How Rainbows Form

How Rainbows Form

Surely you’ve seen a rainbow. Maybe you were chasing
tornadoes and happened upon one. But did you ever notice that the
sky is darker above a rainbow than below? I never did until
someone pointed it out. We’re going to find
out why this is so with the help of refraction
of light, dispersion, and the incomplete
story you’re usually told about why rainbows form. This story starts
with refraction. The bending of light
as it moves from one medium, like air,
to another medium, like water, at an angle. Light actually
slows down in water. Visible light would
take 11 minutes, instead of the usual eight, to get
from the sun to the Earth if there were a giant
ocean in between. So as light slows down
when it enters water, the beam is forced to turn. But unlike the constant
speed of light in a vacuum, the speed of light in
water depends on the color. Violet light moves slower
in water than red light. So it will bend more
as it enters the water. An incoming beam
of white light will be spread out and separated
into a spectrum of color. We call this dispersion. Now, let’s take this
white light beam and send it through
a droplet of water. Some of the light will
refract into the droplet, then reflect off the
back, and refract back out to travel down to
your eye, typically, at an angle of 42 degrees
between the incoming light and the outgoing light. This can occur in a
circle of droplets. So if the ground
weren’t in the way, rainbows would be a full circle. Here’s the fun part–
violet light refracts more in the water droplet than
red light, as we discussed. So violet light should
end up exiting higher in the water droplet than red. But if we look in
the sky, red light is clearly higher
than violet light. What’s going on? Well, if you’re
observing from down here, this water droplet will only
send red light to your eyes and the violet light
will pass above you. It’s the droplets down here
that send violet light to you. And the violet light will
appear to be coming from lower in the sky, as it does. And that’s how we get our
colors in the rainbow from rain and sun. End of story. But there’s one part
I never understood. Why should all of the light
enter the water droplet right at this point? Well, the answer is, it doesn’t. Some red rays enter
here, some enter here, and they all exit in
different directions. If we add back in
the other colors, you end up with an existing
jumbled mess of color. This mess of color
should look white, but it definitely doesn’t. So what’s going on? This is where it gets more
complicated than you usually hear. As this red light ray enters
higher from the midpoint, it exits at an
increasingly large angle. But watch what happens
here– eventually, the ray turns around. No matter where red
light enters the droplet, it can’t go past this angle. That’s the maximum angle and
it’s 42 degrees for red light. It’s slightly less for
orange and yellow and so on, until you reach violet, where
the max angle is 40 degrees. So it turns out that
at that maximum angle, you also get a
maximum brightness. So even though all the
colors can reach here round violet’s max angle,
because their maximum is beyond that, violet is at
its maximum brightness. So it stands out against
the other colors. Same goes for the max
of the other colors. Below violet, no colors
are at a maximum. So all the colors mix
again and form white. That’s why the sky appears
lighter below the rainbow than above. None of the light can
make it out above red, so the sky appears darker. Then how did this second
rainbow sneak up there? Well, you’ll notice the
colors are flipped in order and, again, that the sky
is lighter above them. That indicates that the
light had to reflect twice inside the droplet. In order to get the right
angle down to your eye, it actually had to
enter the droplet from below the midpoint. And you get a double reflection
causing a second rainbow. Now this story is complete. The end. [THEME MUSIC]

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100 Replies to “How Rainbows Form”

  1. A Brief summary of everything she really means. Save myself, and YOU some time!! 🙂

  2. Science can’t explain how rainbows are formed because our Heavenly Father created it and only he knows 🙏☝️❤️

  3. Before seeing the video – "I already know the science behind rainbows so lets just skip." But then I realise "Its PHYSICS GIRL. There's definitely a catch here".

    After the video "Aaaah so that's how it is. As expected of Miss Diana"

    Happy Physicsing !

  4. Surely a firmament above us explains this so much better. Sorry but any explanation I hear for rainbows never rings true. Probably because it isn't true.

  5. There Are also some further effects from DIFfraction as well as the REfraction mentioned here. These show up when the raindrops are quite small and mostly the same size.

  6. I'm surprised she didn't use this video to reveal she is from Hawaii. Hawaii has a high incidence of rainbows. I saw on average 3 per day there where as back in the midwest I may see 3 per year. After she spent a lot of time in Boston at MIT she probably realized how rare they are other places.

  7. Interesting facts, there is a nice video of an full circle rainbow optical phenomenon filmed from airplane perspective at the luftfahrtportal YouTube channel.

  8. 😕😕😕 so there is group of rain droplets in the sky which done this bt how? If they fall then 😟😡😦

  9. The maximum angle made by a light ray with the normal of a surface to accomplish refraction is called critical angle. If the angle is made greater than this point it would reflect. This phenomenon is also called total internal reflection(TIR).
    Anyways thanks a lot for your explanatory videos

  10. When a ray of light changes a medium which in turn changes its velocity……why is it that the frequency remains constant and the wavelength changes but not the other way around??????

  11. Some parts of this woman's dispute is an actual mince. I can believe she encouraged herself to say it publicly. Poor kids

  12. There's always reflected light from a wet ground or river/lake/sea in a rainbow. This is not mentioned. I don't think modern science understands rainbows yet.

  13. Here's some math:


  14. This is not true, you need light water and a refractor to create a rainbow, inside experiments you need a mirror as the refractor the make one, the firmament is the mirror outside

  15. Physics girl is useless, dishonest and gutless. Wont say how the colours form into a bow. She knows they will not do that without a reflector. And that means admitting there is a dome above. She cannot admit the truth. No credibility physics girl..

  16. How can we have a rainbow from light bouncing from moving droplet suspended in midair, troll. It thinks it knows everything, yet only talks..

  17. Gee. I almost feel smarter, even more knowledgeable, but, no. What can I do with this intriguing tidbit? Oy! What a sharp cookie is @thephysicsgirl

  18. The truth is , the rainbow is God's promise he will not destroy the world with water again. Genesis 9:13 . God is the maker of the rainbow.

  19. BS – The Story is not complete as you left out one main factor you can't make a Rainbow with water without a mirror which happens to be the Firmament above the Earth outside. So your story is a bunch of hogwash as you didn't say anything about the Firmament.

  20. still can't understand…water drops are dropping constantly making their own rainbow …how that adds to a giant one?

  21. Thanks Physics Girl, my Physics students always tell me that they love how you explain complex Physics terms, with simple language they can relate to, absolute gold.

  22. we see rainbow color as red color from one drop and violet from other but what if the observer change its position slightly towards violet

  23. Enlightening content, sparkling delivery, full spectrum scope. Critics are either too dense to absorb the burst or misogynistic interference pattern turns their eyes into double slits.

  24. Rubbish
    The light is reflecting off millions of droplets of water in the sky all in different positions
    It would be like you suggested a mess of colour
    Full circle rainbows are visible from above only
    Proof for the dome ?
    You can’t create a rainbow without a dome artificially

  25. Many many years ago I was flying an aeroplane quite low, about 3000ft in rainy conditions when I saw a rainbow completing a full circle.
    It didn't show up that well over the ground but it was there.
    Every time I have seen a rainbow from the air since I have looked but never again seen this phenomenon.
    It happens but why is it so rare.

  26. Very good video on the formation of rainbows! One of the first I saw years ago. And I still learned something seeing it this time!

  27. Genesis 9:12-13 
    And Yahweh said, “This is the sign of the covenant that I make between me and you and every living creature that is with you, for all future generations: I have set my bow in the cloud, and it shall be a sign of the covenant between me and the earth

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