Rainbows as Discs of Colour

$0.00

This is one of a set of almost 40 diagrams exploring Rainbows.


Each diagram appears on a separate page and is supported by a full explanation.

  • Follow the links embedded in the text for definitions of all the key terms.
  • For quick reference don’t miss the summaries of key terms further down each page.

Description

Rainbows as Discs of Colour

TRY SOME QUICK QUESTIONS AND ANSWERS TO GET STARTED
Yes! If each of the bands of colour that forms a primary rainbow is thought of as a disc then the brightness of each colour drops off sharply at its circumference.
It is useful to think of the bands that form a rainbow as discs of colour because the brightness of any specific colour seen by an observer drops off to nothing at the circumference but drops off more slowly towards the centre.

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:
    • 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.
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.

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.

Rainbows are at their best early morning and late afternoon when a shower has just passed over and the Sun is illuminating the curtain of raindrops formed on the trailing edge of the falling rain.

Diagrams are free to download

Downloads: Slides or Illustrations


DOWNLOAD DIAGRAMS
  • SLIDES are optimized for viewing on-screen.
  • ILLUSTRATIONS are optimized for printing on A4 pages in portrait format.
SLIDES
  • Slides are available in JPG and AI (Adobe Illustrator) file formats.
  • Titles: Slides have titles.
  • Backgrounds: Black.
  • Size: 1686 x 1124 pixels (3:2 aspect ratio).
ILLUSTRATIONS
  • Illustrations are available in JPG and AI two file formats.
  • Titles: No titles.
  • Backgrounds: White.
  • Size: 1686 x 1124 (3:2 aspect ratio). So all illustrations reproduce at the same scale when inserted into Word documents etc.
  • Labels: Calibri 24pt Italic.

File formats: JPG & AI


DOWNLOAD THE DIAGRAM ON THIS PAGE AS A JPG FILE
  • JPG (JPEG) diagrams are 1686 x 1124 pixels (3:2 aspect ratio).
  • If a JPG diagram doesn’t fit your needs, you can download it as an AI (Adobe Illustrator) file and edit it yourself.
  • JPG files can be placed or pasted directly into MS Office documents.
DOWNLOAD THE DIAGRAM ON THIS PAGE AS AN AI file
  • All AI (Adobe Illustrator) diagrams are 1686 x 1124 pixels (3:2 aspect ratio).
  • All our diagrams are created in Adobe Illustrator as vector drawings.
  • Save as or export AI files to other formats including PDF (.pdf), PNG (.png), JPG (.jpeg) and SVG(.svg) etc.

Spelling: UK & US


We use English (UK) spelling by default here at lightcolourvision.org.

COPY & PASTING TEXT
  • After copy/pasting text please do a spell-check to change our spelling to match your own document.
DOWNLOAD DIAGRAMS
  • Download AI versions of diagrams to change the spelling or language used for titles, labels etc.
  • We are adding American English (US) versions of diagrams on request. Just contact us and let us know what you need.
  • When downloading JPG versions of diagrams, look out for JPG (UK) or JPG (US) in the download dialogue box.

Download agreement


DOWNLOAD AGREEMENT

Light, Colour, Vision & How To See More (https://lightcolourvision.org) : Copyright © 2015-2022 : MediaStudies Trust.

Unless stated otherwise the author of all images and written content on lightcolourvision.org is Ric Mann.

ALL RIGHTS RESERVED

No part of this website may be copied, displayed, extracted, reproduced, utilised, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical or otherwise including but not limited to photocopying, recording, or scanning without the prior written permission of MediaStudies Trust.

EXCEPTIONS

Exceptions to the above statement are made for personal, educational and non-profit purposes:

Before downloading, cutting and pasting or reproducing any information, images or other assets found on lightcolourvision.org we ask you to agree to the following terms:

  1. All information, images and other assets displayed and made available for download on the lightcolourvision.org website are copyright. This means there are limitations on how they can be used.
  2. All information, images and other assets displayed or made available for download are solely and exclusively to be used for personal, educational and non-profit purposes.
  3. When you find the resources you need, then part of the download process involves you (the user) ticking a box to let us (at lightcolourvision.org) know we both agree on how the material can be used.
  4. Please contact [email protected] before considering any use not covered by the terms of the agreement above.

The copyright to all information, images and all other assets (unless otherwise stated) belongs to:

The Trustees. MediaStudies Trust
111 Lynbrooke Avenue
Blockhouse Bay
Auckland 0600
New Zealand
[email protected]

We love feedback

Your name and email address will be used solely to provide you with information you have specifically requested. See our privacy policy at https://lightcolourvision.org/privacy/.


We welcome your feedback 🙂









    Note: The feedback form records the URL of the current page


    Thank you so much for your time and effort