Absorbtion vs refraction

[cirwin]

here's the rundown on the green flash:

Although the sun appears yellow, it actually emits light of all wavelengths (red, orange, green, blue, ultraviolet, etc.). Each of these wavelengths of light is refracted slightly differently (in physics parlance, each wavelength has a different index of refraction). When sunlight is passed through raindrops, the difference in the way each color bends has the effect of spreading the light out into the familiar rainbow. The spreading of the light due to the differences in refractive index is called dispersion. Due to the angle of the sun and the amount of atmosphere it has to travel through, we normally don't notice this from sunlight. At sunset, however, the amount of atmosphere that the sunlight travels through is large enough such that the dispersion becomes apparent at the fringes of the sun. Therefore, at sunrise or sunset (when we see the edge of the sun), we can sometimes see the light split into its component colors. This would normally look like several rainbow-colored crescent slivers of the sun but, as I mentioned in my previous post, the short wavelengths (blue and violet) tend to get scattered by the atmosphere and do not come straight through to the eye. This leaves green as the next color to be seen, and it appears slightly above the red disc of the sun (since green bends by a different amount). This is the so-called green flash and it only lasts as long as it takes for enough sun to set (or rise) so that we aren't just seeing a fringe of the sun.

I hope this makes sense!

 

[Crimson Ghost]

Thanks - it does make sense and I really appreciate you taking the time to post it. I hadn't taken into consideration the position of the sun and the angle to which the light is originating. I am leaving for Hawaii (yea me) this Saturday - I need to swing into boarders to get some reading material...I think I am going to look for some light reading on this -- I get a little crazy when I want to understand something outside my comfort zone thanks again !!!

 

[discrepancy]

OK, maybe this should be a new thread and maybe someone will move it for me, but it seems all the competent Scubaboard atmospheric physicists are here, so can I please pose a related meteorological question?

The other day I was standing on a point overlooking Sydney Harbour, just on sunset. The sky was overcast, with low cloud cover everywhere and a 2000-3000' base. Myself and a colleague stood looking East across the harbour and out to sea as the sun sank behind and slightly to our left. We watched incredulously as a double rainbow formed, with the end of the rainbow a few hundred yards away right there on the water.

We stood there gobsmacked. The end of the rainbow was so close I suggested that jumping into the harbour for a quick photo op.

There was no pot of gold at the end of the rainbow, just still, black water amid the moored boats. Over the next few minutes as the sun set over my left shoulder, we watched the end of the rainbow move directly away from us. It moved directly away from us right over the top of a small island in the middle of the harbour, and it kept on moving away behind the island and toward another headland and finally out into the ocean. When it was just behind the island and then again behind the headland, I could not confirm that the rainbow ended in the water because it was obscured where it hit the horizon. For the rest of the time we could see it terminating right in the water in front of us.

It only took about 10 minutes for the rainbow to move right across the harbour and all the way out to sea. All the time there was a faint double rainbow high and to the left of the strong main rainbow.

I have no idea what I saw. But it was spectacular, and I will remember it for life.

P.S. There were no little green Leprachauns in sight either.

 

[cirwin]

discrepancy,

the rainbow you saw at sunset sounds like quite a sight! here is the physics behind what happened: rainbows are caused by a combination of relfection off of the back of water droplets (rain) and refraction (and since the different colors refract slightly differently, this causes dispersion (spreading out) of the white light into different colors (the familiar red, orange, yellow, green, blue, violet). any time the sun is at your back, and there is water in front of you, rainbow phenomena can be seen. you can replicate this on a sunny day with a spray of water from a hose. the most surprising part of the rainbow you saw was probably that it occured when there was no rain. the cloud cover you described makes me think that the rainbow was caused not by rain, but by virga. virga is precipatation (rain) that falls from clouds, but evaporates before it hits the ground.

as far as the movement of the rainbow, this is easily explained by the fact that the sun was moving (setting). the reflection/refraction phenomenon happens at a very specific angle. if you were to measure the angles, you would find that there is approximately a 42 degree angle between your vantage point and the rainbow in any direction (any water 42 degrees away from you will look like a rainbow. in fact, in an airplane, if you see a rainbow, it will be a full circle, since there is no ground to block the water 42 degrees below the horizon of your field of vision). if you move 100 feet to your left or right, you will still see that there is a 42 degree angle between you and the rainbow. when you move around, you are seeing light coming from a different set of raindops, so the rainbow moves as you do. this means two interesting things: 1) everyone sees a different rainbow (it looks the same, but comes from different water drops); 2) you can never get to the end of the rainbow - it will just keep moving away from you. as the sun set and moved lower toward the western horizon, the 42 degree angle between the water droplets, you, and the sun has to occur from water droplets that are farther away from you. since the movement of the sun is much more apparent at sunset (you will notice that it really moves a quite a clip in the last 30 seconds), the rainbow you saw was moving at a relatively high speed.

finally, the double rainbow occurs in very strong sunlight when all of the light bounces off of the back of a raindrop, about 96% of it comes out of the side facing you, and the other 4% bounces back in the other direction and gets fed into another raindrop and produces its own rainbow. since this secondary rainbow has been reflected, its colors are reversed. the faintness is due to the fact that only about 4% of the light goes into the second rainbow. this partial reflection is easy to see when you look out of a window from a lighted room at night. anybody outside can see you (the light from you is transmitted by the glass), but you can also see your own reflection (because some of the light bounces off of the glass instead of going through).

sorry for the lengthy reply, but you saw alot of cool physics!

cheers,

chris

 

[Diver Dennis]

Chris, I learned a lot from your posts. Thanks.

 

[Cacia]

yes, nice green flash explanation! I wait for those every chance I get.

 

[pete340]

finally, the double rainbow occurs in very strong sunlight when all of the light bounces off of the back of a raindrop, about 96% of it comes out of the side facing you, and the other 4% bounces back in the other direction and gets fed into another raindrop and produces its own rainbow. since this secondary rainbow has been reflected, its colors are reversed. the faintness is due to the fact that only about 4% of the light goes into the second rainbow. this partial reflection is easy to see when you look out of a window from a lighted room at night. anybody outside can see you (the light from you is transmitted by the glass), but you can also see your own reflection (because some of the light bounces off of the glass instead of going through).

For what it's worth, I had heard a different explanation: The second rainbow comes from light that's reflected twice within a single droplet.

About thirty years ago I saw a triple rainbow over Boston. Didn't think to take a picture.

 

[cirwin]

pete340,

right you are. in my haste to post that explanation, i made a typo. i meant to write that 4% "bounces back in the other direction and gets fed into the raindrop." for whatever reason, i wrote "another," instead. mea culpa...

thank you for setting the record straight.

cheers!

 

[discrepancy]

Thanks for that cirwin. You were right. There had been a lot of drizzle about during the day, and the harbour air was hanging thick with water. A rain front had moved out to sea to the East, and the setting sun behind us was shining strongly through clear new air. Sounds a bit like your garden hose example.

If good things come in threes, then I'm due for one more spectacular rainbow soon. A week before the harbour rainbow, I saw a rainbow at night! First time in my life. Coming back in the boat after a night dive in Vanuatu there was a very clear rainbow over the sea. This time, it was one huge white moon shining all the light. And again there was a faint double rainbow.

What more could you ask for...drinking up the spectacle of a tropical moon rainbow, while basking in the afterglow of walm, balmy night dive in the South Pacific, a cold beer waiting at the bar, and the prospect of the same joys all over again the next day?