Atmospheric rainbows summary

  • Rainbows form when sunlight encounters a curtain of rain.
  • The sunlight enters raindrops at one angle and then emerges at another.
  • The water droplets have to be in just the right place to reflect rays into an observer’s eyes.
  • Each raindrop is made of liquid water and acts as a tiny prism.
  • Raindrops break sunlight into distinct red, orange, yellow, green, blue and violet rays.
  • Rainbows can be described as being both atmospheric and optical phenomena.
Remember that:
  • If the Sun is directly behind you, rain is falling in front of you, and you look straight ahead, then you will see that the rainbow forms around a centre-point.
  • The centre-point of a rainbow is often referred to as the anti-solar point.
  • The anti-solar point, your eyes and the Sun are always in line with one another – on the same axis.
  • Anti means opposite, opposed, or at 1800. So anti-solar means a point opposite to the Sun as seen by an observer.
  • The axis of a rainbow is an imaginary line drawn between the Sun, observer and anti-solar point.
  • When sunlight and raindrops combine to make a rainbow, they can make a whole circle of light in the sky.
  • Rainbows only form a complete circle when the ground doesn’t get in the way. This only happens when you are on a plane.
  • Whenever something blocks sunlight then a shadow forms and a portion of a rainbow disappears.
  • Even if you stand on a mountain peak, the bow forms less than a circle because the mountain creates a shadow.
  • Your own shadow can get in the way of a rainbow formed by the spray from a hose or lawn sprinkler.
  • Seen from the air, the shadow of your plane is often visible at the centre of the rainbow. The further away the curtain of rain is on which the bow forms, the smaller the plane appears.
  • At ground level, the main reason rainbows don’t form a complete circle is because when droplets hit the ground they stop reflecting light so the rainbow comes to an end.