Higher the Sun Lower the Rainbow
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This is one of a set of almost 40 diagrams exploring Rainbows.
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Description
Higher the Sun, Lower 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 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
Some key terms
Visual perception is the human ability to see and understand our surroundings by virtue of the sensitivity of our eyes to wavelengths of light across the entire visible spectrum, from red to violet.
- Visual perception is a complex process that relies on the intricate interaction between our eyes, the brain, and the interpretation of light signals. It enables us to perceive various visual attributes such as shapes, sizes, textures, depths, motions, and spatial relationships, all of which contribute to our comprehensive understanding and interpretation of the visual world around us.
In the field of optics, dispersion is shorthand for chromatic dispersion which refers to the way that light, under certain conditions, separates into its component wavelengths, enabling the colours corresponding with each wavelength to become visible to a human observer.
- Chromatic dispersion refers to the dispersion of light according to its wavelength or colour.
- Chromatic dispersion is the result of the relationship between wavelength and refractive index.
- When light travels from one medium (such as air) to another (such as glass or water) each wavelength is refracted differently, causing the separation of white light into its constituent colours.
- When light undergoes refraction each wavelength changes direction by a different amount. In the case of white light, the separate wavelengths fan out into distinct bands of colour with red on one side and violet on the other.
- Familiar examples of chromatic dispersion are when white light strikes a prism or raindrops and a rainbow of colours becomes visible to an observer.
The angle of refraction measures the angle to which light bends as it passes across the boundary between different media.
- The angle of refraction is measured between a ray of 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.
- See this diagram for an explanation: Refraction of a ray of light
- If the boundary between the media is curved, the normal is drawn perpendicular to the boundary.
Visible light refers to the range of wavelengths of electromagnetic radiation that is perceived as colour by human observers. While the range of visible light is generally considered to be 400-700 nm, the exact range of colours perceptible can vary slightly between individuals.
- Visible light is one form of electromagnetic radiation. Other forms of electromagnetic radiation include radio waves, microwaves, infrared, ultraviolet, X-rays, and gamma rays. Visible light ranges from approximately 400 nanometres (nm) for violet to 700 nm for red.
- A human observer perceives visible light as a combination of all the spectral colours between red and violet, as well as a vast range of other colours produced from the blending of different wavelengths in varying proportions.
Colour is not a property of electromagnetic radiation, but a feature of visual perception by an observer.
- The human eye and so human visual perception are tuned to the visible spectrum and so to spectral colours between red and violet.
- There are no properties of electromagnetic radiation that distinguish visible light from other parts of the electromagnetic spectrum.
- Objects appear to be different colours to an observer depending on the wavelengths, frequencies and amplitude of visible light at the moment it strikes the retina at the back of the eye.
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 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.
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.
- 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 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.
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