When discussing the formation of rainbows, the angle of deflection measures the angle between the initial path of a light ray before it hits a raindrop, and the angle of deviation, which measures how much the ray bends back on itself in the course of refraction and reflection towards an observer.
- The angle of deflection and the angle of deviation are always directly related to one another and together add up to 180 degrees.
- The angle of deflection equals 180 degrees minus the angle of deviation. So, it’s clear the angle of deviation is always equal to 180 degrees minus the angle of deflection.
- In any particular case, the angle of deflection is always the same as the viewing angle because the incident rays of light that form a rainbow all follow paths that run parallel with the rainbow axis.
Related Points
- Any ray of light (stream of photons) travelling through empty space, unaffected by gravity, travels in a straight line forever.
- When light travels from a vacuum or from one transparent medium into another, it undergoes refraction causing it to change both direction and speed.
- The more a light ray changes direction when it passes through a raindrop, the smaller the angle of deflection will be.
- It is the optical properties of raindrops that determine the angle of deflection of light as it exits a raindrop.
- Because of the optical properties of primary rainbows, no rays of light within the visible part of the electromagnetic spectrum exit a raindrop at an angle of deflection larger than 42.4 degrees.
- Because of the optical properties of secondary rainbows, no rays of light within the visible part of the electromagnetic spectrum exit a raindrop at an angle of deflection larger than 50.4 degrees.
Implications of the Angle of Deflection for Raindrops
- For a single incident ray of light of a known wavelength striking a raindrop at a known angle:
- To appear in a primary rainbow it cannot exceed an angle of deflection of more than 42.4 degrees. This corresponds with the minimum angle of deviation.
- 42.40 is the angle of deflection that results in red appearing along the outside edge of a primary rainbow from the point of view of an observer.
- 180 degrees minus 137.6 degrees equals 42.4 degrees, which is the biggest angle of deflection for any visible light ray if it is to appear within a primary rainbow.
- 180 degrees minus 139.3 degrees equals 40.7 degrees, which is the angle of deflection for a light ray that appears violet and forms the inside edge of a primary rainbow.
- Angles of deviation between 137.6 degrees and 139.3 degrees match viewing angles and angles of deflection between 42.4 degrees (red) and 40.7 degrees (violet).
- An angle of deviation of 137.6 degrees (so viewing angles of 42.4 degrees) matches the appearance of red light with a wavelength of about 720 nm.
- The range of angles of deflection that create the impression of colour for an observer is not related to droplet size.
- The laws of refraction (Snell’s law) and reflection can be used to work out the angle of deviation of white light in a raindrop.
- The angle of deviation can be fine-tuned for any specific wavelength by making small adjustments to the refractive index.
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