Higher the Sun, Lower the Rainbow
Higher the Sun, Lower the Rainbow
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About the Diagram
An overview of rainbows
- Atmospheric rainbows:
- 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.
About the diagram
- In this diagram the light source is the Sun, the observer is shown as an eye and the rainbow appears to the observer as a small arc of colours low on the horizon.
- The diagram shows that when a rainbow forms, the light source, observer and the centre-point of the arcs of rainbow colours are always on the same axis.
- In this case, the lower half of the rainbow is missing because when droplets of rain hit the ground they stop reflecting light.
- A small arc of the rainbow is visible to the observer because the Sun is high in the sky.
- In the right conditions, a rainbow can form a complete circle but the ground usually gets in the way.
- The position of a rainbow is always determined by the fact that the Sun, observer and the anti-solar point (the centre of a rainbow) are all on the same axis.
- The red arc is on the outside of the bow because the illustration is of a primary rainbow.
- An arrow marks the angle between the axis and the red arc seen by the observer. The angular distance for red is always around 420. The angle for violet is always around 400. The exact angle depends on the strongest wavelengths of light visible to the observer at any particular moment.
About the diagram: Sun, observer and anti-solar point
The exact position at which an atmospheric rainbow will appear in the sky can be anticipated by imagining a 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 imaginary line that joins the Sun, observer and the centre of the rainbow is called the rainbow axis.
- The point on the rainbow axis around which a rainbow appears is called the anti-solar point. The centre of a rainbow coincides with the anti-solar point.
- Stand with the Sun on your back and look at the ground on a sunny day, the shadow of your head marks the point called the antisolar point, it is 180° away from the Sun.
- The red arc of a primary bow forms at an angle of 42.40 from the rainbow axis.
- Seen from an observer’s point of view, the angle outwards from the rainbow axis to the coloured arcs is called the viewing angle.
- In diagrams, the same angle between the axis and a line extended from an observer’s eyes to the arcs of a rainbow is called the angular distance.
- With the Sun behind you, spread out your arms to either side or up and down to get a sense of where a rainbow should appear if the conditions are right.
- Unless seen from the air, the centre of a rainbow and the anti-solar point will always be below the horizon.
- 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 at an angle of 53.40 and red on the inside.
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