The term impact parameter refers to a scale 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.
- For a primary rainbow, all the incident rays of interest strike a raindrop between its horizontal axis (0.0 on the impact parameter scale) and the upper-most point (1.0 on the impact parameter scale). In the second case, the ray grazes the surface at 900 to the normal and continues on its course without deviation.
- For a secondary rainbow, all the incident rays of interest strike a raindrop between its horizontal axis (0.0 on the impact parameter scale) and the lowest point (1.0 on the impact parameter scale). In the second case, the ray grazes the surface at 900 to the normal and continues on its course without deviation.
- An impact parameter is useful because it allows the relationship between equidistant incident rays, the angle at which they strike the surface and their angle of refraction to be plotted.
A typical atmospheric rainbow includes six bands of colour from red to violet but there are other bands of light present that don’t produce the experience of colour for human observers.
- It is useful to remember that:
- Each band of wavelengths within the electromagnetism spectrum (taken as a whole) is composed of photons that produce different kinds of light.
- Remember that light can be used to mean visible light but can also be used to refer to other areas of the electromagnetism spectrum invisible to the human eye.
- Each band of wavelengths represents a different form of radiant energy with distinct properties.
- The idea of bands of wavelengths is adopted for convenience sake and is a widely understood convention. The entire electromagnetic spectrum is, in practice, composed of a smooth and continuous range of wavelengths (frequencies, energies).
- Radio waves, at the end of the electromagnetic spectrum with the longest wavelengths and the least energy, can penetrate the Earth’s atmosphere and reach the ground but are invisible to human eyes.
- Microwaves have shorter wavelengths than radio waves, can penetrate the Earth’s atmosphere and reach the ground but are invisible to human eyes.
- Longer microwaves (waves with similar lengths to radio waves) pass through the Earth’s atmosphere more easily than the shorter wavelengths nearer the visible parts of spectrum.
- Infra-red is the band closest to visible light but has longer wavelengths. Infra-red radiation can penetrate Earth’s atmosphere but is absorbed by water and carbon dioxide. Infra-red light doesn’t register as a colour to the human eye.
- The human eye responds more strongly to some bands of visible light between red and violet than others.
- Ultra-violet light contains shorter wavelengths than visible light, can penetrate Earth’s atmosphere but is absorbed by ozone. Ultra-violet light doesn’t register as a colour to the human eye.
- Radio, microwaves, infra-red, ultra-violet are all types of non-ionizing radiation, meaning they don’t have enough energy to knock electrons off atoms. Some cause more damage to living cells than others.
- The Earth’s atmosphere is opaque to both X-rays or gamma-rays from the ionosphere downwards.
- X-rays and gamma-rays are both forms of ionising radiation. This means that they are able to remove electrons from atoms to create ions. Ionising radiation can damage living cells.
- All forms of electromagnetic radiation can be thought of in terms of waves and particles.
- All forms of light from radio waves to gamma-rays can be thought to propagate as streams of photons.
- The exact spread of colours seen in a rainbow depends on the complex of wavelengths emitted by the light source and which of those reach an observer.