Refraction of Red Green and Blue Rays in a Raindrop




To find out more about the diagram above . . . . read on!

Paths of Red Green and Blue Rays in a Raindrop

Look carefully at the diagram at the top of the page. Now check out the following questions (and answers)!

  1. Does light undergo refraction as it enters a raindrop?
  2. Does light undergo dispersion as it enters a raindrop?
  3. Are rainbow colours spectral colours?
  4. What happens when light undergoes refraction?
  5. Is magenta a spectral colour?

About the Diagram

Introducing the diagram! Read back and forward between the image at the top of the page and the explanation below!

Have you already checked out An Introduction to Reflection, Refraction and Dispersion?

It is the opening page of our Reflection, Refraction and Dispersion Series and contains masses of useful information. This is the table of contents:

The diagram

This diagram considers what happens to a ray of incident light containing wavelength corresponding with red, green and blue when it strikes a raindrop.

  • It shows that refraction causes each wavelength within the ray to change direction by a different amount resulting in dispersion of the component colours.
  • The effects of refraction, reflection and dispersion all help to explain why rainbows appear when sunlight strikes falling rain.
  • The sections below help to explain other factors that affect the appearance of rainbows.
  • Remember that when light strikes the boundary between two different media it may be partially reflected and partially refracted.
  • If both reflection and refraction take place:
    • A proportion of the light bounces off the surface of the new medium it encounters and returns into the medium from which it originated.
    • A proportion crosses the boundary and undergoes refraction, so changes speed and direction.

What is a rainbow?

A rainbow takes the form of circular arcs of spectral colour that appear in the sky in the presence of strong sunshine and rain.

Rainbows often appear just after rain has passed over when the edge of the shower forms a wall of droplets.

To see a rainbow, the rain must be in front of the observer and the sun must be in the opposite direction, at their back.

Rainbows are caused by sunlight reflecting, refracting and dispersing inside water droplets before they are seen by an observation.

Rainbows can sometimes be produced by waterfalls, lawn sprinklers and other things that create a spray of water.

A rainbow is an optical effect

A rainbow isn’t an object in the sense that we see and recognise most things in the world around us. A rainbow is an optical effect, a trick of the light, and is caused by the behaviour of light waves travelling through transparent spherical droplets of water. Rainbows only appear at certain times and have no fixed location. Where they appear depends on the location of the observer.

Preconditions for seeing a rainbow

There are three basic preconditions for seeing a rainbow.

  • An observer who is in the right place at the right time.
  • Bright sunlight shining through a clear sky.
  • A wall of rain falling between clouds and the ground.

Understanding rainbows

To fully understand rainbows involves referring to different fields of enquiry and areas of knowledge.

The field of optics tells us that rainbows are all about the paths that light rays take through different media and are principally caused by the reflection, refraction and dispersion of different wavelengths of light in water droplets.

A weather forecaster might explain rainbows in meteorological terms because they depend on sunlight and only appear in certain weather conditions and times of day.

A hydrologist who studies the movement and distribution of water around the planet might refer to the water-cycle and so to things like evaporation, condensation and precipitation.

A vision scientist will need to refer to visual perception in humans and the biological mechanisms of the eye.

An optometrist may check for colour blindness or eye disease.

Water droplets

When a raindrop is in free-fall and not buffeted by the wind, it forms a sphere. The more perfect the sphere the better the rainbow because each droplet will affect the incoming sunlight in the same way.

The size of raindrops is important. When all the droplets are the same size, they produce rainbows with intense bands of colour. But if the droplets are too large (over 3 to 4 mm) then air resistance affects their shape and causes a blurring of colours. If the droplets are too small and lightweight they float in the air and form mist or fog which produces faint, fuzzy rainbows.

The distribution of raindrops is also important. If they are passing across the observer’s entire field of view the rainbow may appear continuous from end to end. Where and when rain falls is however the result of endless changes as clouds are blown across the landscape, so long arcs may shrink down to nothing in seconds.

Equally, a rainbow that looks almost close enough to touch may be visible for minutes on end before receding slowly into the distance.

Primary rainbow

The most common rainbow is a primary bow that appears when sunlight is refracted as it enters raindrops, reflects once off the opposite interior surface, and is then refracted again as it escapes back into the air towards the observer.

The colours in a primary rainbow are always arranged with red on the outside and violet on the inside.

The outside of a primary rainbow forms an angle of 420 to its axis. The axis of a rainbow is an imaginary line drawn between the light source and observer during the period the bow is visible.

Secondary rainbow

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

A secondary rainbow usually appears alongside a primary rainbow and forms a second larger arc with the colours reversed. So, a secondary rainbow has violet on the outside and red on the inside.

When both primary and secondary bows are visible at the same time they are often referred to as a double rainbow.

The outside of a secondary rainbow forms an angle of 50 to 530 to its axis. The axis of a rainbow is an imaginary line drawn between the light source and observer during the period the bow is visible.

Supernumerary rainbow

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 supernumerary means additional to the usual number. The first supernumerary rainbow forms near the edge of the primary bow and is normally the sharpest. Each subsequent supernumerary bow is a little fainter. They often look like fringes of pastel colours and can change in size, intensity and position from moment by moment.

Supernumerary rainbows are clearest when raindrops are small and of equal size.

On rare occasions, supernumerary rainbows can be seen on the outside a secondary rainbow.

Supernumerary rainbows are produced by water droplets with a diameter of around 1 mm or less. The smaller the droplets, the broader the supernumerary bands become, and the less saturated are their colours.

Supernumerary rainbows are caused by interference between light waves that contribute towards the main bow but are out of phase with one another by the time they leave a raindrop and travel towards the observer.

The area inside the primary bow where supernumerary bows appear is usually much lighter than the area outside because it contains reflected light of all wavelengths mixed to produce white light.

The theory is that rays of a similar wavelength have slightly different distances to travel through misshapen droplets affected by turbulence, and this can cause them to get slightly out of phase with one another. When they are in phase, they reinforce one another, but when they get out of phase is when they produce an interference pattern that appears inside the primary bow.

Fogbows, dewbows and moonbows

There are many optic effects similar to rainbows.

A fogbow is a similar phenomenon to a rainbow. As its name suggests, it appears as a bow in fog rather than rain. Because of the very small size of water droplets that cause fog a fogbow has only very weak colours.

A dewbow can form where dewdrops rather than rain droplets reflect and disperse sunlight. Dewbows can sometimes be seen on fields in the early morning when the temperature drops below the dew point and the ground is covered with cobwebs.

A moonbow is produced by moonlight rather than direct sunlight but appears for the same reasons. Moonbows are often too faint to excite the colour receptors (cone cells) but sometimes appear in photographs taken at night with a long exposure.

A light source for rainbows

The best light source for rainbows is a strong point-source such as sunlight. A human observer with binocular vision has a 1200 field of view from side to side whilst in clear conditions the Sun fills only 0.50.

When atmospheric conditions defuse sunlight, it causes too much scattering of rays before they reach raindrops, so no bow is produced.

A wide range of visible wavelengths of light is needed to produce rainbows because each wavelength corresponds with a different colour. The Sun produces a continuous range of wavelengths across the entire visible spectrum.

Rainbow colours

Rainbow colours are the colours seen in rainbows and in other situations where visible light separates into its component wavelengths and the colours corresponding with each wavelength become visible to the human eye.

Rainbow colours contain all the spectral colours. A spectral colour is a colour evoked in normal human vision by a single wavelength of visible light, or by a narrow band of adjacent wavelengths. When spectral colours are mixed together in equal amounts, they produce white.

The rainbow colours (ROYGBV) in order of wavelength are red (longest wavelength), orange, yellow, green, blue and violet (shortest wavelength).

It is the sensitivity of the human eye to this small part of the electromagnetic spectrum that results in our perception of colour. Whilst the visible spectrum and its spectral colours are determined by wavelength, and/or frequency, it is our eyes and brains that interpret these differences in electromagnetic radiation to produce our colour perceptions.

Naming rainbow colours is a matter more closely related to the relationship between perception and language than anything to do with physics or optics. Even the commonplace colours associated with rainbows defy easy definition. They are concepts we generally agree on but are not strictly defined by anything in the nature of light itself.

Modern portrayals of rainbows show six colours – ROYGBV. This leaves out other colours such as indigo and cyan. In reality, rainbows contain all spectral colours and the most intense can vary depending upon conditions at the time of observation.

Bands of colour

There is no property belonging to electromagnetic radiation that causes bands of colour to appear to a human observer. The visible spectrum is made up of a smooth and continuous range of wavelengths that correspond with a smooth and continuous range of colours.

The fact that we do see distinct bands is often described as an artefact of human colour vision. We see them because the human eye can distinguish between some ranges of wavelengths of visible light better than others because they appear more intense. It is our eyes in conjunction with our brain that sees a boundary between different bands of colour and gives each a different name.

The best conditions for seeing rainbows

The right weather conditions are important if you hope to see a rainbow.

Rainbows are rare in areas with little or no rainfall such as dry, desert conditions with few clouds.

Hills and mountains often create ideal conditions because clouds form quickly, and the weather can change several times a day – especially during spring and autumn.

But too much cloud is not good because it blocks direct sunlight.

The best rainbows appear in the morning and evening when the sun is strong but low in the sky. Far northern and southern latitudes are good for rainbows because the sun remains lower in the sky all day and rises and sets more slowly than nearer the equator.

Winter is not the best season for rainbows because the days are shorter, the sun isn’t as strong and there can be too much cloud.

Rainbows are less common in summer and around midday because the whole bow is below the horizon.

Follow the blue links for definitions . . . . or check the summaries of key terms below!

Some Key Terms

Move to the next level! Check out the following terms.


Dispersion (or chromatic dispersion) refers to the way that light, under certain conditions, separates into its component wavelengths and the ...
Read More


Any material through which an electromagnetic wave propagates (travels) is called a medium (plural media). In optics, a medium is ...
Read More


A rainbow is an optical effect produced by illuminated droplets of water. Rainbows are caused by reflection, refraction and dispersion ...
Read More

Rainbow colours

Rainbow colours are the bands of colour seen in rainbows and in other situations where visible light separates into its ...
Read More


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


Refraction refers to the way that electromagnetic radiation (light) changes speed and direction as it travels across the interface between ...
Read More

Spectral colour

A spectral colour is a colour evoked in normal human vision by a single wavelength of visible light, or by ...
Read More

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