# Raindrop Anatomy

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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.
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## Description

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

#### Raindrop Anatomy

###### TRY SOME QUICK QUESTIONS AND ANSWERS TO GET STARTED
Yes! The arcs of a rainbow always appear at the same angle from its centre point.
Seen from an observer's point of view, the angle between the centre of a rainbow and the coloured arcs is called the viewing angle. In diagrams, the same angle between the rainbow axis and a line extended from an observer's eyes to the arcs of a rainbow is called the angular distance.
The angle between the red band on a primary rainbow and its centre-point is always 42.4 degrees.
Yes! The centre of a rainbow is always below the horizon when an observer looks over water or level ground.

###### Overview of the diagram
• This diagram aims to identify a simple and closely interconnected set of terms that explain how individual raindrops form rainbows.
• The terms used here underpin our Introduction to Rainbows and our resource Rainbows: In detail.
• A quick internet search will reveal that the way terms are defined and used varies. We have made every effort to align ourselves with research that provides an accessible and straightforward introduction to rainbows.
• The diagram shows some of the geometry that determines the path of light through a raindrop.
• This is a measured diagram and the angles shown are indicative of the path of a single yellow ray with a wavelength of 589.29nm travelling through water at 200C.
• The table below provides a key to the notation used on the diagram.
• Beyond identifying a simple and closely interconnected set of terms, the aim of the diagram is to:
• highlight the geometric points used to explain changes in the direction and speed of light when it strikes the surface of a raindrop.
• Show where and how angles are measured.
• Show connections between angles.
• Note that:
• The notation used in the diagram is not associated with any other system of identification.
• Standard mathematical symbols have not been used for this diagram.
LabelDescription
AirEarth's atmosphere is generally considered to be composed of nitrogen, oxygen, argon with trace amounts of carbon dioxide, hydrogen, methane and neon.
Average humidity, pressure and a temperature of 20C produce a refractive index of 1.000293.
RaindropAn idealised raindrop forms a geometrically perfect sphere. Although such a form is one in a million in real-life, simplified geometrical raindrops help to make sense of rainbows and reveal general rules governing why they appear.
Impact parameter scaleAn impact parameter scale is used on a ray-tracing diagram to measure the point at which incident rays strike the surface of a raindrop. Rays are given a value between 0.0 and 1.0 depending upon their point of impact.
Horizontal axis of a raindropThe horizontal axis of a raindrop is usually drawn on a ray diagram (showing the Sun, observer and anti-solar point in side-elevation) as running parallel with incident rays and the rainbow axis.
Light sourceA light source is a natural or man-made object that emits one or more wavelengths of light.
Anti-solar pointThe 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.
AThe point at which an incident ray strikes a raindrop
BThe point at which a refracted ray is reflected off the inside of a raindrop.
CThe point at which a reflected ray strikes the inside of a raindrop undergoes refraction and exits towards an observer.
Incident rayIncident ray refers to incoming light that is travelling towards an object or medium
Refracted rayA refracted ray is a ray that has undergone refraction.
Reflected rayA reflected ray is a ray that has undergone reflection.
Deflected rayA deflected ray is a ray that has been deflected from its original path in the direction of an observer.
Angle of incidenceThe angle of incidence measures the angle at which incoming light strikes a surface.
Angle of refractionThe angle of refraction measures the angle to which light bends as it passes across the boundary between different media.
Angle of reflectionThe angle of reflection measures the angle at which reflected light bounces off a surface.
Angle of deflection (d)The angle of deviation measures the degree to which the path of a light ray is bent back by a raindrop in the course of refraction and reflection towards an observer.
Angle of deviationThe angle of deviation measures the degree to which the path of a light ray is bent back by a raindrop in the course of refraction and reflection towards an observer.
Angular distanceAngular distance is the angle between the rainbow axis and the direction in which an observer must look to see a specific colour within the arcs of a rainbow.
Viewing angleThe viewing angle of a rainbow is the angle between a line extended from an observer’s eyes to a bow’s centre point and a second line extended out towards the coloured arcs.
y1The angle of deviation at point A
y2The angle of deviation at point B
y3The angle of deviation at point C
y1 + y2 + y3 =The total angle of deviation
Tangent to surfaceA tangent to a circle is a straight line that touches but does not intersect the circle and is at right angles to a radial line drawn from the centre of the circle.
NormalIf one line is normal to another, then it is at right angles. In geometry, the normal (or a normal) refers to a line drawn perpendicular to and intersecting another line, plane or surface.
>>The symbol used to mark parallel lines.

Key to Rainbow Anatomy diagram

#### Some key terms

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.
• 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.
###### References

https://en.wikipedia.org/wiki/Reflection_(physics)

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.
• In optics, the normal is a line drawn on a ray diagram perpendicular to, so at a right angle to (900), to 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 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 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.

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.

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