What is a rainbow?
Everyone loves a rainbow! Even the grumpiest person cannot fail to feel lifted when they see this magical splash of colors in the sky.
Rainbows are a meteorological and optical phenomenon and have inspired, mystified and awed people across the world for thousands of years. Even though we now know its scientific explanation, the rainbow never fails to lift a heart and cause people to pause in wonder.
This page looks at the scientific reasons for why a rainbow occurs, how a rainbow is formed and the various characteristics a rainbow can have.
Table of Contents
- When will you see a rainbow
- The colors of the rainbow
- Why rainbows are an arc
- Rene Descartes
- Primary and Secondary Rainbows
- Alexander's dark band
- Supernumerary bows
- Twin rainbows
- Rainbows in mist
- Red rainbows, moon and fog bows
- Ice halos
- Circumhorizon arc
- Circumzenithal arc
When will you see a rainbow?
For a rainbow to occur there needs to be three things:
1. You need the sun to be shining behind you.
2. There needs to be a rain cloud or moisture in the air in front of you.
3. The sun must be shining through the rain cloud at an anti solar point of 42 degrees. An anti solar point is where the shadow of your head is, directly away from the sun.
Look at this photo below....
Can you see the people sitting on the bench? They have the sun shining behind them. You can see there's rain in the cloud in front of them. The sunlight is shining through millions and millions of tiny water drops, and as it does this the light refracts and disperses to show a beautiful spectrum of colors. Read on to find out how and why this happens!
You will see a rainbow at an anti-solar point of 42 degrees
The colors of the rainbow
Why do we always see the same colors in a rainbow in the same order?
Humans see light as white, but it is actually made up of many colors that we can't see because light moves so fast.
When the beam of sunlight passes through the water droplet it slows down and spreads out, separating each color that makes up white light into a spectrum of colors. This is called "dispersion". The colors of a rainbow are always seen in this order:
RED, ORANGE, YELLOW, GREEN, BLUE, INDIGO and VIOLET
There are other many other colors, but we can't see them through our human eyes - like infrared and ultraviolet!
Each color in light is measured in wavelengths - some travel on longer wavelengths than others. Red, for example, is on a longer wavelength than violet. Red bends and changes direction when it travels through the water droplet much less than violet does. The index of refraction is the measure of speed of the wavelengths of color. You will see the colors of the rainbow in this same particular order because of the index of refraction.
If you want to learn more about the mathematics of rainbows , read this excellent guide The Calculus of Rainbows
You can see how light is dispersed through a raindrop by experimenting with a glass prism....see this video
Light refracting through a prism
Why Rainbows are an arc - understanding light refraction
As the light travels through the water drop and disperses, it also does something called refraction.
Refraction is what happens to the light when it hits the water drop, it changes speed and bends. You can see this happening in the photo here of the glass prism on the right, refraction is happening as the white light (left) bends and disperses through the droplet.
So the reason why rainbows are an arc is because of the bending of and dispersing of light through millions of water droplets.
A more in depth article about refraction can be found here: "Snell's Law of the Refraction of Light."
The discovery of why rainbows happen
Rene Descartes (1596 - 1650) was a famous philosopher who came up with the modern theory for how rainbows occur in 1637. He was the first person to make the relation between round water drops and the interaction of refracting light. This is his sketch for how primary and secondary rainbows are formed.
A primary rainbow
Primary and secondary rainbows and the anti solar point
Primary rainbows shows red as the color on the outside, leading through to violet on the inside. You will see a primary rainbow at an anti solar point of 42 degrees. An anti solar point is the shadow of your head, directly away from the sun. In the photo above you can see the primary rainbow very clearly.
Secondary rainbows occur when the water droplets in the primary rainbow reflect light twice. Because it is the reflection of the primary, the colors are reversed. The colors are not so clear in the secondary and it is generally fuzzier and flatter than the primary. This rainbow occurs at 51 degrees from your anti solar point. Take a look at the image below, this shows both primary and secondary rainbows clearly.
Double rainbows showing primary and secondary bows
Characteristics of rainbows - Alexander's dark band
Alexander's Dark Band is the name for the dark sky between the primary and secondary rainbows you can see it really clearly in the photo above. It was named by Alexander of Aphrodisias, a Greek Philosopher who first noticed these optical phenomena in 200AD.
Alexander wrote commentaries on Aristotle's works and it was in writing a commentary to Aristotle's "Meteorology" book that he mentioned the dark sky between rainbows.
Primary bows light up the sky inside of the arc, the secondary rainbow is a reflection of the primary so it brightens up the sky on the outside of the arc. This makes the sky appear darker between the bands.
Can you see Alexander's dark band here?
A Supernumerary Rainbow is a selection of smaller multiple green, pink and purple colors in the inside of the main rainbow. The extra bands are created by the interference of light waves on the water drops. The colors are lighter than those in the primary rainbow, and can change color too.
The photo above shows the Supernumerary Rainbow in full, and the photo below is a close up of the extra bands.
You can sometimes see a supernumerary on the outside the secondary rainbow, but these bands are very faint.
A very good and detailed explanation of how these bands are formed can be found at www.atopics.co.uk
How many bands can you see in this supernumerary?
Twin rainbows are joined together at the base, unlike double rainbows which are separate.
There is currently no agreed explanation for twin rainbows, there could be a variety of reasons why they occur. Some scientists believe that they may be caused by a mixture of ice crystals and water droplets. Or twin bows could because of non spherical water droplets, this makes the light refract slightly differently, resulting in a twin rainbow.
Another explanation is that the second bow is a reflection of the first, but this hasn't been concretely proved because there have been rainbows that have a reflection and a twin too!
Rainbows in dew and mist - and a rainbow experiment for kids!
This photo below shows a rainbow of crepuscular rays in sprinkler spray. A crepuscular ray is light that appears from a single point in the sky, and look like columns of light. The columns of light here are from the trees above and as they shine onto the water droplets from the sprinkler they make a rainbow.
A fun experiment is to make a rainbow with hose water spray! On a sunny day go outside and set up a hose to spray a fine mist, stand with your back to the sun and turn the water on. Remember that the anti solar point is the shadow of your head and that the angle for a rainbow is 42 degrees from this point. Move the hose slowly from the shadow of your head to what you perceive 42 degrees will be - you will then make a rainbow! The brightest rainbows are later in the day, so try this as late afternoon for the best results.
Rainbow in water spray
Red rainbows can be seen at sunset, the low sun lengthens the red and yellow wavelengths in light and scatters the shorter greens and blues.
Moonbow, Kula, Hawaii
A moon bow is a very rare occurrence and you are lucky to see one! It is caused by the moon behind you, rather than the sun. It looks like something from a dream doesn't it?
A Fog bow is formed the same way as a rainbow, but the water droplets are smaller in fog so you cannot see the colors. This ghostly arc is most commonly seen on mountains and in cold sea mists. This photo was taken on a hillside above San Francisco.
These amazing halos are made up of tiny ice crystals known as diamond dust. These ice crystals are found in cirrus clouds, which are very high up wispy clouds at about 3 -6 miles up in the sky. Each minuscule particle of diamond dust refracts the light at 22 degrees. Just like the raindrops act like prisms in dispersing the light, so do the ice crystals and a rainbow colored halo appears around either the sun or the moon. Halos are also known as glorioles or icebows and folklore says that when you see one, rain is on its way.
An ice halo made from diamond dust
The circumhorizon arc
This stunning image below is a Circumhorizon Arc, nicknamed a "fire rainbow". But it is not a rainbow! The circumhorizon arc is created by a high sun, at about 58 degrees and is created by the "diamond dust" ice particles found in high level cirrus clouds. This optical phenomena is seen in mid latitude countries, so whilst it is very rare to see this in countries nearer to the Northern Hemisphere. Do you live in Los Angeles, Houston or Melbourne? If so then you can see this beautiful sight up to 6 times over the summer, if you live in Northern Europe then to see this would be very unusual indeed!
A circumhorizon arc, or fire rainbow
A circumzenithal arc is also created by the tiny particles of diamond dust ice crystals and the sun. The ice crystals must be flat and six sided, the sun has to be angled at 32 degrees - with these conditions you can see why this is a rare occurrence! It is most commonly seen in colder climates and will often stay visible in the sky for up to 30 minutes.
A smiling sky - the circumzenithal arc
Different parts of the world see different bows more often than others, have you seen any unusual rainbows? Vote below!
© 2010 LadyFlashman
Rainbows can be explained by science, but people find a lot of pleasure in seeing them. What do rainbows mean to you?
MJ Martin aka Ruby H Rose from Washington State on June 16, 2014:
Awesome rainbows and lessons. A symbol of colorful light to brighten our world with peace. Water rainbows, my favorite.
Frugal-UK LM on June 09, 2014:
We home educate and this lens was very useful when we studied rainbows. Thankyou
seodress on July 09, 2013:
anonymous on May 14, 2013:
I love rainbows there so peaceful
Johanna Eisler on April 09, 2013:
Every time I see a rainbow, it reminds me of the promise given at the time of the first rainbow...
God said to Noah: "This is the sign of the covenant that I am giving between me and you and every living soul that is with you, for the generations to time indefinite. My rainbow I do give in the cloud, and it must serve as a sign of the covenant between me and the earth. And it shall occur that when I bring a cloud over the earth, then the rainbow will certainly appear in the cloud. And I shall certainly remember my covenant which is between me and you and every living soul among all flesh; and no more will the waters become a deluge to bring all flesh to ruin. And the rainbow must occur in the cloud, and I shall certainly see it to remember the covenant to time indefinite between God and every living soul upon all flesh that is upon the earth." (Genesis 9:12-16) :)
Maria Burgess from Las Vegas, Nevada on March 17, 2013:
Rainbows are a reminder that God is there for me and there is a blessing to be had if I just work out the challenge I am facing. Rainbows are beautiful and they make me very happy when they appear! They usually show up when I need that kind of a message. Great lens!
dean_w on March 17, 2013:
What do rainbows symbolize to me? God's love and His promise to us.
anonymous on January 13, 2013:
Rainbows remind me of happiness, because whenever i see one, my spirits lift and I feel carefree.
xtianfriborg13 on November 25, 2012:
Fascinating! I love rainbows!!
jenny-archer-9 on November 19, 2012:
peace and awe,the wonder of childhood.
FashionMommy on November 09, 2012:
it's always been a joy to see a rainbow. this just made my day. thanks for sharing!
anonymous on November 06, 2012:
What incredible photographs! I don't know that rainbows symbolize anything specific to me, but when I see one I always wonder at nature and the beauty we get to enjoy if we just take the time to do so.
fred69 on November 06, 2012:
Rainbows...the sun is back.
Tamara14 on November 05, 2012:
Too much rain in Croatia today. If only sun would interfere a little and make a nice big rainbow :)
jlshernandez on November 02, 2012:
Whenever I see a rainbow, I feel blessings are coming my way.
irminia on October 30, 2012:
Absolutely comprehensive and fantastic lens! Even moonbows and everything ...
RinchenChodron on October 30, 2012:
Celticep from North Wales, UK on October 29, 2012:
To me rainbows are a symbol of good things to come
TravelEase on October 26, 2012:
Glorious rainbows. Really enchanting.
anonymous on October 21, 2012:
@anonymous: if you'd like to read my blog post or see m pic's please use the following in you search engine and hopefully you'll be able to find me ... "20120711_Coombe Abbey Photo Wander Walk" ... cheers, Gary
anonymous on October 21, 2012:
Thanks for a super posting - Made a link on my blog as fits nicely with the end a photographic walk I did earlier in the year, Cheers again ...
UKMarkWilliam on October 19, 2012:
This post is really awesome. This is excellent and informative. Thanks for this post.
anonymous on October 18, 2012:
when we stand on a moon we dont see rainbow , why so ?
anonymous on October 14, 2012:
I don't know that much about rainbow thanks for sharing
kindoak on October 12, 2012:
Rainbows make me happy. Don't know why, just is that way. I like them even better than the northern lights we get here sometimes!
sherioz on September 23, 2012:
What fabulous photos. I always try to capture a rainbow and never manage it. Great scientific explanation also.
Tony Bonura from Tickfaw, Louisiana on September 19, 2012:
Your poll only allows for one choice, and I have seen several different types of rainbows.
irenemaria from Sweden on September 15, 2012:
Such lovely lens! I saw a full rainbow yesterday and was in awe!
radhanathswamifan on September 15, 2012:
A ray of hope:)
SheilaMilne from Kent, UK on September 14, 2012:
I was brought in Ireland up on the "pot of gold" legend. I still look for it. :) We've had several wonderful secondary rainbows over us recently so this has been a very timely as well as fascinating read.
RonandKaren on September 13, 2012:
I always wondered how a rainbow is formed so thanx for letting me know! I'll be sure to share it with little ones around me. :) Great photos too.
irminia on September 10, 2012:
It is simply uplifting to see a rainbow, I enjoy this very much. I had no idea there are so many phenomena related to rainbows. thank you.
BillyPilgrim LM on September 10, 2012:
Very nice. Still searching for that pot of gold, tho...x
Ellen Gregory from Connecticut, USA on September 09, 2012:
It's just so rare to see one after the rain, that it's a special treat. It's a splotch of different colors across the sky
Leah J. Hileman from East Berlin, PA, USA on September 09, 2012:
That God will never flood the earth again. And that light refraction is a really cool phenomenon.
Mary Norton from Ontario, Canada on September 09, 2012:
You have some unusual rainbow pictures here. Rainbows for me are dreams come true.
Doreen Katzaman on September 07, 2012:
I am always in awe of rainbows! I really enjoyed your lens. Beautiful and informative.
jmchaconne on September 05, 2012:
Rainbows are awesome, thank you for a wonderful lens. I learned somethings I'd not known about them.
anonymous on August 29, 2012:
Very interesting. I love rainbows. Whenever it is raining in the afternoon and the sun pops out, I run to the back yard to see if there is one.
Dusty2 LM on August 14, 2012:
Dew and mist bows have seen. Red, fog and moon bows have not. What interesting photos, especially the moon bow. Thank you for sharing this lens and information about rainbows. Always fascinated with rainbows and still looking for the "Pot O' Gold".
gregorymbrooks on August 14, 2012:
I remember learning about this in calculus, and then again in physics. What a great lens!
DMVAgent on August 07, 2012:
Very cool rainbow, i really like it. Rainbow of possibilities here, rainbow is life!
Jillynn on August 01, 2012:
Spirit and beauty and joy and refreshment, at a minimum. Lovely lens.
JamesDWilson on July 31, 2012:
Beautiful natural phenomena. Understanding makes them more beautiful, not less.
writerkath on July 29, 2012:
Hope, faith... Also, just JOY! I've seen double rainbows, and also parahelions (at South Pole ages ago). (I think that's what it was called. We called them Sun Dogs)
anooptu on July 26, 2012:
i like rainbow from my childhood.
7 colors making wonder on the sky.
thanks for sharing science behind rainbow !
lizholy on July 22, 2012:
Love the rainbow in Sprinker Spring... thanks for the post, I didn't even know that there were many different kinds of rainbows. Blessings!
Tigerstarr830 on July 22, 2012:
We were in New Mexico on vacation and a perfect double rainbow in it's entirety
formed in a shopkeeper's front yard!!!! Very unusual and I felt blessed to see it.
Lynne Modranski from Ohio on July 21, 2012:
My daughters and I had a rainbow "follow" us down the road while driving one day. We saw it from a distance, and then when we reached it, it stayed about one foot ahead of our front bumper for about a mile. I was in awe (still am, just thinking about it)
Thanks for a great lens!
srsddn lm on July 20, 2012:
Rainbows have always attracted me. But the information about rainbows in the lens will make rainbows a more thrilling experience after reading this lens.
OrlandoTipster on July 20, 2012:
I think rainbows are god's way of smiling on those who happen to catch sightof one
Elsie Hagley from New Zealand on July 20, 2012:
I love rainbows. You have done a great job with this lens of rainbow science.
Your photos are beautiful, enjoyed it very much. Blessed.
Fantastic Voyages from Texas on July 20, 2012:
Gosh, I didn't realize there were so many kinds of rainbows. So informative, and love the photos. Thanks for sharing!
Gayle Dowell from Kansas on July 20, 2012:
I love the sight of a rainbow after a storm. To me it symbolizes that there is hope after hardship. Blessed~
anonymous on July 20, 2012:
Awesome lens. Rainbows are such a beautiful sight to behold. Thanks for sharing.
casquid on July 20, 2012:
An appearance of a rainbow makes my day, too! This was the right article to luck up on today! Thanks!!
Tia Novak on July 18, 2012:
I feel very lucky when I see a rainbow.
Melissa Miotke from Arizona on July 10, 2012:
I like to think they're my Mom smiling down on me. I loved this lens and your beautiful pictures!
Diva2Mom on July 09, 2012:
Like a smile from Heaven, meant just for me, to cheer me up. Very educational lense, never knew so many types.
WriterJanis2 on June 25, 2012:
Incredible images and loads of info. Blessed!
Zebedee32 on June 23, 2012:
Some lovely pictures and things I did not know about. Thanks for sharing.
Lori Green from Las Vegas on June 22, 2012:
I love seeing Rainbows. I have a great picture of one over Lousiana right after Katrina.
johnny6toes on June 16, 2012:
Beautiful topic. Rainbows are just one of those things that makes you step back in awe and appreciate everything our natural world is capable of.
Indigo Janson from UK on June 13, 2012:
I came back to enjoy more beautiful rainbows.... fog bows and moon bows just make me say wow but all rainbows are magical.
Kathy McGraw from California on June 13, 2012:
I so remember this page, and that Circumhorizon Arc, which American Indians call by a different name. I saw one in the desert, and one in the San Diego area of CA...they are beautiful, and indeed rare. This is a great resource for homeschool science lessons as well as just a beautiful page for all of us ;)
randomthings lm on June 12, 2012:
Rainbows are so cool, especially when one part of the sky is dramatically stormy and the other part is sunny. What an amazing creation of nature! Very good explanation here, thanks!
waldenthreenet on June 09, 2012:
Valuable topic. Rainbows are important from both arts and science perspectives and has been so in history. Conversations for deeper understanding most welcome. Thanks.
makemoneyonline5 on May 12, 2012:
I love rainbows. I don't think anyone tires of seeing a beautiful rainbow. Wonderful lens, thank you for sharing.
avigarret on May 03, 2012:
I never knew learning about the science behind this could be so entertaining, thank you for sharing this information.
anonymous on April 27, 2012:
THE SUN OR SURYA IS THE GOD OF OUR UNIVERSE HE IS THE CREATOR FOR ALL LIVINGS IN THE EARTH.THE SINGLE RAY THAT CONTAINS COLOURS TO ALL THE OTHER UNKNOWN THINGS TO LIVES OF EARTH LIVINGS.
LynetteBell from Christchurch, New Zealand on April 22, 2012:
I love rainbows:)
Stephanie Tietjen from Albuquerque, New Mexico on April 15, 2012:
You have a great way of explaining the science. Thanks
mistyblue75605 lm on April 12, 2012:
nicely done! Thanks for sharing the info :)P
Rob Hemphill from Ireland on April 10, 2012:
Rainbows are colorful and beautiful, and make me rush for my camera. With our climate in Ireland, we see rather a lot of them!
julieannbrady on March 26, 2012:
Rainbows are colorful works of art ... like a colorful prism up in the sky!
anonymous on March 19, 2012:
I love rainbows, a marvelous creation of nature.
pinto2011 on March 19, 2012:
It is always enthralling to see a rainbow and carried away with the mythical stories but to know the truth is the essence of life and you have really carry forwarded the whole truth in the nicest way.
Dana Marie from St. Peters, MO on March 12, 2012:
God given beauty for the world to see and enjoy!
MarkHansen on March 07, 2012:
One of the most beautiful phenomenas.
Miha Gasper from Ljubljana, Slovenia, EU on February 23, 2012:
Knowing all the facts behind rainbows makes them even more beautiful.
reasonablerobby on February 20, 2012:
Brilliant. One of my favourite books in Unweaving The Rainbow by Richard Dawkins who takes issue with the romantic poets who said science was destroying the romanticism of rainbows by explaining their physics.
jimmyworldstar on February 09, 2012:
Rainbows are just beautiful works of nature. I haven't seen any fogbows but they still look aesthetically pleasing.
cmadden on December 13, 2011:
There is beauty in the science which parallels the beauty of the natural phenomenon it explains. Very nice lens.
franstan lm on August 19, 2011:
I love rainbows.
GrinningFool on August 12, 2011:
Love your favorite rainbow photo!
JoshK47 on June 30, 2011:
What a wonderful lens - and while not rainbows, that circumhorizon arc looks *amazing*. I'd make a double rainbow joke here, but I feel it's been played out. Though I suppose that still counts as a reference. lol
Linda Hahn from California on May 14, 2011:
Magic in physical form.
BeaGabrielle1 on May 12, 2011:
I love rainbows, and their biblical significance. But I never knew there were so many different types of rainbows!
sushilkin lm on May 10, 2011:
Its nice to read your informative knowledge. Thanks
anonymous on April 22, 2011:
Rainbows are an awesome sight - definite beauty of nature- thanks so much for bringing us such great information and wonderful updates !!! You made my day - its raining in MIchigan today !!!
Blackspaniel1 on April 17, 2011:
Nice one. I also have one called rainbows explained.
sheriangell on April 16, 2011:
Well this certainly cheered me up on a rainy Saturday! Thanks!
pheonix76 from WNY on April 10, 2011:
Lovely page, I am so happy I found it!! You have selected some beautiful photos, thank you for teaching us about rainbows. Cheers.
anonymous on January 08, 2011:
I love rainbows now more than ever! Amazing, wonderful, interesting, informative, beautiful and so very well done!
WebIsFun on January 07, 2011:
Rainbows for me, mean that something wonderful is about to happen
puerdycat lm on December 16, 2010:
Thanks! Wonderful! Love multiple rainbows. And I remember moonbow over Tantalus in Hawaii!
jgelien on November 23, 2010:
To me rainbows are a symbol of hope. You have some gorgeous pictures here
and I enjoyed reading all the information about the different types of rainbows.
anonymous on November 06, 2010:
WOW! These rainbows are awesome! Gorgeous!
kt_glasses on October 12, 2010:
I wish all the science teachers can teach in such a clear and fun way! Great lens!
anonymous on October 07, 2010:
Favorite colour: rainbow