A Moment of Science

Why Are Rainbows Curved?

The narrow range of angles at which the colors are reflected is the key to why rainbows must be curved.

Double rainbow over lake

Photo: Kansas Poetry (Patrick) (flickr)

Rainbows, like this double rainbow seen on Clinton Lake, are curved because of the angle at which the light is reflected.

To see why rainbows are curved it helps to understand what causes them in the first place.  We see rainbows when rain is falling in front of us and the sun is shining behind us.

When sunlight enters raindrops most of it passes straight through.  But some of the light strikes the drop at such an angle that it’s separated into the rainbow colors and reflected back out the front of the drop.  Each color is reflected at a slightly different angle, but all between forty and forty-two degrees.

The narrow range of angles at which the colors are reflected is the key to why rainbows must be curved.  The next time you see a rainbow notice that you are in the center of the arc, and that if you move the rainbow moves with you.  This is because even though all the raindrops out there are reflecting rainbow colors, they do so in very narrow beams.  You see the colors from only those drops whose beams shine on you.

The only shape that will allow the light beams to shine on you at the precise angles is an arc or a circle with you at the center so that you are always the same distance and angle from any point on the arc.

  • http://profiles.yahoo.com/u/IL3OISGI4NFVXCCS77IZV55AMI Roslyn

    This rainbow looks like it's actually framing the rising or setting sun. Is it possibly a sun dog?

  • Anonymous

    that was very clear, thanks

Stay Connected

What is RSS? RSS makes it possible to subscribe to a website's updates instead of visiting it by delivering new posts to your RSS reader automatically. Choose to receive some or all of the updates from A Moment of Science:

Support for Indiana Public Media Comes From

About A Moment of Science

Search A Moment of Science