Human Eye in Cross Section –
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Human Eye in Cross Section
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About the Diagram
Some Key Terms
A human observer is a person who engages in observation by watching things.
- In the presence of visible light, an observer perceives colour because the retina at the back of the human eye is sensitive to wavelengths of light that fall within the visible part of the electromagnetic spectrum.
- The visual experience of colour is associated with words such as red, blue, yellow, etc.
- The retina’s response to visible light can be fully described in terms of wavelength, frequency and brightness.
- Other properties of the world around us must be inferred from patterns of light.
- The human eye and so human visual perception are tuned to the visible spectrum and so to spectral colours between red and violet.
- There are no properties of electromagnetic radiation that distinguish visible light from other parts of the electromagnetic spectrum.
- Objects appear to be different colours to an observer depending on the wavelengths, frequencies and amplitude of visible light at the moment it strikes the retina at the back of the eye.
Optical illusions and other visual anomalies are caused by the way the human visual system processes information.
Physical illusions result from the limitations and assumptions of the human visual system when interpreting the external world. Examples include:
- The Sun and Moon appear larger near the horizon as a result of the brain’s interpretation of distance cues.
- Rainbows are composed of a continuous range of wavelengths across the visible spectrum but appear to be formed from a series of bands of colour.
Physiological illusions are often connected with the different attributes of visual perception and occur when visual stimuli are beyond our brain’s processing ability.
Physiological illusions arise due to the way that the human eye and visual system process information from the outside world, such as lighting, contrast, and colour. Examples include:
- After-images occur when the eye’s photoreceptor cells become fatigued due to overstimulation, resulting in an image appearing after the stimulus is removed.
- Moiré patterns occur when two similar patterns with slightly different frequencies overlap, creating a new pattern that appears to move or vibrate.
Cognitive illusions result from the brain’s inability to correctly interpret visual information, leading to uncertainties or errors in perception. Examples include:
- Ambiguous illusions are images that can be read in more than one way, depending on contextual cues and the viewer’s past experiences. They often cause a perceptual “switch” between alternative interpretations.
- Geometrical illusions occur when the brain uses contextual cues and assumptions to interpret visual stimuli, leading to distortions in size, length, position, or curvature.
- Paradox illusions occur when visual stimuli contain conflicting information that cannot be resolved by the brain, leading to a perceptual paradox.
- Fictions are created when the brain fills in missing visual information based on contextual cues and past experiences, leading to the perception of additional content that is not actually present.
- An object is intuitively assumed to exist and to be responsible for a unified experience, consisting of visual and other sensations and perceptions.
- Every object, material, medium or substance that we can see is made of matter of one kind or another. The key differentiating factor is the elements and molecules they are constructed from.
- You will have come across the elements that make up the periodic table.
- A close look at molecules reveals that they are made up of atoms composed of electrons surrounding a nucleus of protons and electrons.
- Light illuminates objects. In a nutshell, different elements and molecules react to light in different ways because of their atomic structure and the particular way they combine to form mixtures or compounds.
- In the case of an opaque object, it is the molecules that form its surface that determine what happens when light strikes it. Translucent and transparent objects behave differently because light can travel through them.
- Another factor that needs to be taken into account when light strikes an object is surface finish. A smooth and polished surface behaves differently from one that is rough, textured or covered in ripples.
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