The area inside a primary rainbow
The area inside the arcs of a primary rainbow, from its centre-point out to the violet band at 40.70 appears tonally lighter or brighter than the area of sky on the outside.
- The area inside the arcs of a primary rainbow contains:
- Light that has been reflected off the outside surface of raindrops towards an observer. This light has not undergone refraction or dispersion so reflects white sunlight back towards an observer.
- Light that has been randomly scattered after being intercepted and refracted by multiple droplets in succession. The result is a mixture of different wavelengths inside a primary rainbow that produces a whiter or lighter appearance to an observer.
The area outside a primary rainbow
Very little light is directed into the area outside a primary rainbow. As a result, rainbows appear to be brightly coloured and stand out against the sky beyond.
- The outer red edge of a primary rainbow corresponds with the minimum angle of deviation of light for the raindrops that contribute to an observer’s perception of a rainbow.
- The minimum angle of deviation is the minimum number of degrees that light must bend back on itself after it passes through a raindrop if it is to form part of a rainbow seen by an observer.
- The minimum angle of deviation of light through a raindrop is measured between the original path of incident light before it strikes a raindrop and the path along which an observer looks towards a rainbow.
- The area outside a primary rainbow corresponds with angles that are less than the minimum angle of deviation. Relatively few photons are directed towards an observer from this area.
- The coloured arcs of a primary rainbow correspond with angles greater than the minimum angle of deviation.
- The brightness of the arcs of a primary rainbow results from the fact that as light of any particular wavelength passes through a raindrop it tends to concentrate near the minimum angle of deviation (see rainbow rays).
The area between a primary and a secondary rainbow
- As refraction and dispersion take place in raindrops that form a primary rainbow, light is directed inwards towards its centre and so away from Alexander’s band.
- As refraction and dispersion take place in raindrops that form a secondary rainbow, light is directed outwards away from Alexander’s band.
The area inside a secondary rainbow
The inner red-coloured edge of a secondary rainbow corresponds with the minimum angle of deviation.
- Relatively few photons are directed towards an observer from the area between primary and secondary rainbows (Alexander’s band).
The area outside a secondary rainbow
Some light is reflected or refracted into the area outside a secondary rainbow but it does not significantly lighten the sky.
- The area outside a secondary rainbow corresponds with the area inside a primary rainbow.
- Light is widely dispersed during the formation of a secondary rainbow as a result of the second internal reflection that proceeds their observation.
- Wider dispersion of wavelengths involved in the formation of a secondary rainbow and then the random scattering of light into the area outside a secondary rainbow produces no equivalent to the lighter or brighter look of the sky inside a primary rainbow.
- White light, containing all wavelengths within the visible part of the electromagnetic spectrum in equal proportions and at equal intensities, separates into spectral colour as refraction and dispersion take place.
- It is the small difference in the refractive index of different wavelengths of incident light that causes dispersion and separation of white light into rainbow colours.
- As light travels through the air it is invisible to our eyes. White light is what an observer sees when all the colours that make up the visible spectrum reflect off a neutral coloured surface or particles of dust and vapour.
- Colour is what a human observer sees when a single wavelength, a band of wavelengths or a mixture of different wavelengths strike neutral coloured surfaces or particles of dust and vapour.