Higher the Sun Lower 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.
  • For quick reference don’t miss the summaries of key terms further down each page.

Description

Higher the Sun, Lower the Rainbow

TRY SOME QUICK QUESTIONS AND ANSWERS TO GET STARTED
Rainbows appear when bright sunshine is refracted, reflected and dispersed in raindrops in the presence of an observer.
Yes! If droplets are large, 1 millimetre or more in diameter, red, green, and violet are bright but blue is hardly visible.
Yes! A rainbow can form a complete circle when seen from a plane.

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 a small arc of colours low on the horizon.
  • 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.
  • In this case, the lower half of the rainbow is missing because when droplets of rain hit the ground they stop reflecting light.
Notice that:
  • A small arc of the rainbow is visible to the observer because the Sun is high in the sky.
  • In the right conditions, a rainbow can form a complete circle but the ground usually gets in the way.
  • 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 the illustration is of a primary rainbow.
  • 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 on the strongest wavelengths of light visible to the observer at any particular moment.
About the diagram: Sun, observer and anti-solar point

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 you, passes through the back of your head, out through your eyes and extends in a straight line into the distance.

  • The imaginary line that joins the Sun, observer and the centre of the rainbow is called the rainbow axis.
  • The point on the rainbow axis around which a rainbow appears is called the anti-solar point. The centre of a rainbow coincides with the anti-solar point.
  • Stand with the Sun on your back and look at the ground on a sunny day, the shadow of your head marks the point called the antisolar point, it is 180° away from the Sun.
  • The red arc of a primary bow forms at an angle of 42.40 from the rainbow axis.
  • Seen from an observer’s point of view, the angle outwards from the rainbow axis to the coloured arcs is called the viewing angle.
  • In diagrams, the same angle between the axis and a line extended from an observer’s eyes to the arcs of a rainbow is called the angular distance.
  • With the Sun behind you, spread out your arms to either side or up and down to get a sense of where a rainbow should appear if the conditions are right.
  • Unless seen from the air, the centre of a rainbow and the anti-solar point will always be 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.
  • To see a secondary rainbow look for the primary bow first – it has red on the outside. The secondary bow will be a bit larger with violet on the outside at an angle of 53.40 and red on the inside.

Some key terms

Colour is not a property of electromagnetic radiation, but a feature of visual perception by an observer.

The angle of refraction measures the angle to which light bends as it crosses the boundary between different media.

  • The angle of refraction is measured between the bent ray and an imaginary line called the normal.
  • In optics, the normal is a line drawn on a ray diagram perpendicular to, so at a right angle to (900), the boundary between two media.
  • Snell’s law is a formula used to describe the relationship between the angle of incidence and the angle of refraction when light crosses the boundary between transparent media, such as water and air or water and glass.

The angle of incidence measures the angle at which incoming light strikes a surface.

  • When light is travelling towards something it is said to be incident to that surface or object.
  • The angle of incidence is measured between a ray of incoming light and an imaginary line called the normal.
  • In optics, the normal is a line drawn on a ray diagram perpendicular to, so at a right angle to (900), the boundary between two media.
  • Expressed more formally, in optics, the normal is a geometric construct, a line drawn perpendicular to the interface between two media at the point of contact. This conceptually defined reference line is crucial for characterizing various light-matter interactions, such as reflection, refraction, and absorption.
  • Incident light may have travelled from the Sun or a man-made source or may have already been reflected off another surface such as a mirror.
  • When incident light strikes a surface or object it may undergo absorption, reflection, refraction, transmission or any combination of these optical effects.
  • The angle of incidence measures the angle at which incoming light strikes a surface.
  • The angle of incidence is measured between a ray of incoming light and an imaginary line called the normal.
  • See this diagram for an explanation: Reflection of a ray of light
  • In optics, the normal is a line drawn on a ray diagram perpendicular to, so at a right angle to (900), the boundary between two media.
  • If the boundary between the media is curved, then the normal is drawn at a tangent to the boundary.

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

  • The visible spectrum is the range of wavelengths of the electromagnetic spectrum that correspond with all the different colours we see in the world.
  • As light travels through the air it is invisible to our eyes.
  • Human beings don’t see wavelengths of light, but they do see the spectral colours that correspond with each wavelength and colours produced when different wavelengths are combined.
  • The visible spectrum includes all the spectral colours between red and violet and each is produced by a single wavelength.
  • The visible spectrum is often divided into named colours, though any division of this kind is somewhat arbitrary.
  • Traditional colours referred to in English include red, orange, yellow, green, blue, and violet.

The angle of reflection measures the angle at which light rebounds from a surface after being reflected.

  • The angle of reflection is measured between a ray of light which has been reflected off a surface and an imaginary line called the normal.
  • In optics, the normal is a line drawn on a ray diagram perpendicular to, so at a right angle to (900), the boundary between two media.
  • The angle of reflection can be used to understand how light will behave when it interacts with different types of surfaces and objects.

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

  • Visible light is a form of electromagnetic radiation.
  • Other forms of electromagnetic radiation include radio waves, microwaves, infrared, ultraviolet, X-rays, and gamma rays.
  • Visible light is perceived by a human observer as all the spectral colours between red and violet plus all other colours that result from combining wavelengths together in different proportions.
  • A spectral colour is produced by a single wavelength of light.
  • The complete range of colours that can be perceived by a human observer is called the visible spectrum.
  • The range of wavelengths that produce visible light is a very small part of the electromagnetic spectrum.

The angle of reflection is the angle between the incident light ray and the reflected light ray, both measured from an imaginary line called the normal.

  • According to the law of reflection, the angle of incidence (the angle between the incident ray and the normal) is always equal to the angle of reflection.
  • The angle of reflection is measured between the reflected ray of light and an imaginary line perpendicular to the surface, known as the normal.
  • In optics, the normal is a straight line drawn on a ray-tracing diagram at a 90º angle (perpendicular) to the boundary where two different media meet.
  • Expressed more formally, in optics, the normal is a geometric construct, a line drawn perpendicular to the interface between two media at the point of contact. This conceptually defined reference line is crucial for characterizing various light-matter interactions, such as reflection, refraction, and absorption.
  • If the boundary between two media is curved, the normal is drawn perpendicular to the tangent to that point on the boundary.
  • Reflection can be diffuse (when light reflects off rough surfaces) or specular (in the case of smooth, shiny surfaces), affecting the direction of reflected rays.
  • The angle of reflection measures the angle at which reflected light bounces off a surface.
  • The angle of reflection is measured between a ray of light which has been reflected off a surface and an imaginary line called the normal.
  • See this diagram for an explanation: Reflection of a ray of light
  • In optics, the normal is a line drawn on a ray diagram perpendicular to, so at a right angle to (900), the boundary between two media.
  • If the boundary between the media is curved then the normal is drawn perpendicular to the boundary.
  • Colour vision is the human ability to distinguish between objects based on the wavelengths of the light they emit, reflect or transmit. The human eye and brain together translate light into colour.
  • Colour is not a property of electromagnetic radiation, but a feature of visual perception.
  • The human eye, and so human perception, is tuned to the range of wavelengths of light that make up the visible spectrum and so to the corresponding spectral colours between red and violet.
  • Light, however, is rarely of a single wavelength, so an observer will usually be exposed to a spread of different wavelengths of light or a mixture of wavelengths from different areas of the spectrum.
  • An observer’s perception of colour is a subjective process as the eyes and brain respond together to stimuli produced when incoming light reacts with light-sensitive cells within the retina at the back of the eye.
  • The perception of colour can be influenced by various factors, such as lighting conditions, surrounding colours, and individual differences in colour perception.

The angle of incidence refers to the angle at which incoming light strikes a surface and is measured between a ray of incoming light and an imaginary line called the normal.

  • In optics, the normal is a line drawn on a ray diagram perpendicular to, so at a right angle to (900), the boundary between two media.
  • The angle at which incident light from the Sun or a light bulb strikes a surface can affect the outcome. For instance, when incident light hits a mirror, the angle of incidence determines the angle of reflection.

The perception of colour by an observer results from properties of light that are visible to the human eye. The visual experience of colour is associated with terms like red, blue and yellow.

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