Rainbow Anatomy

An atmospheric rainbow is an arc or circle of spectral colours and appears in the sky when an observer is in the presence of strong sunshine and rain.

• Atmospheric rainbows:
  • Are caused by sunlight reflecting, refracting and dispersing inside raindrops before being seen by an observer.
  • Appear in the section of the sky directly opposite the Sun from the point of view of an observer.
  • Become visible when millions of raindrops reproduce the same optical effects.
• Atmospheric rainbows often appear as a shower of rain is approaching, or has just passed over. The falling raindrops form a curtain on which sunlight falls.
• To see an atmospheric rainbow, the rain must be in front of the observer and the Sun must be in the opposite direction, at their back.
• A rainbow can form a complete circle when seen from a plane, but from the ground, an observer usually sees the upper half of the circle with the sky as a backdrop.
• Rainbows are curved because light is reflected, refracted and dispersed symmetrically around their centre-point.
• The centre-point of a rainbow is sometimes called the anti-solar point. ‘Anti’, because it is opposite the Sun with respect to the observer.
• An imaginary straight line can always be drawn that passes through the Sun, the eyes of an observer and the anti-solar point – the geometric centre of a rainbow.
• A section of a rainbow can easily disappear if anything gets in the way and forms a shadow. Hills, trees, buildings and even the shadow of an observer can cause a portion of a rainbow to vanish.
• Not all rainbows are ‘atmospheric’. They can be produced by waterfalls, lawn sprinklers and anything else that creates a fine spray of water droplets in the right conditions.

There are several particularly noticeable things to see when looking closely at rainbows:

• The arcs of spectral colours curving across the sky with red on the outside and violet on the inside, this is a primary rainbow. The arcs appear between the angles of approx. 40.7° and 42.4° from the centre (anti-solar point) as seen from the point of view of an observer.
• There may be another rainbow, just outside the primary bow with violet on the outside and red on the inside, this is the secondary rainbow. The arcs appear between the angles of approx. 50.4° and 53.4° from its centre as seen from the point of view of an observer.
• Faint supernumerary bows often appear just inside a primary rainbow and form shimmering arcs of purples and cyan-greens. These bands appear at an angle of approx. 39° to 40° from the centre so just inside the violet arc of the primary bow.
• The remaining area inside a rainbow from its centre out to approx. 39° often appears lighter or brighter in comparison to the sky outside the rainbow. There are three main causes:
  • Light strikes multiple droplets in succession and randomly scatters in all directions.
  • Small amounts of light of all wavelengths are deflected towards the centre and combine to produce the appearance of weak white light.
  • Almost no light is deflected to the area outside a rainbow.
• When a secondary rainbow appears, the area between the two often appears to be darker in tone than any other area of the sky. This is called Alexander’s band. The effect is the result of rays being deflected away from this area as primary and secondary bows form.

To understand rainbows it is important to sort out what an observer is actually looking at.

• Rainbows only exist in the eyes of an observer.
• Every observer sees a different rainbow produces by a unique set of raindrops that happen to be in the right place at the right time.
• The individual raindrops that result in the appearance of a rainbow for one observer are always different from the raindrops that produce a rainbow for someone else.
• As an observer moves, their rainbow moves with them. Seen from a car window, the rainbow appears stationary whilst the landscape rushes past.

From an observer’s point of view

• Atmospheric rainbows appear to an observer as arcs of colour across the sky. From an aeroplane, a rainbow can appear as entire circles of colour.
• Even from the ground, it is easy to deduce that every rainbow has a centre point around which the arcs of a rainbow are arranged.
• The exact position in the sky where an atmospheric rainbow will appear can be anticipated by working out where its centre will be.
• The centre of a rainbow is always on an imaginary straight line that starts at the centre of the Sun behind you, passes through the back of your head, out through your eyes and extends in a straight line into the distance.
• The eyes of an observer are always aligned with the rainbow axis.
• To an observer, the rainbow axis appears as a point, not a line, and that imaginary point marks the centre of where every rainbow will appear.
• The idea that a rainbow has a centre corresponds with what an observer sees in real-life.
• The idea of a rainbow axis or anti-solar point corresponds with a diagrammatic view showing the scene in side elevation.

Looking for rainbows

• To work out where a rainbow might appear:
  • Turn your back on the Sun.
  • If you can see your shadow, look at the head. The axis of the rainbow runs from the Sun behind you, through your eyes and through the head of your shadow. Imagine where your eyes might be in your shadow. If a rainbow appears that point will be its centre.
  • If you can’t see your shadow, just try and imagine the line from the Sun, passing through your head and then extend it away from you till it reaches the landscape. At whatever point it touches, that will be the centre.
  • Unless you are in a plane, the centre point is always below the horizon so on the ground or in the landscape in front of you.
  • Now, with the Sun behind you spread out your arms to either side or up and down at 45⁰ from the centre point.
  • Swing them round like the blades of a windmill. That is where your primary rainbow will appear.

Remember that:

• Every observer has a rainbow axis and a centre-point on that axis that moves with them as they change position. It means that their rainbow moves too.
• The centre of a secondary rainbow is always on the same axis as the primary bow and shares the same anti-solar point.
• To see a secondary rainbow look for the primary bow first – it has red on the outside. The secondary bow will be a bit larger with violet on the outside and red on the inside.

Rainbows as discs of colour

• Close consideration of why rainbows appear as arcs or circles can be explained by the idea that an observer is looking at superimposed, concentric discs of colour.
• Think in terms of each observed band of colour within a rainbow forming on the edge of a separate coloured disc.
• The area close to the circumference of each disc produces the most intense and brilliant colour.
• The intensity of each colour drops sharply away from the circumference of its disc and towards the centre.
• The observed colour of each disc corresponds with the band of wavelengths that produces it.
• The fact that we see distinct bands of colour in a rainbow is often described as an artefact of human vision.
• Each disc contributes small amounts of its own colour to the area towards the shared centre of the six concentric discs making the sky appear lighter.