A Rainbow is an Optical Phenomenon


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.


A Rainbow is an Optical Phenomenon

The axis of a rainbow, the rainbow axis, is an imaginary straight line that connects the light source, observer and anti-solar point. The centre of a rainbow is always on its axis. The centre of a rainbow always corresponds with the anti-solar point.
Yes! A rainbow can form a complete circle when seen from a plane.
Yes! If droplets are large, 1 millimetre or more in diameter, red, green, and violet are bright but blue is hardly visible.
Rainbows appear when bright sunshine is refracted, reflected and dispersed in raindrops in the presence of an observer.

About the Diagram

An 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.
About the diagram

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.
Atmospheric rainbow summary
Visual processing

Visual processing is a complex and dynamic process that involves interactions between various retinal cells, neural pathways, and brain regions, ultimately leading to conscious visual perception.

Visual processing begins the moment light enters the human eye. It then progresses through multiple stages as signals travel towards the visual cortex, where the neural activity is integrated, resulting in conscious visual experience.

As visual processing begins the retina starts to process information about colors, as well as basic information about the shape and movement associated with those colors. By the end of this stage, multiple forms of information about a visual scene are ready to be conveyed to higher brain regions.

Let’s examine two major forms of processing, trichromatic and opponent-processing, which occur within the eyeball as visual information is gathered from light entering our eyes.

Trichromacy, also known as the trichromatic theory of colour vision, explains how three types of cone receptors in the retina work together with bipolar cells to perform their role in the initial stage of colour processing. Rod cells also play a significant role in this form of processing visual information, particularly in low-light conditions.

Opponent-processing, also known as the opponent-process theory of colour vision, explains the second form of processing. Opponent-processing involves ganglion cells that process the data received from trichromatic processing and combine it with other intercellular activities.

It is interesting to note that as both trichromatic and opponent-process theories developed over the last century, researchers and authors have often pitted one theory against the other. However, both processes are crucial for understanding how colour vision occurs.

Trichromatic theory explains the encoding of visual information when light hits the retina, while opponent-processing explains a subsequent stage of information convergence, assembly, and coding before the data leaves the retina via the optic nerve.

Note that:

  • Both trichromatic and opponent-processing occur independently within each retina, without comparing with the other.
  • Each eye gathers information from a specific viewpoint, approximately 50 mm to the left or right of the nose.
  • The two impressions are later compared and combined to provide us with a single three-dimensional, stereoscopic view of the world, rather than two flattened images.

We can consider the layers of retinal cells involved in trichromatic and opponent-processing as examining, interpreting, and transmitting visually relevant information. However, it would be incorrect to view this as a straightforward linear process due to the intricate neural networking, cross-referencing, and feedback loops within the retina.

Some key terms

The visible part of the electromagnetic spectrum is called the visible spectrum.

A human observer is a person who engages in observation by watching things.

Refraction refers to the way that electromagnetic radiation (light) changes speed and direction as it travels across the interface between one transparent medium and another.

  • As light travels from a fast medium such as air to a slow medium such as water it bends toward the normal and slows down.
  • As light passes from a slow medium such as diamond to a faster medium such as glass it bends away from the normal and speeds up.
  • In a diagram illustrating optical phenomena like refraction or reflection, the normal is a line drawn at right angles to the boundary between two media.
  • A fast (optically rare) medium is one that obstructs light less than a slow medium.
  • A slow (optically dense) medium is one that obstructs light more than a fast medium.
  • The speed at which light travels through a given medium is expressed by its index of refraction.
  • If we want to know in which direction light will bend at the boundary between transparent media we need to know:
  • Which is the faster, less optically dense (rare) medium with a smaller refractive index?
  • Which is the slower, more optically dense medium with the higher refractive index?
  • The amount that refraction causes light to change direction, and its path to bend, is dealt with by Snell’s law.
  • Snell’s law considers the relationship between the angle of incidence, the angle of refraction and the refractive indices (plural of index) of the media on both sides of the boundary. If three of the four variables are known, then Snell’s law can calculate the fourth.

Visible light is the range of wavelengths of electromagnetic radiation perceived as colour by human observers.

Reflection takes place when incoming light strikes the surface of a medium, obstructing some wavelengths which bounce back into the medium from which they originated.

Reflection takes place when light is neither absorbed by an opaque medium nor transmitted through a transparent medium.

If the reflecting surface is very smooth, the reflected light is called specular or regular reflection.

Specular reflection occurs when light waves reflect off a smooth surface such as a mirror. The arrangement of the waves remains the same and an image of objects that the light has already encountered become visible to an observer.

Diffuse reflection takes place when light reflects off a rough surface. In this case, scattering takes place and waves are reflected randomly in all directions and so no image is produced.

Internal reflection takes place when light travelling through a medium such as water fails to cross the boundary into another transparent medium such as air. The light reflects back off the boundary between the two media.

  • Internal reflection is a common phenomenon so far as visible light is concerned but occurs with all types of electromagnetic radiation.
  • For internal refraction to occur, the refractive index of the second medium must be lower than the refractive index of the first medium. So internal reflection takes place when light reaches air from glass or water (at an angle greater than the critical angle), but not when light reaches glass from air.
  • In most everyday situations light is partially refracted and partially reflected at the boundary between water (or glass) and air because of irregularities in the surface.
  • If the angle at which light strikes the boundary between water (or glass) and air is less than a certain critical angle, then the light will be refracted as it crosses the boundary between the two media.
  • When light strikes the boundary between two media precisely at the critical angle, then light is neither refracted or reflected but is instead transmitted along the boundary between the two media.
  • However, if the angle of incidence is greater than the critical angle for all points at which light strikes the boundary then no light will cross the boundary, but will instead undergo total internal reflection.
  • The critical angle is the angle of incidence above which internal reflection occurs. The angle is measured with respect to the normal at the boundary between two media.
  • The angle of refraction is measured between a ray of light and an imaginary line called the normal.
  • In optics, the normal is an imaginary line drawn on a ray diagram perpendicular to, so at a right angle to (900), to the boundary between two media.
  • If the boundary between the media is curved then the normal is drawn perpendicular to the boundary.

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