A rainbow is an optical effect, a trick of the light, caused by the behaviour of light waves travelling through transparent water droplets towards an observer.
- Sunlight and raindrops are always present when a rainbow appears but without an observer, there is nothing, because eyes are needed to produce the visual experience.
- A rainbow isn’t an object in the sense that we understand physical things in the world around us. A rainbow is simply light caught up in raindrops.
- A rainbow has no fixed location. Where rainbows appear depends on where the observer is standing, the position of the Sun and where rain is falling.
- The exact paths of light through raindrops is so critical to the formation of rainbows that when two observers stand together their rainbows are produced by different sets of raindrops.
This DICTIONARY OF LIGHT, COLOUR & VISION contains a vocabulary of closely interrelated terms that underpin all the resources you will find here at lightcolourvision.org.
- Each term has its own page in the DICTIONARY and starts with a DEFINITION.
- Bullet points follow that provide both context and detail.
- Links embedded in the text throughout the site (highlighted in blue) take you directly to DICTIONARY entries.
- Shorter SUMMARIES of terms appear on DIAGRAM PAGES under the heading SOME KEY TERMS. These entries strip definitions back to basics and can be viewed without leaving the page.
Why a dictionary of light, colour & vision
- One of the practical objectives of this website is to make the connections between the topics of light, colour and vision accessible to students and researchers of all ages.
- Our DICTIONARY aims to avoid a problem faced by websites such as Wikipedia where articles are often composed by contributors with narrow specialisation and their own topic-specific vocabulary.
- The layout of the DICTIONARY also aims to avoid situations where a single unknown word or phrase makes it difficult, if not impossible, for our visitors to find the information they need (as explained below).
Terms, definitions and explanations
- All the terms we have selected for the DICTIONARY are widely used and are applied consistently across the topics of light, colour and vision.
- The aim is to avoid definitions and explanations with different meanings in different fields.
- As far as possible definitions contain no more than two short sentences.
- The explanations that follow each definition are arranged as short bullet points that avoid paragraphs of information completely.
- Each bullet makes a stand-alone point and is intended to deal with a single piece of information that we believe is likely to be important to our readership.
- The writing style across all terms aims to be clear, accessible and engaging.
- The idea is to enable our visitors to find and digest information quickly and to confirm facts one at a time.
- Because our readership and their concerns are diverse, bullet points sometimes provide different perspectives on a single term or topic.
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The angle of deflection measures the angle between the original path of a ray of incident light prior to striking a raindrop and the angle of deviation which measures the degree to which the ray is bent back on itself in the course of refraction and reflection towards an observer.
- The angle of deflection and the angle of deviation are always directly related to one another and together add up to 1800.
- The angle of deflection is equal to 1800 minus the angle of deviation. So clearly the angle of deviation is always equal to 1800 minus the angle of deflection.
- In any particular example, the angle of deflection is always the same as the viewing angle because the incident rays of light that form a rainbow all approach on a trajectory running parallel with the rainbow axis.
Remember that:
- Any ray of light (stream of photons) travelling through empty space, unaffected by gravitational forces, travels in a straight line forever.
- When light travels from a vacuum or from one transparent medium into another, it undergoes refraction causing it to change both direction and speed.
- The more a ray changes direction as it passes through a raindrop the smaller will be the angle of deflection.
- It is the optical properties of raindrops that determine the angle of deflection of incident light as it exits a raindrop.
- It is the optical properties of raindrops that prevent any ray of visible light from exiting a primary raindrop at an angle of deflection larger than 42.70.
Now consider the following:
- For a single incident ray of light of a known wavelength striking a raindrop at a known angle:
- To appear in a primary rainbow it cannot exceed an angle of deflection of more than 42.70. This corresponds with the minimum angle of deviation.
- 42.70 is the angle of deflection that produces the appearance of red along the outside edge of a primary rainbow from the point of view of an observer.
- 1800 – 137.60 = 42.0 4 is the maximum angle of deflection for any ray of visible light if it is to appear within a primary rainbow.
- 1800 -139.30 = 40.70 is the angle of deflection for a ray that appears violet along the inside edge of a primary rainbow.
- Angles of deviation between 137.60 and 139.30 correspond with viewing angles and angles of deflection between 42.40 (red) and 40.70 (violet).
- An angle of deviation of 137.60 (so viewing angles of 42.40) corresponds with the appearance of red light with a wavelength of approx. 720 nm.
- The range of angles of deflection that create the impression of colour for an observer is not related to droplet size.
- The laws of refraction (Snell’s law) and reflection and the law of reflection can be used to calculate the angle of deviation of white light in a raindrop.
- The angle of deviation can be fine-tuned for any specific wavelength by fine adjustment of the refractive index.
Viewing angle, angular distance and angle of deflection
- The term viewing angle refers to the number of degrees through which an observer must move their eyes or turn their head to see a specific colour within the arcs of a rainbow.
- The term angular distance refers to the same measurement when shown in side elevation on a diagram.
- The angle of deflection measures the angle between the original path of a ray of incident light prior to striking a raindrop and the angle of deviation.
- The term rainbow ray refers to the path taken by the deflected ray that produces the most intense colour experience for any particular wavelength of light passing through a raindrop.
- The term angle of deviation measures the degree to which the path of a light ray is bent back by a raindrop in the course of refraction and reflection towards an observer.
- In any particular example of a ray of light passing through a raindrop, the angle of deviation and the angle of deflection are directly related to one another and together add up to 1800.
- The angle of deviation is always equal to 1800 minus the angle of deflection. So clearly the angle of deflection is always equal to 1800 minus the angle of deviation.
- In any particular example, the angle of deflection is always the same as the viewing angle because the incident rays of light that form a rainbow are all approaching on a trajectory running parallel with the rainbow axis.