Rainbows as Discs of Colour
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This is one of a set of almost 40 diagrams exploring Rainbows.
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Description
Rainbows as Discs of Colour
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About the diagram
Overview of rainbows
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:
- 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.
Thinking of rainbows as discs of colour
Rainbows can be modelled as six concentric two-dimensional discs as seen from the point of view of an observer. Each disc has a different radius and contains a narrow spread of colours. The red disc has the largest radius and violet the smallest.
- The colour of each disc is strongest and most visible near its outer edge because this is the area into which light is most concentrated from the point of view of an observer.
- This concentration of light near the outer edge of each disc results from the path of rainbow rays.
- The term rainbow ray describes the path that produces the most intense experience of colour for any particular wavelength of light passing through a raindrop.
- The intensity of the colour of each disc reduces rapidly away from the rainbow angle because other rays passing through each raindrop diverge from one another and so are much less concentrated.
- The divergence of rays of light after exiting a raindrop is often called scattering.
- From the point of view of an observer, the six discs are superimposed upon one another and appear to be in the near to middle distance in the opposite direction to the Sun.
- There is no property belonging to electromagnetic radiation that causes a rainbow to appear as bands or discs of colour to an observer. The fact that we do see distinct bands of colour in the arc of a rainbow is often described as an artefact of human colour vision.
- To model rainbows as discs allows us to think of them as forming on flat 2D curtains of rain.
- Rainbows are often modelled as discs for the same reason the Sun and Moon are represented as flat discs – because when we look into the sky, there are no visual cues about their three-dimensional form.
- Each member of the set of discs has a different radius due to the spread of wavelengths of light it contains. This can be explained by the fact that the angle of refraction of rays of light as they enter and exit a droplet is determined by wavelength. As a result, the radius of the red disc is the largest because wavelengths corresponding with red are refracted at a larger angle (42.40) than violet (40.70).
- From the point of view of an observer, refraction stops abruptly at 42.40 and results in a sharp boundary between the red band and the sky outside a primary rainbow.
- The idea of rainbows being composed of discs of colour fits well with the fact that there is a relatively clear outer limit to any observed band of colour.
Some key terms
A human observer is a person who engages in observation by watching things.
- In the presence of visible light, an observer perceives colour because the retina at the back of the human eye is sensitive to wavelengths of light that fall within the visible part of the electromagnetic spectrum.
- The visual experience of colour is associated with words such as red, blue, yellow, etc.
- The retina’s response to visible light can be fully described in terms of wavelength, frequency and brightness.
- Other properties of the world around us must be inferred from patterns of light.
Rainbow colours are the bands of colour seen in rainbows and in other situations where visible light separates into its component wavelengths and the spectral colours corresponding with each wavelength become visible to the human eye.
- The rainbow colours (ROYGBV) in order of wavelength are red (longest wavelength), orange, yellow, green, blue and violet (shortest wavelength).
- The human eye, and so human perception, is tuned to the visible spectrum and so to spectral colours between red and violet. It is the sensitivity of the eye to this small part of the electromagnetic spectrum that results in the perception of colour.
- Defining rainbow colours is a question more closely related to the relationship between perception and language than to anything to do with physics or scientific accuracy.
- Even the commonplace colours associated with the rainbow defy easy definition. They are concepts we generally agree on, but are not strictly defined by anything in the nature of light itself.
- Whilst the visible spectrum and spectral colour are both determined by wavelength and frequency it is our eyes and brains that interpret these and create our perceptions after a lot of processing.
On a sunny day, stand with the Sun on your back and look at the ground, the shadow of your head coincides with the antisolar point.
- The anti-solar point is the position on the rainbow axis around which the arcs of a rainbow appear.
- 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.
- The idea that a rainbow has a centre corresponds with what an observer sees in real-life.
- As seen in side elevation, the centre-point of a rainbow is called the anti-solar point.
- ‘Anti’, because it is opposite the Sun with respect to the location of an observer.
- Unless seen from the air, the anti-solar point is always 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.
- First, second, fifth and sixth-order bows all share the same anti-solar point.
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