THERE’S SCIENCE IN THE SETTING SUN (2-5): Green Flash.
Image Credit & Copyright: Allen Vinson. CLICK image for larger view and look below for information and related links.
Another great event in sunsets/rises is the optical phenomena known as the Green flash!
As the sun sets lower and lower on the horizon many of the shorter light wavelengths are scattered away as we discussed in part one. Also in part one do you remember what I left you with? That’s right the next longest of the ROY G BIV wavelength colors; GREEN. Starting to make sense now right? As the Sun begins to touch the horizon these colors come into view again as the bottom of the Sun’s disk will be a dark red. As you work your way up it changes to orange then yellow in the order of longest to shortest color wavelengths.
When using magnification you may notice a faint green ring form around the top of the Sun’s disk known as the “Green Rim” while it’s setting. As the last fraction of the Sun is setting the mirage effect on the atmosphere momentarily magnifies the Green rim’s light and presents it as a flash of green at the top of the Sun. It sometimes appears as a ray or a “fire” detached above the solar disk itself.
This isn’t the entire explanation as there are actually a few different types of Green rays, rings, flashes etc. each having slightly different causes. In the extremely rare case, enough blue light remains to show up as a Blue flash but not much is known about the details of their appearance yet.
Green flashes can be seen at any latitude or altitude. In fact, when flying westward many pilots claim to see brilliant Green flashes as the westward flight slows the sunset. It’s preferred that you have a clear open horizon with no obstructions though they have been seen over mountain tops etc. It’s never recommended that you observe the Sun with the unaided eye, so when looking or photographing it’s much safer to look through the display screen and avoid the viewfinder altogether. Just use the daytime Moon or some other distant object (ship, land etc.) to focus at infinity. Seeing a green flash is pretty difficult but knowing where and when they occur will really help you score one. If you look too soon, the Sun will blind you and if you look too late or even blink at the wrong time you will miss it. That’s why sunset is almost always preferred over sunrise because you have milestones in the form of the Sun itself to alert you to how close this one to two second event will be. Clouds, haze and heavy pollution will all but end your Green flash attempts.
Hyperphysics Red Sunset & Green Flash: http://hyperphysics.phy-astr.gsu.edu/hbase/atmos/redsun.html
Introduction to Green Flashes: http://mintaka.sdsu.edu/GF/