Author Topic: physics of Rainbow  (Read 606868 times)

candacedwan

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Re: physics of Rainbow
« Reply #15 on: May 25, 2008, 01:24:11 pm »
The effect of brightness inside the rainbow and darkness outside the rainbow intrigues me. Can you help me to understand this? Does the rainbow behave as a giant lens?

Fu-Kwun Hwang

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Re: physics of Rainbow
« Reply #16 on: May 26, 2008, 12:55:30 am »
Did you noticed that there is a intensity vs angle diagram in the simulation?
Check out the diagram and think how it is related to your question.

Nick Lu

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physics of Rainbow
« Reply #17 on: June 01, 2008, 02:30:02 am »
Dear Admin. Fu-Kwun Hwang
I've already read your article about the rainbow, and I love it pretty much!! However, I still don't quite understand why the rainbow always looks like an arch as raindrops are everywhere, why do I only receive the refraction light spectrum from that arch not anywhere else? Besides, I've no idea how the hell this happened in these pictures as I tried to think about it but just had no idea... and in picture 721e67.jpg, the color of the cloud between the two rainbows is darker than the outside part, odes it have something to do with this phenomenon?
Thank you very much!

Fu-Kwun Hwang

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Re: physics of Rainbow
« Reply #18 on: June 01, 2008, 12:51:20 pm »
Registed user can get files related to this applet for offline access.
Problem viewing java?Add http://www.phy.ntnu.edu.tw/ to exception site list
If java program did not show up, please download and install latest Java RUN TIME


We can drag the sun ray up and down and we will find ray #3 can be any angle.
If all the intensity of ray #3 are all equal, we would not be able to see rainbow.
But the intensity of ray #3 has a maximum value at particular angle (for red light near 42 degree).
The diagram in the simulation shows intensity of ray #3 and #4(secondary/outer and weaker rainbow) as function of angle.
There is a gap in which no ray will be refracted, so there is a region which will be darker in the space.

         

If we accept for horizontal sun ray, the view angle for red light will be near 42 degree (different angle for different color sun ray)
The light might enter our eye for us to see it. So all the light enter our eye with 42 degree is a cone.


If you move to a different position, you are viewing another rainbow. So the rainbow saw by different people are coming from sun ray refracted from different rain drop. (Your rainbow is for your eyes only!)

But 42 degree is for horizontal sun ray. If the sun is located at higher angle, then the angle you will find rainbow will be smaller. We will not be able to find rainbow at ground if the sun is above 42 degree (for example: at noon).
So we only find rainbow in the early morning or before sun set.

I hope the above explanation can help you understand more about rainbow. 



Registed user can get files related to this applet for offline access.
Problem viewing java?Add http://www.phy.ntnu.edu.tw/ to exception site list
If java program did not show up, please download and install latest Java RUN TIME

Fu-Kwun Hwang

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Re: physics of Rainbow
« Reply #19 on: June 26, 2008, 11:47:31 am »
Quote
Dear Fu-Kwun Hwang,
I was looking for some good explanation about rainbow physics when I found this problem and still i cannot figure out an answer to this. I've been studying some optics this year and I've been told that the primary rainbow has always the red colour on its external part. Now, as seen in picture "Rainbow1.png" the red ray is refracted downward; so the observer should perceive it as the lowest one. Could you please give me the answer to this?
 

Different color of sun light refracted from the same water drop are refracted at slightly different angle. The distance between the person who saw the rainbow and the water drop are normally very large. So different color light refracted from the same water drop would not enter the same person's eye. The red light and the blue light saw by the same person are sun light refracted from different water drop. Please check out rainbowcolor.jpg

If you move to different position, the rainbow you saw was not the same one. And different person will see rainbow refracted from different water drops. Everyone saw his own rainbow. The rainbow you saw is for your eyes only.

Fu-Kwun Hwang

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Re: physics of Rainbow
« Reply #20 on: April 02, 2009, 08:39:56 am »
The following is a snap shot from a movie try to explain "physics of rainbow". However, the explaination is not correct. Can you find out what is wrong in the attached image?
Another incorrect statement in the movie : light enter the raindrop is "totally internal reflected" which is not true.
 

Shamar712

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Re: physics of Rainbow
« Reply #21 on: May 04, 2009, 04:02:12 am »
Being new to this site and email was what I found first, I sent Mr. Hwang an email. The following is a copy of that email:

Mr. Hwang,
 
From my little understanding of rainbows, the following is not supposed to happen... but it did. In defense of myself I will say that I am an objectively led person with a background of finding problems missed by others. I am not given to fantasy and I assure you the following is true.
 
I walked through the end of a rainbow first noticed in my backyard at a distance of twenty yards. An eight foot cedar tree approx three feet behind the rainbow had a wavy appearance and its color had shaded towards grey from its normal green. Approximately ten feet from the ground the colors of the rainbow had faded and gave the apprearance of the air 'dancing'. Something like looking through a wine glass after swirling the wine and allowing it to settle down the sides of the glass.
 
As I walked through the thin bow the air turned clear on the far side and the cedar tree again took on its normal appearance. I had approached the bow at an angle of about 120 degrees (from approx twenty yards and the rainbow did not change positions) and walked thru it at an angle of about 90 degrees after observing for a moment. There was no sensation, other than awe, as I did so. I repeated the walk thru only twice as the bow dissipated.
 
I am not 'seeeeeing things'! How is this possible???
 
Note: Since writing this I have had the witness of others who state they also walked thru the end of a rainbow and found this: http://www.thenakedscientists.com/forum/index.php?topic=18551
 
Your help would be appreciated.
Shamar

Fu-Kwun Hwang

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Re: physics of Rainbow
« Reply #22 on: May 04, 2009, 09:46:41 am »
The rainbow in the above simulation is due to light enter raindrop/waterdrop , refracted twice and reflected once or twice.
The angle of the rainbow is nearly 42 degree.  And the rainbow is always at the same relative position no matter how you move. You can not pass it. Because see rainbow from different raindrop when you move.
However, there are several way to produce rainbow light pattern. e.g. light pass through a prism, the image in the URL you have provided, ...etc.

May I know what is the direction of the sun when you saw the rainbow? Is there any other strong light source around?

Shamar712

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Re: physics of Rainbow
« Reply #23 on: May 04, 2009, 10:19:54 am »
<<<Dr. Hwang, "...May I know what is the direction of the sun when you saw the rainbow? Is there any other strong light source around?">>>

I do not remember the time of day, but think it was early afternoon. The attitude of the bow was approx NE to SW, me being on the SW end of the bow. The sun was shinning at my location and the rain clouds were just to the East of me.

There was no other light source.

Shamar

lookang

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Re: physics of Rainbow
« Reply #24 on: May 04, 2009, 10:26:40 am »
found in youtube is these great videos to complement the java applet.

The applet is great but i still cannot understand some parts of the physics of rainbow.

a quick view of this video provided me with a 3-D understanding of how the history and the current day understanding of rainbow is able to account for what we see.

http://www.youtube.com/watch?v=r_hFjFM91C4
The Scientific Explanation of Rainbows Part 1





enjoy!

Shamar712

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Re: physics of Rainbow
« Reply #25 on: May 04, 2009, 11:23:35 am »
Your post is most appreciated, Lookang. I found it very interesting.

It does not seem to explain what I saw, however. I actually approached 'my bow' from outside the SW end of the bow at about 120 degrees, not in front of it. I do not recall for certain if I could see the air disturbance from the back side, but I think so - the air had a wavy appearance. I should have made my observation more precise, but... I was awe struck while trying to record what I was seeing.

I did not specifically note the arc of the rainbow as to its height, but as I remember the arc was more shallow than normally what I see.

The disturbance of the air really makes me wonder what? and why?, it is as though the air were ionized in some way. Is there a 'channel' of some sort generated in the air upon which to produce the bow itself???

Shamar712

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Re: physics of Rainbow
« Reply #26 on: May 06, 2009, 07:51:59 am »
The rainbow in the above simulation is due to light enter raindrop/waterdrop , refracted twice and reflected once or twice.
The angle of the rainbow is nearly 42 degree.  And the rainbow is always at the same relative position no matter how you move. You can not pass it. Because see rainbow from different raindrop when you move.
However, there are several way to produce rainbow light pattern. e.g. light pass through a prism, the image in the URL you have provided, ...etc.

May I know what is the direction of the sun when you saw the rainbow? Is there any other strong light source around?


Mr. Hwang,
I do hope you have not dismissed my experience (or perhaps I did not understand your answer), some of the peripheral info may be in error, but I assure you the basic info is fact - I stood in that rainbow and passed through it twice. On the back side I did not see any other rainbow nor did I see a different bow at any time during the experience. That bow I did see was stationary the entire time.

Facts:
1. I stood in the bow and it was stationary
2. Standing in front of the bow, on the sun side, I could see the air 'slowly dancing' all the way to the ground. It was obvious, there was a rippling effect which distorted the air.
3. The cedar tree behind the bow had shaded towards grey when looking thru the bow and had a rippled effect.
4. The colors of the bow faded to clear from a few feet above my head (at least I could see no colors from ten feet down)

The following is a copy of an email I sent, in response to a request for additional info, to Kathy Coil @ noao.edu, there is more info:

Kathy,

I'll do the best I can, below, from memory, most of the info will only be
close. I have a rough understanding of  rainbow formation and the various
angles involved..., but, no understanding of how I am able to stand in a
rainbow. Is there any known explanatory geometry of a rainbow that would
allow for this???
.....I assure you, I stood in that rainbow, walked thru it and observed the
'wavy' effect in the air very closely (standing close I pondered what effect
it might have if I walked into it, so I first put my hand into the bow)...
as well as its effects on the cedar tree, as described, behind the bow.
.....The bow had faded its color from about ten feet off the ground and what
I was seeing was an obvious wavy effect in the air - no colors, just 'slow
dancing' air.

Time of day, not certain, but early afternoon in summer - sun had to be to
the NW. Weather: A light rain had passed from west to east, cloudy bright
and the sun was coming full - the remainder of the rain clouds were thin and
passing. It was not raining on me and the rain had passed by about a mile.
There were no other light sources, large trees or obstacles near, the
closest, a tree, at about forty yards to the NW and well below the angle of
the sun. (Sun angle? - I think it was in June near the equinox. Angle of the
arc to the sun (?), not sure, seems it was not straight on. It appeared to
me the bow was 90 degrees to the ground, or straight up. )

Stepping from a covered deck, going east, I saw the bow and that its end was
twenty yards from me to the SE. As I walked to the bow, it did not change
position, it remained stable.The bow ran from NE to SW, I was at the SW end
with the other end disappearing over a hill (width of the bow inknown - no
real reference) at perhaps a half mile. (I was in a valley approx a quarter
mile wide, on the west side of the valley.) Drawing a line from the base, to
the NW, of the bow perpendicular to the arc and calling it 90 degrees with
zero degrees being inside the arc, I approached the bow from 120 to 135
degrees and then stood directly in front of it within two feet observing the
interference in the air..

The base of the bow was narrow, about four feet wide, and the bow was very
thin from front to back, only a few inches. I stepped back and forth thru it
twice, slowly, before it disipated... and as I remember the sun was coming
full at that time.


Fu-Kwun Hwang

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Re: physics of Rainbow
« Reply #27 on: May 06, 2009, 10:22:47 pm »
I understand that you are talking about the experience you have (what you saw and what you feel).
The reason that I asked those question was I hope to be able to find more information.

If you saw "rainbow pattern" at the same location as you walk through. It means that there is a some kind of screen so that light can be reflected back to your eye.
We see object because light reflected from surface of the object and enter our eye (unless object is a light source).
Because the "rainbow pattern" you saw is not a light source, so it must be some kind of "screen" to be able to reflect light.
And I have not figure out why/how the rainbow pattern was formed from your information.
I hope someone else can provide a better explanation.
In case you see the same "rainbow" again, please remember to take some picture.
I believe it will help if we can see those picture!

Shamar712

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Re: physics of Rainbow
« Reply #28 on: May 07, 2009, 03:09:15 am »
I understand that you are talking about the experience you have (what you saw and what you feel).
The reason that I asked those question was I hope to be able to find more information.

If you saw "rainbow pattern" at the same location as you walk through. It means that there is a some kind of screen so that light can be reflected back to your eye.
We see object because light reflected from surface of the object and enter our eye (unless object is a light source).
Because the "rainbow pattern" you saw is not a light source, so it must be some kind of "screen" to be able to reflect light.
And I have not figure out why/how the rainbow pattern was formed from your information.
I hope someone else can provide a better explanation.
In case you see the same "rainbow" again, please remember to take some picture.
I believe it will help if we can see those picture!


"...must be some kind of "screen" to be able to reflect light": My perception or thought at the time was that the air was inonized in some way and wondered if there were a 'channel' formed in the air - the ripple effect continued upward but was no longer visible as the colors of the bow became visible... not that it wasn't there. It was as though the colors of the bow were set upon the 'ionized' air. (???)

The following link, also hearsay, was something of interest I found:
http://jenniverse.com/impossible%20things%20rainbow.html

As circumstancial as the evidence is, it does appear there are stable rainbows that must have some sort of 'anchor' to the ground and to a location in the air... and perhaps over the entire course of the bow. I've often wondered what the other end of the rainbow looked like. (?)

Also, another witness to the same effect, I had asked if he noticed any anomaly in the air..... I did have to prompt him, however, he said after he thought about it there was also a 'ripple' effect in the air. He also stated that the air was not without color at the base as was my experience, rather there were colors to the ground as in the above link.

There is something 'strange' going on here - my curiosity is certainly piqued. My thought is there must be some sort of channel/"screen" or 'circuit' generated in the air as a foundation for the bow to permit such an experience to happen. (???)

Believe me, if such an occurrence happens again, and I am looking :o), I will find my camera in a hurry! You will have a video.

I appreciate the dialogue
Al

Emosie

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Re: physics of Rainbow
« Reply #29 on: August 23, 2009, 10:37:10 am »

re: rainbow halos and other

hiya i was hoping u could give me a bit of info about certain rainbows
i have a rainbow in my backyard its allways there no matter what time it is
(but u see it best if there is a bit of shade as when it is very bight its almost clear) if u spray the hose in my backyard
a rainbow appears but not just any rainbow one of these halo rainbows and its stands vertically not horizontally n close to the  ground
some times several more halo rainbows appear in side of it.
i thought that was  strange enough but Ive also noticed that there is one at night around 1am that circles the moon
any info is highly appreciated