Dictionary of light, colour & vision

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 ...

Alexander's band (Alexander's dark band) is an optical effect associated with rainbows. The term refers to the area between primary and secondary bows that often appears to be noticeably darker ...

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 ...

(1) The angle of deviation measures the angle between the direction of an incident ray and the direction of a refracted ray when light travels from one medium to another ...

Angular distance is the angle between the rainbow axis and the direction in which an observer must look to see a specific colour within the arcs of a rainbow. Angular ...

The area inside a primary rainbow The area inside the arcs of a primary rainbow, from its centre-point out to the violet band at 40.70 appears tonally lighter or brighter ...

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: ...

Rainbows form when sunlight encounters a curtain of rain. The sunlight enters raindrops at one angle and then emerges at another. The water droplets have to be in just the ...

The fact that we see a few distinct bands of colour in a rainbow, rather than a smooth and continuous gradient of hues, is sometimes described as an artefact of ...

There are three basic conditions that have to be fulfilled before an atmospheric rainbow appears: Bright sunlight shining through clear air. A curtain of falling rain in the near to ...

Rainbows can be modelled as a set of six nested cones with the apex of each aligned with the lenses of an observer's eyes. Each cone has a different radius ...

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 ...

Distance to a rainbow Rainbows are formed from the millions of individual raindrops that happen to be in exactly the right place at the right time, so it is difficult ...

The form and composition of rainbows are often explained in terms of electromagnetic waves. Electromagnetic waves consist of coupled oscillating electric and magnetic fields orientated at 900 to one another ...

There are many optical effects similar to rainbows. A fog bow is a similar phenomenon to a rainbow. As its name suggests, it is associated with fog rather than rain ...

An idealised raindrop forms a geometrically perfect sphere. Although such a form is one in a million in real-life,  simplified geometrical raindrops help to make sense of rainbows and reveal ...

The term impact parameter refers to a scale used on a ray-tracing diagram to measure the point at which incident rays strike the surface of a raindrop. Rays are given ...

A typical atmospheric rainbow includes six bands of colour from red to violet but there are other bands of light present that don't produce the experience of colour for human ...

The path of light through a raindrop is a key factor in determining whether it will direct light towards an observer and contribute to their perception of a rainbow. This ...

The best light source for a rainbow is a strong point source such as sunlight. Sunlight is ideal because it is so intense and contains all the wavelengths that make ...

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 weather, season and time of day are all important if you hope to see an atmospheric rainbow. The best rainbows appear in the morning and evening when the Sun ...

When an observer looks up into the sky and sees an atmospheric rainbow they are looking at tiny images of the Sun mirrored in millions of individual raindrops. This is ...

The minimum angle of deviation of a ray of light of any specific wavelength passing through a raindrop is the smallest angle to which it must change course before it ...

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 ...

Primary rainbows are sometimes referred to as first-order bows. First-order rainbows are produced when light is reflected once as it passes through the interior of each raindrop. Secondary rainbows are ...

The most common atmospheric rainbow is a primary bow.  Primary rainbows appear when sunlight is refracted as it enters raindrops, reflects once off the opposite interior surface, is refracted again ...

The term rainbow angle is often paired with rainbow ray to measure the angle at which light is deflected back towards an observer as it passes through a raindrop. At ...

Rainbow colour refers to the colours seen in rainbows and other situations where visible light separates into its component wavelengths and the corresponding hues become visible to the human eye ...

Rainbows are composed of rainbow rays. Rainbow rays are responsible for an observer's perception of a rainbow. Rainbow rays are rays of light of a single wavelength that have their ...

Rainbows result from light encountering raindrops in the presence of an observer. The phenomenon of rainbows offers many clues as to the nature of light. Light is a form of ...

A ray of light (light ray or just ray) is a common term when talking about how and why rainbows appear. The idea that light is made up of rays ...

Tiny images of the Sun mirrored in millions of individual raindrops create the impression of bands of colour arching across the sky when an observer sees an atmospheric rainbow. Rainbows ...

Rainbows can be formed by droplets of liquids other than water, or even by a cloud of solid transparent microspheres. The table below shows the viewing angles for primary rainbows ...

An idealized raindrop forms a sphere. These are the ones that are favoured when drawing diagrams of both raindrops and rainbows because they suggest that when light, air and water ...

Raindrops, incident light and primary rainbows Let's look at the rays of incident light that contribute to a primary rainbow. All rays of light that contribute to a primary rainbow ...

Polarization of electromagnetic waves refers to the geometrical orientation of their oscillations. Polarization restricts the orientation of the oscillations of the electric field of electromagnetic waves to a single plane ...

In real-life, full-size raindrops don't form perfect spheres because they are composed of water which is fluid and are only held together by surface tension. In normal atmospheric conditions, the ...

Not all incident light striking a raindrop crosses the boundary into the watery interior of a droplet. Some of the incident light is reflected off the surface and a small ...

An important optical effect that explains how raindrops produce rainbows is refraction. Refraction refers to the way that electromagnetic radiation (light) changes speed and direction as it travels across the ...

Scattering takes place when streams of photons (or waves of light) are deflected in different directions.  In this resource, the term is used to refer to the different forms of ...

A secondary rainbow appears when sunlight is refracted as it enters raindrops, reflects twice off the inside surface, is refracted again as it escapes back into the air, and then ...

Make sure you are always carrying your camera or phone with you. Natural rainbows are rarer than you might think? You may get your photo on the first day, or ...

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 ...

Supernumerary rainbows are faint bows that appear just inside a primary rainbow. Several supernumerary rainbows can appear at the same time with a small gap between each one. The word ...

To properly understand rainbows involves referring to different fields of enquiry and areas of knowledge. The field of optics tells us that rainbows are about the paths that light takes ...

The viewing angle of a rainbow is the angle between a line extended from an observer's eyes to a bow's centre point and a second line extended out towards the ...

Wavefronts Parallel electromagnetic waves with a common point of origin, the same frequency and phase, and propagating through the same medium, produce an advancing wavefront perpendicular to their direction of ...

Perhaps the most common of atmospheric effects, the blueness of the sky, is caused by the way sunlight is scattered by tiny particles of gas and dust as it travels ...

If we understand why the sky is usually blue it's easier to understand why it can be filled with reds and pinks at sunrise and sunset. Let's review why the ...