A mirage is a naturally occurring optical phenomenon in which light rays are bent to produce a displaced image of distant objects or the sky.
The word comes to English via the French mirage, from the Latin mirari, meaning “to look at, to wonder at”. This is the same root as for “mirror” and “to admire”.
In contrast to a hallucination, a mirage is a real optical phenomenon that can be captured on camera, since light rays are actually refracted to form the false image at the observer’s location.
What the image appears to represent, however, is determined by the interpretive faculties of the human mind. For example, inferior images on land are very easily mistaken for the reflections from a small body of water.
Mirages can be categorized as:
“inferior” (meaning lower)
“superior” (meaning higher)
“Fata Morgana”, one kind of superior mirage consisting of a series of unusually elaborate, vertically stacked images, which form one rapidly changing mirage.
Mirage. (2016, December 18). In Wikipedia, The Free Encyclopedia.
According to legend, Erik the Red sailed from Iceland and discovered Greenland after he had seen the island in a mirage. Describe how the mirage might have occurred.
Well, that answer from our textbook teacher editions, however true, isn’t very helpful. It’s not clear what we are looking at. Let’s look at a much better picture to see both the problem and the solution.
Problem: Erik the Red shouldn’t be able to see Greenland from where he is standing, on Iceland. Greenland is so far away that it is over the curve of the Earth (over the horizon.)
The superior mirage, also know in northern polar regions as the arctic mirage — or in Icelandic, the hillingar effect — causes the light from distant objects to be optically refracted downward
Thus it becomes possible for objects lying beyond the normal horizon to be seen.
(They even appear, at times, to rise up over the horizon, a condition known to mariners as looming, and look much closer in distance.)
Fata Morgana Mirage in Greenland, 1999, by Jack Stephens
SEE BELOW FOR THE FAMOUS MOBY DICK MIRAGE
The arctic mirage, on the other hand, occurs when the light rays are refracted downward by cold, dense air near the earth into an arc bending toward the observer. (In the diagrams accompanying this article, the dark lines indicate the actual light ray path and the white dashed lines the path our mind thinks it sees.)
The refractivity of air — a measure of the air’s ability to bend the path of light rays — is dependent upon its density, and the density of air is inversely related to its temperature (decreasing as temperature increases). The atmospheric conditions for producing the arctic mirage occur when cool air adjacent to the surface underlies warm air. When the air temperature increases with altitude, the condition is known meteorologically as a temperature inversion.
When the temperature of the lower atmosphere increases with altitude at a rate of 11.2 C° per 100 metres (6.0 F° per 100 ft), the refractive capacity of the air is great enough to cause the path of light rays to bend in an arc equal to the curvature of the Earth. This curvature can present an observer with the image of a flat horizon receding to infinity. A temperature gradient greater than 11.2 C° per 100 m causes light ray paths to exceed the curvature of the Earth, and thus the horizon would appear to be raised upward giving the Earth’s surface a saucer-shaped appearance. Under this latter condition, images of objects located at or below the normal optical horizon, such as mountains, glaciers, cliffs or sea-ice rise (loom) into the field of vision, overcoming the normal visual restrictions of the curvature of the Earth.
The normal viewing distance at the surface of the earth depends upon the height of the object being observed and the height of the observer. Disregarding atmospheric effects on light rays, the curvature of the earth restricts the distance one can see from the surface. For example, a beach or small iceberg rising 3.0 to 3.7 m (10 to 12 ft) above the sea surface can be seen from the surface at a distance of no more than 19.2 km (12 miles) through a clear, normal atmosphere. A mountain peak of 914 metres (3,000 feet) would disappear at 115 kilometres (72 miles) distant, one 1520 m (5,000 ft) tall at 150 km (94 miles).
The maximum viewing distance under arctic mirage conditions, on the other hand, is limited only by the light absorption of the atmosphere. Near sea level, the transmission of light is generally of sufficient quality to enable the naked eye to potentially see objects at a distance of up to 400 km (250 miles). However, when the refracting layer is at the upper boundary of a very deep cold layer, the thinner air may permit more light to be transmitted, thus making visibility in excess of 400 km possible.
Under arctic mirage conditions, instances of atmospheric visibility extending 320 km (200 miles) have been reported. In 1937 and 1939, W.H. Hobbs documented several occasions during which objects were sighted at distances well in excess of those possible under normal viewing conditions.
Answer text from The Arctic Mirage. Aid to Discovery. The Weather Doctor.
Moby Dick illusion
One famous literary description of a Fata Morgana occurs in Chapter 135 of Herman Melville’s masterpiece, Moby Dick. As Ahab is pulled overboard, and the White Whale rams the Pequod, Melville writes:
“The ship? Great God, where is the ship? Soon they through dim, bewildering mediums saw her sidelong fading phantom, as in the gaseous Fata Morgana.”
But, of course the ship was sinking, the vision was an illusion.
A Ship Floating In Mid-Air At A Scottish Golf Tournament?
Just like the above mentioned mirages!
image: Tom Phillips/BuzzFeed/ilyast/syntika/Thinkstock
Floating boats and islands
Fata Morgana Mirage at Cocoa Beach, FL!
Lake Superior Marquette, MI 05.23.15 – first scene is real time, freighter in to Marquette, second is timelapse, Granite Island looking like a lava lamp
2016 Massachusetts Science and Technology/Engineering Curriculum Framework
HS-PS4-3. Evaluate the claims, evidence, and reasoning behind the idea that electromagnetic radiation can be described by either a wave model or a particle model, and that for some situations involving resonance, interference, diffraction, refraction, or the photoelectric effect, one model is more useful than the other.
Core Idea PS4: Waves and Their Applications in Technologies for Information Transfer
When a wave passes an object that is small compared with its wavelength, the wave is not much affected; for this reason, some things are too small to see with visible light, which is a wave phenomenon with a limited range of wavelengths corresponding to each color. When a wave meets the surface between two different materials or conditions (e.g., air to water), part of the wave is reflected at that surface and another part continues on, but at a different speed. The change of speed of the wave when passing from one medium to another can cause the wave to change direction or refract. These wave properties are used in many applications (e.g., lenses, seismic probing of Earth).
The wavelength and frequency of a wave are related to one another by the speed of travel of the wave, which depends on the type of wave and the medium through which it is passing. The reflection, refraction, and transmission of waves at an interface between two media can be modeled on the basis of these properties.
All electromagnetic radiation travels through a vacuum at the same speed, called the speed of light. Its speed in any given medium depends on its wavelength and the properties of that medium. At the surface between two media, like any wave, light can be reflected, refracted (its path bent), or absorbed. What occurs depends on properties of the surface and the wavelength of the light.
SAT Subject Area Test in Physics
Waves and optics:
- Reflection and refraction, such as Snell’s law and changes in wavelength and speed
- Ray optics, such as image formation using pinholes, mirrors, and lenses