. Stonehenge for astronomical purposes The photoreceptors are a bit more sensi- must surely have observed the green tive to the wavelengths of green and flash, yet it was not described in writing slightly less so to red. As the sun sets, the until 1836 when Captain George Back photoreceptors sensitive to red will be on the HMS Terror wrote about seeing it stimulated to fire, and are bleached while on an expedition to the Arctic somewhat, and this adaptation will pro- (Meinel, Meinel, Sunsets, Twilights, and duce the sensation of a more intense Evening Skies, Cambridge: Cambridge green than is actually present. University Press, 1983). Later, the phe- Sometimes, the green flash is merely nomenon was mythologised when Jules a green rim just above the sun’s yellow Verne wrote of it in a book entitled Le upper limb, but sometimes it is a brief Rayon Vert. He described a controversial emerald green so captivating and Norse legend regarding the phenom- charming as to be nearly mystical, as enon which states that anyone who can be seen on the cover. There can be a witnesses a green flash will always be variety of subtle phenomena, including true ‘‘in matters of the heart.’’ Little rays, rims, and other ethereal sights, evidence, however, exists that this although these phenomena are usually indeed was a Norse legend. Sometimes there are even multiple very brief or very difficult to see. With Nevertheless, the green flash is layers of alternately cold and warm air, proper atmospheric conditions and tele- legendary and many suspect that it does much like a wedding cake with layers of scopic magnification (be careful, not actually exist, that it is an after- denser icing between layers of spongy remember that Galileo developed solar image. This suspicion is belied by the cake. retinopathy because of his telescopic two images on the cover showing Light rays are slowed and bent observation of sunspots, BJO June 2001 different aspects of the same phenom- towards the denser medium and thus cover), a green rim can frequently be http://bjo.bmj.com/ enon. A camera cannot record an after- can be ducted in a curved manner. seen along the upper limb of the setting image, and furthermore, the Additionally, when there are layers of or rising sun for several seconds. But phenomenon has also been witnessed cold dense air topped by warmer less remember, the green flash is just that— at dawn before sunrise. So, what else dense air, the equivalent of a prism a quiet, peaceful, emotionally powerful might be at work? exists—an atmospheric prism. glimpse so extraordinary as to be unbe- Our atmosphere has density that When the sun sets, white light is lievable. Perhaps that is why it was not varies with altitude, but other factors directed tangentially through the earth’s described until Captain Back. on September 24, 2021 by guest. Protected copyright. can change the density too. Usually, atmosphere towards the observer. As Our cover images, then, show two there is a gradually declining tempera- these light rays pass through the atmo- different aspects of this curious event. ture gradient with altitude, but in sphere, they are bent towards the surface The photograph at the top was taken certain circumstances, such as presence (cooler, denser) especially over cold water. over the Malagasy strait and shows a of an inversion layer or layers, there may This bending of the light rays will displace true flash the instant after sunset. The be significantly different, and warmer, the image of the sun vertically, making it photograph at the bottom was taken temperatures in various cake-like layers appear to the observer to be higher in the over a cold Pacific Ocean following a hot well above the surface. This is especially sky than it actually is. Simultaneously, summer day with little air mass move- true above surface features that are this white light is divided into its colour ment. This created a layered atmosphere consistent in temperature and contour, components with red being refracted the with at least two significant layers of such as large bodies of water or deserts. least, as the longest wavelength, and cold and warm air alternating with one Over large bodies of open water, such violet being refracted the most, as shortest another creating two prisms resulting in as the ocean, especially where there are visible wavelength. As the sun sets, red this beautiful double green flash. Both currents creating colder than expected rays will be the first to disappear because water, there can be significantly differ- the longer wavelengths are not refracted photographs were taken using a tele- ent temperatures in the layered atmo- andbentasmuchastheshorterwave- scope. Better one or two? sphere. The first few metres of air lengths. The next colour is yellow and is I R Schwab immediately above the water level will followed by green, blue and the last, University of California, Davis, Sacramento, be colder than the air above it. Since air violet. But the molecular grouping of the CA, USA; [email protected] is an excellent insulator, and cold air is atmosphere will scatter shorter wave- Photograph at the top by E Vic & Jen Winter heavier and denser, the air mass nearest lengths more, and hence the shorter (www.icstars.com). Photograph at the bot- the ocean tends to stay in place even wavelengths—blue and beyond––are tom by the author. Thanks to Jay Pasachoff when the sun warms the air above it. almost never seen. What is left, as the for review of the essay. www.bjophthalmol.com.
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