Viewing angles, angular distance and angles of deflection

The term viewing angle refers to the angle, measured in degrees, between the direction an observer looks in to see the centre of the rainbow and the direction they look to see a specific colour within the rainbow’s arc.
The term angular distance refers to the same measurement as the viewing angle, especially when depicted on a side elevation diagram.
The angle of deflection measures the change in direction that a light ray undergoes as it strikes, refracts into, reflects inside, and refracts out of a raindrop towards an observer.
The term rainbow ray refers to the path taken by a deflected light ray that results in the most intense colour perception for a specific wavelength of light passing through a raindrop.
The term angle of deviation measures the change in direction a light ray undergoes due to refraction and reflection inside a raindrop, relative to its original direction towards an observer.
- In any specific case of a light ray passing through a raindrop, the angle of deviation and the angle of deflection are interrelated and their sum equals 180⁰.
- The angle of deviation is equal to 180⁰minus the angle of deflection, and vice versa, so the angle of deflection is equal to 180⁰minus the angle of deviation.
- In any specific instance, the angle of deflection is approximately the same as the viewing angle, because the incident light rays that contribute to a rainbow all approach parallel to the axis of the rainbow.