MOTION MOUNTAIN

images – transporting light 155
The (inverted) superior mirage, neglecting Earth’s curvature:
hot air
cold air
The (inverted) inferior mirage, neglecting Earth’s curvature:
cold air
hot air
Ref. 112
Ref. 113
F I G U R E 103 The basis of mirages is an effective reflection due to refraction in a hot air layer; it can lead
to spectacular effects, such as the inverted superior mirage (top left and right) and the inferior image
(bottom left and right) (photographs © Thomas Hogan and Andy Barson).
index of around 0.2 for visible light, and thus a phase velocity of around5c for such waves.
In fact, almost all materials have refractive indices below 1 for some wave frequencies,
including table salt.
In short, refraction of light, the change of the direction of light motion, is due todifferent
phase velocities of light in different materials. Material changes bend light paths.
Refraction is so common because it is extremely rare to have different adjacent materials
with the same refractive index.
Gases have refractive indices close to the vacuum value 1. Nevertheless, also gases lead
to refraction. In particular, the refractive index of gases depends on temperature. In air of
varying temperature, refraction leads to curved light paths and produces a well-known
effect: the mirage, also called fata morgana. Figure 103 showsphotographs of a superior
mirage, which relies on an inversion layer in the atmosphere above the object and the
observer, and a inferior mirage, due to a hot layer of air below the observer, just above
the ground. Inferior mirages are also regularly seen on hot highways. All mirage types
are due to refraction; their detailed appearance depends on the given temperature profile
in the air, and the relative heights of the observer, the inversion layer and the observed
object. Often, the curvature of the Earth also plays a role.
Above all, refraction is used in the design of lenses. With glass one can produce precisely
curved surfaces that allow us to focus light. All focusing devices, such as lenses,
can be used to produce images.The two main types of lenses, with their focal points and
the images they produce, are shown in Figure 104; they are called converging lenses and
divergent lenses. When an object is more distant from a single converging lens than its
focus, the lens produces a real image, i.e., an image that can be projected onto a screen.
In all other cases single converging or diverging lenses produce so-calledvirtualimages:
such images can be seen with the eye but not be projected onto a screen. For example,
whenan object isput between a converging lens and its focus, the lens works as a mag-
Motion Mountain – The Adventure of Physics copyright © Christoph Schiller June 1990–November 2015 free pdf file available at www.motionmountain.net