Emmert's law
Encyclopedia
Emmert's Law bears the name of Emil Emmert (1844–1911), since it was first described by him in 1881. Emmert noted that an afterimage appeared to increase in size when projected to a greater distance. It is unclear whether he intended this to mean physical distance or perceived distance, but most authors assume the latter. A modern version of the law states that objects that generate retinal images
of the same size will look different in physical size (linear size) if they appear to be located at different distances. Specifically, the perceived linear size of an object increases as its perceived distance from the observer increases. This makes intuitive sense: an object of constant size will project progressively smaller retinal images as its distance from the observer increases. Similarly, if the retinal images of two different objects at different distances are the same, the physical size of the object that is farther away must be larger than the one that is closer.
The effect of viewing distance on perceived size can be observed by first obtaining an afterimage,
which can be achieved by viewing a bright light for a short time, or staring at a figure for a longer time. It appears to grow in size when projected to a further distance. However, the increase in perceived size is much less than would be predicted by geometry, which casts some doubt on the geometrical interpretation given above.
Emmert's law is closely related to size-constancy, and has been used to investigate the moon illusion
(the apparent enlargement of the moon or sun near the horizon compared with higher in the sky).
Retina
The vertebrate retina is a light-sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...
of the same size will look different in physical size (linear size) if they appear to be located at different distances. Specifically, the perceived linear size of an object increases as its perceived distance from the observer increases. This makes intuitive sense: an object of constant size will project progressively smaller retinal images as its distance from the observer increases. Similarly, if the retinal images of two different objects at different distances are the same, the physical size of the object that is farther away must be larger than the one that is closer.
The effect of viewing distance on perceived size can be observed by first obtaining an afterimage,
which can be achieved by viewing a bright light for a short time, or staring at a figure for a longer time. It appears to grow in size when projected to a further distance. However, the increase in perceived size is much less than would be predicted by geometry, which casts some doubt on the geometrical interpretation given above.
Emmert's law is closely related to size-constancy, and has been used to investigate the moon illusion
Moon illusion
The Moon illusion is an optical illusion in which the Moon appears larger near the horizon than it does while higher up in the sky. This optical illusion also occurs with the sun and star constellations. It has been known since ancient times, and recorded by numerous different cultures...
(the apparent enlargement of the moon or sun near the horizon compared with higher in the sky).
See also
- Visual perceptionVisual perceptionVisual perception is the ability to interpret information and surroundings from the effects of visible light reaching the eye. The resulting perception is also known as eyesight, sight, or vision...
- Optical illusionOptical illusionAn optical illusion is characterized by visually perceived images that differ from objective reality. The information gathered by the eye is processed in the brain to give a perception that does not tally with a physical measurement of the stimulus source...