Rainbows as Cones of Colour
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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 Cones of Colour
TRY SOME QUICK QUESTIONS AND ANSWERS TO GET STARTED
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.
No! The outermost colour on a secondary rainbow is violet because the colours of a secondary rainbow are reversed.
It is the small difference in the refractive index of different wavelengths of incident light that causes dispersion and separation of white light into rainbow colours.
Yes! 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.drops.
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:
- 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 cones of colour
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 and each is composed of a narrow spread of wavelengths of light that determines its apparent colour. Red fills the cone with the largest radius and violet fills the smallest.
- The cones do not have a simple 2D base. At their nearest, droplets may be within reach of an observer. At the other extreme, distant raindrops also refract and reflect light back towards an observer.
- Modelling a rainbow as a cone that shows depth, as well as height and width, demonstrates that all the raindrops contained within one of the cones at any moment can contribute to the visual experience of an observer regardless of how far they are away.
- Whilst modelling rainbows as discs corresponds with what an observer sees, the idea that rainbows are formed from cones of colour corresponds with a diagram showing a side elevation with the Sun, observer and rainbow arranged along the rainbow’s axis.
Some key terms
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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.
A light source is a natural or man-made object that emits one or more wavelengths of light.
- The Sun is the most important light source in our lives and emits every wavelength of light in the visible spectrum.
- Celestial sources of light include other stars, comets and meteors.
- Other natural sources of light include lightning, volcanoes and forest fires.
- There are also bio-luminescent light sources including some species of fish and insects as well as types of bacteria and algae.
- Man-made light sources of the most simple type include natural tars and resins, wax candles, lamps that burn oil, fats or paraffin and gas lamps.
- Modern man-made light sources include tungsten light sources. These are a type of incandescent source which means they radiate light when electricity is used to heat a filament inside a glass bulb.
- Halogen bulbs are more efficient and long-lasting versions of incandescent tungsten lamps and produce a very uniform bright light throughout the bulb’s lifetime.
- Fluorescent lights are non-incandescent sources of light. They generally work by passing electricity through a glass tube of gas such as mercury, neon, argon or xenon instead of a filament. These lamps are very efficient at emitting visible light, produce less waste heat, and typically last much longer than incandescent lamps.
- An LED (Light Emitting Diode) is an electroluminescent light source. It produces light by passing an electrical charge across the junction of a semiconductor.
- Made-made lights can emit a single wavelength, bands of wavelengths or combinations of wavelengths.
- An LED light typically emits a single colour of light which is composed of a very narrow range of wavelengths.
A human observer is a person who engages in observation by watching things.
- In the presence of visible light, an observer perceives colour because the retina at the back of the human eye is sensitive to wavelengths of light that fall within the visible part of the electromagnetic spectrum.
- The visual experience of colour is associated with words such as red, blue, yellow, etc.
- The retina’s response to visible light can be fully described in terms of wavelength, frequency and brightness.
- Other properties of the world around us must be inferred from patterns of light.
A rainbow is an optical effect produced by illuminated droplets of water. Rainbows are caused by reflection, refraction and dispersion of light in individual droplets and results in the appearance of an arc of spectral colours.
- Rainbows only appear when weather conditions are ideal and an observer is in the right place at the right time.
- Waterfalls, lawn sprinklers and other things that produce water droplets can produce a rainbow.
- A rainbow is formed from millions of individual droplets each of which reflects and refracts a tiny coloured image of the sun towards the observer.
- It is the dispersion of light as refraction takes place that produces the rainbow colours seen by an observer.
- When the sun is behind an observer then the rainbow will appear in front of them.
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