Lower the Sun Higher the Rainbow
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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.
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Description
Lower the Sun, Higher the Rainbow
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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
- In this diagram the light source is the Sun, the observer is shown as an eye and the rainbow appears to the observer as arcs of rainbow colours.
- The diagram shows that when a rainbow forms, the light source, observer and the centre-point of the arcs of rainbow colours are always on the same axis.
Notice that:
- A little over half of the rainbow is visible to the observer because the Sun is very low in the sky.
- In the right conditions, a rainbow can form a complete circle but the ground usually gets in the way.
- When droplets of rain hit the ground they stop deflecting light. Without deflected light, there is no rainbow.
- The position of a rainbow is always determined by the fact that the Sun, observer and the anti-solar point (the centre of a rainbow) are all on the same axis.
- The red arc is on the outside of the bow because a primary rainbow is shown.
- An arrow marks the angle between the axis and the red arc seen by the observer. The angular distance for red is always around 420. The angle for violet is always around 400. The exact angle depends of the strongest wavelengths visible to the observer at a particular moment.
About the diagram: Sun, observer and anti-solar point
Some Key Terms
On a sunny day, if you stand with the Sun at your back and look at the ground, the shadow of your head will align with the antisolar point.
- The antisolar point is the position directly opposite the Sun, 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 antisolar point, which is the geometric centre of a rainbow.
- This concept corresponds with what an observer sees in real life: the idea that a rainbow has a center. From a side view, the centre of a rainbow is called the antisolar point, so named because it is opposite the Sun relative to the observer’s position.
- Unless observed from the air, the antisolar point is always below the horizon. Both primary and secondary rainbows share the same antisolar point, as do higher-order bows, such as fifth and sixth-order rainbows.
Reflection is the process where light rebounds from a surface into the medium it came from, instead of being absorbed by an opaque material or transmitted through a transparent one.
- The three laws of reflection are as follows:
- When light hits a reflective surface, the incoming light, the reflected light, and an imaginary line perpendicular to the surface (called the “normal line”) are all in the same flat area.
- The angle between the incoming light and the normal line is the same as the angle between the reflected light and the normal line. In other words, light bounces off the surface at the same angle as it came in.
- The incoming and reflected light are mirror images of each other when looking along the normal line. If you were to fold the flat area along the normal line, the incoming light would line up with the reflected light.
Incident light refers to light that is travelling towards an object or medium.
- Incident light refers to light that is travelling towards an object or medium.
- Incident light may come from the Sun, an artificial source or may have already been reflected off another surface, such as a mirror.
- When incident light strikes a surface or object, it may be absorbed, reflected, refracted, transmitted or undergo any combination of these optical effects.
- Incident light is typically represented on a ray diagram as a straight line with an arrow to indicate its direction of propagation.
The Sun is the star at the centre of our solar system.
- The energy emitted by the Sun is called electromagnetic radiation or solar radiation.
- The solar radiation that the human eye is sensitive to is often called sunlight or visible light.
- The term light is often used to refer to visible light but can also be used to refer to all the different forms of electromagnetic radiation.
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.
Refraction refers to the way that electromagnetic radiation (light) changes speed and direction as it travels across the boundary between one transparent medium and another.
- Light bends towards the normal and slows down when it moves from a fast medium (like air) to a slower medium (like water).
- Light bends away from the normal and speeds up when it moves from a slow medium (like diamond) to a faster medium (like glass).
- These phenomena are governed by Snell’s law, which describes the relationship between the angles of incidence and refraction.
- The refractive index (index of refraction) of a medium indicates how much the speed and direction of light are altered when travelling in or out of a medium.
- It is calculated by dividing the speed of light in a vacuum by the speed of light in the material.
- Snell’s law relates the angles of incidence and refraction to the refractive indices of the two media involved.
- Snell’s law states that the ratio of the sine of the angle of incidence to the sine of the angle of refraction is equal to the ratio of the refractive indices.
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