How does looming occur

Explore how a fish observed in a body of water is actually swimming much deeper than it appears to be. This interactive tutorial enables the observer to vary the water depth, changing the position of the virtual fish image. This concept is nicely illustrated by the illusion, created by refraction effects, of the actual depth of a fish in shallow water when observed from the bank of a lake or pond see Figure 6.

When we peer through the water to observe fish swimming around the pond, they appear to be much closer to the surface than they really are. On the other hand, from the fish's point of view, the world appears distorted and compressed above the water due to virtual images created by refraction of reflected and transmitted light reaching the eyes of the fish.

In fact, due to refraction, a fisherman on the bank appears to be farther away from the fish from the fish's viewpoint than he or she really is. This phenomenon can be used to determine the refractive index of a liquid with an optical microscope.

A flat cell capable of holding liquid with a mark or graduations placed on the inside glass surface is constructed or purchased for this experiment. One of the microscope eyepieces must have a graduated reticle inserted at the primary image plane for line width measurements of the mark in the flat cell.

Before adding the liquid of unknown refractive index to the cell, the microscope is focused on the mark at the bottom of the cell and a measurement of the mark's position on the reticle is noted. Next, a small amount of liquid is added to the cell and the microscope is refocused on the mark through the liquid and a new measurement is taken. The microscope is finally focused on the surface of the liquid, and a third reading is recorded by measuring the position of the mark on the reticle. The refractive index of the unknown liquid can then be calculated using the following equation:.

Although it is generally true that light must pass from one substance into another to undergo refraction, there are circumstances in which perturbations, such as temperature gradients, can produce enough fluctuation in refractive index within a single medium to generate a refractive effect.

If they have significantly different temperatures, overlapping layers of air in the atmosphere are responsible for producing what are often termed mirages , a phenomenon in which the virtual image of an object is observed to be positioned either above or below the actual object.

Layering of warmer and cooler air is especially common over desert areas, the ocean, and hot asphalt pavement such as parking lots and highways. The actual mirage effect that is visualized depends upon whether cooler air overlies warmer air, or vice versa Figure 7 a.

One type of mirage appears as an upside-down virtual image directly beneath the real object, and occurs when a layer of warm air near the ground or water surface is trapped by denser, cooler air lying above. Light from the object traveling downward into the warmer air adjacent to the ground or water is refracted upward toward the horizon. At some point the light reaches a critical angle for the warm air, and is bent upward by total internal reflection , resulting in the virtual image appearing below the object.

Another form of mirage, termed looming , occurs when warm air lies over a layer of cooler air, and is common over large bodies of water that may remain relatively cool when the air above the water is heated during the day see Figure 7 b.

Light rays from an object, such as a ship on the water, traveling upward through the cool air into the warmer air are refracted downward toward an observer's line of sight. The rays then appear to originate from above the object and it appears to "loom" above its actual position. It is common for ships at sea near the horizon to appear to float above the water.

Although reference is usually made to a standard and fixed refractive index for a substance, careful measurements indicate that the index of refraction for a particular material varies with the frequency and wavelength of radiation, or the color of visible light.

In other words, a substance has many refractive indices that may differ either marginally, or to a significant degree, as the color or wavelength of light is changed. This variation occurs for nearly all transparent media and has been termed dispersion. The degree of dispersion exhibited by a specific material is dependent upon how much the refractive index changes with wavelength. For any substance, as the wavelength of light increases, the refractive index or the bending of light decreases.

In other words, blue light, which comprises the shortest wavelength region in visible light, is refracted at significantly greater angles than is red light, which has the longest wavelengths. It is the dispersion of light by ordinary glass that is responsible for the familiar splitting of light into its component colors by a prism.

Discover how the incident angle of white light entering the prism affects the degree of dispersion and the angles of individual light rays exiting the prism. The tutorial also explores how changes in refractive index affect dispersion of light passing through the prism.

In the late seventeenth century, Sir Isaac Newton performed a series of experiments that led to his discovery of the visible light spectrum, and demonstrated that white light is composed of an ordered array of colors starting with blue at one end and progressing through green, yellow, and orange, finally ending with red at the other end. Working in a darkened room, Newton placed a glass prism in front of a narrow beam of sunlight emerging through a hole drilled into a window shutter.

When the sunlight passed through the prism, an ordered spectrum of color was projected onto a screen placed behind the prism. From this experiment, Newton concluded that white light is produced from a mixture of many colors, and that the prism spread or "dispersed" white light by refracting each color at a different angle so they could be easily separated Figure 8. Newton was unable to further subdivide the individual colors, which he attempted by passing a single color of dispersed light through a second prism.

However, when he placed a second prism very close to the first, so that all of the dispersed colors entered the second prism, Newton found that the colors were recombined to produce white light again.

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By using this site you agree to the use of cookies. Check our Cookie Policy for more details. Many analyses assume that a normal driver would and should respond as soon as looming is perceptible. Is that a good assumption? There are several reasons why it probably is not. All of the thresholds for looming originate from car-following paradigms, where the lead vehicle is a relatively short distance away in daylight, the speed is slow and the deceleration occurs suddenly.

It is unclear whether these thresholds generalize to the case of a freeway driver traveling 70 mph at night who encounters a stopped truck. The speeds are higher, the distances are greater, the approach speed is greater, and yet the growth of looming is slower and more gradual, at least at first. The distance is an especially critical variable since drivers rely on different information at short and long distances e.

The output of the primary visual cortex splits into ventral and dorsal streams e. The ventral stream is the "what" system that is responsible for conscious object recognition. It uses spatial information and operates at long distances. It crudely infers relative motion from change in location.

The dorsal stream is the where ambient "where" system that operates outside of consciousness and guides egomotion using optical information. It directly perceives absolute motion from optic flow. Many people mistaken believe that looming specifies closing rate. It does not. It is not a "specifying variable" - an infinite number of closing speeds, distances and lead vehicle sizes can produce the same angular velocity.

Many people also mistakenly believe that it can reveal whether a lead vehicle is stopped. It can't. In addition to not being a specifying variable, it depends entirely on relative speed , not absolute speed.

A 60 mph driver approaching a 20 mph lead vehicle sees the same looming as a 40 mph driver approaching a stopped vehicle. Looming can combine with visual angle to reveal the TTC, but it still does not specifically say whether or not response is necessary.

The driver still must interpret the TTC in terms of the overall situation. For example, drivers brake at longer TTC at higher speeds Host, , although not long enough to compensate for the increased stopping distance. Moreover, drivers commonly travel behind a lead vehicle with a 1. They do this because they do not expect the lead vehicle to suddenly brake hard. There is more to deciding whether to brake than merely perceiving the TTC.

Moreover, our brains evolved prior to the development of motorized vehicles. At foot speed, an object even feet away is not an immediate threat. We don't have a strong innate sense that an object at that distance constitutes a collision affordance. The entire scenario described above is based on the " hypothesis," which posits that drivers use to judge TTC and to avoid collision. However, much research e. While drivers certainly incorporate optical image growth into judgments about when to brake, the judgment is not necessarily based on either alone or with other variables.

A common finding is that braking behavior is better predicted by a weighted combination of expansion rate and image size. Other research suggests that drivers use the temporal derivative of , "tau dot" , rather than or perhaps use the temporal derivative of the expansion rate.

All of these optical variables change systematically with TTC. Many treat looming as a signal to respond, i.

It is not. The data show that drivers do not reliably respond to looming until it reaches 0. The drivers then respond in under a second. The implication is clear: drivers respond to perceived hazards and not simply to motion. They must interpret the motion, so there is a gap between simple perception of motion and perception of a hazard that signals the need to respond. There is good evidence that driver TTC judgments are also influenced by spatial variables such as depth and distance cues, e.

Viewers also experience "motion adaptation," which causes the looming to slow. See Green, et al, for more discussion. There is also ample evidence that driver braking judgments are based on some other variables from the optic flow field, e. These provide information about egospeed. Humans are satisficers.

Time and again I have seen reports and heard testimony that blames a person for failure to act in some idealized, optimal way - a way in which absolutely no real person behaves. Most drivers have approached trucks on the highway thousands of times. At the normal speed differentials, the driver can look in his mirror, scan the scenery, think of dinner, etc. Since this is almost always the existing state-of-affairs, the driver adapts and relies on this degree of fault tolerance as the norm.

This is human nature and what people do: 1 they adapt their behavior to requirements of the situation and 2 they economize their effort to be efficient. Nobel Prize winner Herbert Simon coined the term "satisficing" Simon, to summarize the strong innate human tendency to seek satisfactory solutions that are reasonable tradeoffs between efficiency and outcome.

Humans are not optimizers who attempt unnecessary perfection for its own sake. They learn the system tolerance and determine a satisficing solution that is a good tradeoff between effort and outcome. They don't stare fixedly at the road because such close attention extracts a high mental cost of stress and fatigue and because it is simply not necessary. The person who encounters the stopped truck and fails to avoid is often simply unlucky.

Expected safety margins suddenly vanish. I have explained how the roadway teaches experienced drivers the system tolerances, i. For example, a driver traveling 65 mph sees a vehicle feet, about the lower end of looming perception distance. These are large safety margins. If the vehicle is stopped, however, the driver has only 3.

He is already in a severe emergency situation. The decision to brake also depends on the vehicle capabilities, possible response alternatives and the possible consequences of sharp braking or turning, etc. Green, This last point deserves some amplification. A Fata Morgana is most commonly seen in polar regions, especially over large sheets of ice that have a uniform low temperature.

It may, however, be observed in almost any area. In polar regions the Fata Morgana phenomenon is observed on relatively cold days. In deserts, over oceans, and over lakes, however, a Fata Morgana may be observed on hot days. A Fata Morgana may be observed from any altitude within the Earth's atmosphere, from sea level up to mountaintops, and even including the view from aircraft.

A Fata Morgana may be described as a very complex superior mirage with more than three distorted erect and inverted images. Because of the constantly changing conditions of the atmosphere, a Fata Morgana may change in various ways within just a few seconds of time, including changing to become a straightforward superior mirage. A Fata Morgana superior mirage of a ship can take many different forms.

Even when the boat in the mirage does not seem to be suspended in the air, it still looks ghostly, and unusual, and what is even more important, it is ever-changing in its appearance. Sometimes a Fata Morgana causes a ship to appear to float inside the waves, at other times an inverted ship appears to sail above its real companion. In fact, with a Fata Morgana it can be hard to say which individual segment of the mirage is real and which is not real: when a real ship is out of sight because it is below the horizon line, a Fata Morgana can cause the image of it to be elevated, and then everything which is seen by the observer is a mirage.