OPTICAL MIRAGES
1. Optical Conditions
An optical mirage is a phenomenon associated with the refraction of light in a cloud-free atmosphere, resulting in the apparent displacement of a distant object. The refraction is caused by abnormal air densities, which cause the light to travel in a curvilinear path. Object shapes can be distorted and scintillation can cause rapid changes in the observed colour, shape and position of the object. For a stationary observer the effect can last for hours; for a moving observer only seconds or minutes. Mirages have the following characteristics:
- Mirage images are seen at low grazing angles near the horizontal plane.
- Mirages can only be seen if there is a long uninterrupted sightline.
- Mirages are associated with atmospheric gradients which give anomalous propagation
- Focusing and interference of the wavefronts can give rapid movement and enhanced brightening.
- Distortions and colour changes can be accompanied by multiple images.
- Objects on the surface (e.g. ships) can appear as if they are elevated.
2. Object Position
The amount the object is apparently (visually) displaced from its true position (angle) depends on the variability of the density of the medium (hence its value of refraction index). The effect is well understood in physical optics, and is pressure (P), temperature (T) and wavelength ( $\lambda$ ) dependent. The spatial variations can be expressed in terms of P, T and $\lambda$ and the resulting practical values of refraction (minutes of arc per km) for the ratio of temperature to altitude is significant. In the simplest terms this means that:
- One degree of arc represents the largest angular change between the true and false (mirage) position of an object.
- When the image appears above the true position this is know as a superior mirage. When below the true position, the term used is an inferior mirage.
- There must be the coincident conditions of a temperature inversion and the presence of light to illuminate the object at grazing angle.
- Vertical stretching can lead an observer to underestimate the true distance to the luminous object.
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