Optic chiasm

Optic chiasm

The optic chiasm is the part of the brain where the optic nerves partially cross. It is located at the bottom of the brain immediately below the hypothalamus.

The cross-over of optic nerve fibres at the optic chiasm allows the visual cortex to receive the same hemispheric visual field from both eyes. Superimposing and processing these monocular visual signals allows the visual cortex to generate binocular and stereoscopic vision.

So, the right visual cortex receives the temporal visual field of the left eye, and the nasal visual field of the right eye, which results in the right visual cortex producing a binocular image of the left hemispheric visual field. The net result of optic nerves crossing over at the optic chiasm is for the right cerebral hemisphere to sense and process left-hemispheric vision, and for the left cerebral hemisphere to sense and process right-hemispheric vision.

 

Optic nerve

Optic nerve

The optic nerve is the cable–like grouping of nerve fibres formed from the axons of ganglion cells that transmit visual information towards the lateral geniculate nucleus.

The optic nerve contains around a million fibres and transports the continuous stream of data that arrives from rods, cones and interneurons (bipolar, amacrine cells). The optic nerve is a parallel communication cable that enables every fibre to represent distinct information about the presence of light in each region of the visual field.

Observer’s point of view

To understand rainbows it is important to sort out what an observer is actually looking at.

  • Rainbows only exist in the eyes of an observer.
  • Every observer sees a different rainbow produces by a unique set of raindrops that happen to be in the right place at the right time.
  • The individual raindrops that result in the appearance of a rainbow for one observer are always different from the raindrops that produce a rainbow for someone else.
  • As an observer moves, their rainbow moves with them. Seen from a car window, the rainbow appears stationary whilst the landscape rushes past.
From an observer’s point of view
  • Atmospheric rainbows appear to an observer as arcs of colour across the sky. From an aeroplane, a rainbow can appear as entire circles of colour.
  • Even from the ground, it is easy to deduce that every rainbow has a centre point around which the arcs of a rainbow are arranged.
  • The exact position in the sky where an atmospheric rainbow will appear can be anticipated by working out where its centre will be.
  • The centre of a rainbow is always on an imaginary straight line that starts at the centre of the Sun behind you, passes through the back of your head, out through your eyes and extends in a straight line into the distance.
  • The eyes of an observer are always aligned with the rainbow axis.
  • To an observer, the rainbow axis appears as a point, not a line, and that imaginary point marks the centre of where every rainbow will appear.
  • The idea that a rainbow has a centre corresponds with what an observer sees in real-life.
  • The idea of a rainbow axis or anti-solar point corresponds with a diagrammatic view showing the scene in side elevation.
Looking for rainbows
  • To work out where a rainbow might appear:
    • Turn your back on the Sun.
    • If you can see your shadow, look at the head. The axis of the rainbow runs from the Sun behind you, through your eyes and through the head of your shadow. Imagine where your eyes might be in your shadow. If a rainbow appears that point will be its centre.
    • If you can’t see your shadow, just try and imagine the line from the Sun, passing through your head and then extend it away from you till it reaches the landscape. At whatever point it touches, that will be the centre.
    • Unless you are in a plane, the centre point is always below the horizon so on the ground or in the landscape in front of you.
    • Now, with the Sun behind you spread out your arms to either side or up and down at 450 from the centre point.
    • Swing them round like the blades of a windmill. That is where your primary rainbow will appear.
Remember that:
  • Every observer has a rainbow axis and a centre-point on that axis that moves with them as they change position. It means that their rainbow moves too.
  • The centre of a secondary rainbow is always on the same axis as the primary bow and shares the same anti-solar point.
  • To see a secondary rainbow look for the primary bow first – it has red on the outside. The secondary bow will be a bit larger with violet on the outside and red on the inside.
Rainbows as discs of colour
  • Close consideration of why rainbows appear as arcs or circles can be explained by the idea that an observer is looking at superimposed, concentric discs of colour.
  • Think in terms of each observed band of colour within a rainbow forming on the edge of a separate coloured disc.
  • The area close to the circumference of each disc produces the most intense and brilliant colour.
  • The intensity of each colour drops sharply away from the circumference of its disc and towards the centre.
  • The observed colour of each disc corresponds with the band of wavelengths that produces it.
  • The fact that we see distinct bands of colour in a rainbow is often described as an artefact of human vision.
  • Each disc contributes small amounts of its own colour to the area towards the shared centre of the six concentric discs making the sky appear lighter.

Orders of rainbows

Primary rainbows are sometimes referred to as first-order bows. First-order rainbows are produced when light is reflected once as it passes through the interior of each raindrop.

Secondary rainbows are second-order bows. Second-order bows are produced when light is reflected twice as it passes through the interior of each raindrop.

  • Each subsequent order of rainbows involves an additional reflection inside raindrops.
  • Higher-order bows get progressively fainter because photons escape droplets after the final reflection. As a result, insufficient light reaches an observer to trigger a visual response.
  • Each higher-order of bow gets progressively broader spreading photons more widely and reducing their brightness further.
  • Only first and second-order bows are generally visible to an observer but multi-exposure photography can be used to capture them.
  • Different orders of rainbows don’t appear in a simple sequence in the sky.
  • First, second, fifth and sixth-order bows all share the same anti-solar point.
  • Zero, third and fourth-order bows are all centred on the Sun and appear as circles of colour around it.

https://www.atoptics.co.uk/rainbows/orders.htm

optical density

Optical density is a measurement of the degree to which a refractive medium slows the transmission of light.

  • The optical density of a medium is not the same as its physical density.
  • The more optically dense a medium, the slower light travels through it.
  • The less optically dense (or rare) a material is, the faster light travels through it.
  • A vacuum has the least optical density and so light travels through it at a maximum speed of 299,792 kilometres per second.
  • Optical density accounts for the variation in refractive indices of different media.

https://en.wikipedia.org/wiki/Absorbance

Observer

A human observer is a person who engages in observation by watching things.

Observer

A human observer is a person who engages in observation or watches something.

  • Human subjects observe themselves, each other and the world around them.
  • The act of observation enables us to develop our understanding and engage with the world.
  • When an observer sees something they are engaging in visual perception.
  • An observer can take many forms:
    • A person watching an ocean sunset or the sky at night.
    • A person studying their baby’s face.
    •  A person studying something they can’t see by collecting data from an instrument or machine.
    • A person conducting an experiment in a laboratory.
  • In everyday life, an observer feels themselves to be involved in the things they observe.
  • A scientific observer is someone who causes no unnecessary changes to the object of their observations.

A human observer is a person who engages in observation by watching things.

Observing light and colour
  • Our eyes are sensitive to light as a result of the stimulation of photoreceptors embedded within the retinas of our eyes.
  • In the presence of light, an observer perceives colour.
  • The visual experience of colour by an observer is associated with words such as red, blue, yellow, etc.
  • Things appear coloured to an observer because colour corresponds with a property of light that is visible to the human eye.
The subjective experience of colour
  • The perception of colour is a very subjective experience.
  • Factors that determine the particular colour an observer sees include:
    • The environment in which colours are observed, the type of object and colour associations.
    • The colour of nearby objects.
    • The well-being of an observer. Health, medications, mood, emotions or fatigue can all affect the eye, vision and perception.
  • Different observers may see colour differently because of life experience including educational, social and cultural factors.
  • The term observer has distinct and different meanings within the fields of special relativity, general relativity, quantum mechanics, thermodynamics and information theory.
Perception of colour
  • The perception of colour is a very subjective experience.
  • One factor that determines the particular colour an observer sees is the colour of nearby objects.
  • Another factor is to do with the well-being of an observer. Health, medications, mood, emotions or fatigue can all affect the eye, vision and perception.
  • A further factor is the environment in which colours are observed, the type of object and colour associations.
  • Two different observers may see colour differently because of their different life-experience including educational, social and cultural factors.
  • The term observer has distinct and different meanings within the fields of special relativity, general relativity, quantum mechanics, thermodynamics and information theory.

References

Object

An object is a material thing that can be seen and touched.

  • 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.

Optics

Optics is the branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of instruments that use or detect it.

  • Most optical phenomena can be accounted for using the classical electromagnetic description of light – wavelength, frequency and intensity.
  • Optics is both a science and an area of engineering. It has been used to make many useful things, including eyeglasses, cameras, telescopes, and microscopes. Many of these things are based on lenses, which focus light and can make images of things that are bigger or smaller than the original.
  • While optics is an old science, new things are still being discovered about it. Scientists have learned how to make light travel through a thin optical fibre made of glass or plastic. Light can go long distances in a fibre. Fibres are used to carry phone calls and the Internet between cities.
  • Contemporary specializations within the field of optics include:
    • Geometrical optics treats light as a collection of rays that travel in straight lines and bend when they pass through or reflect from surfaces.
    • Physical optics is a more comprehensive model of light and includes wave effects such as diffraction and interference that cannot be accounted for in geometric optics.
    • Quantum mechanics deals with the fact that light has both wave-like and particle-like properties.
About geometrical optics

Geometrical optics, or ray optics, is a model of optics that describes light propagation in terms of rays. A ray in geometric optics is an abstraction useful for approximating the paths along which light propagates under certain circumstances.

  • The simplifying assumptions of geometrical optics include that light rays:
  • Propagate in straight-line paths as they travel in a homogeneous medium
  • Bend, and in particular, circumstances may split in two, at the interface between two dissimilar media
  • Follow curved paths in a medium in which the refractive index changes
  • May be absorbed or reflected.
  • Geometrical optics does not account for certain optical effects such as diffraction and interference. This simplification is useful in practice; it is an excellent approximation when the wavelength is small compared to the size of structures with which the light interacts. The techniques are particularly useful in describing geometrical aspects of imaging, including optical aberrations.

Optical phenomena

Optical phenomena ( or optical effects) result from the properties and behaviour of light, including its interactions with matter. Optical phenomena include absorption, dispersion, diffraction, polarization, reflection, refraction, scattering and transmission.

  • Optics is the branch of physics that describes the behaviour of visible, ultraviolet, and infrared light.
  • Visible, ultraviolet, and infrared light along with X-rays, microwaves, and radio waves are all forms of electromagnetic radiation.
  • Many optical phenomena can be accounted for using the classical electromagnetic description of light as propagating in the form of waves.
  • Geometric optics treats light as collections of rays that travel in straight lines but bend when they pass through or reflect from surfaces.These behaviours are often mapped out using ray diagrams.
  • Ray diagrams have practical application when describing how everyday objects work, including mirrors, lenses, telescopes, microscopes, lasers, and fibre optic devices.
  • Some optical effects such as diffraction and interference must be explained in terms of the particle-like properties (photons) and with reference to the field of quantum mechanics and terms such as wave-particle duality.

Summary

Oscillation

An oscillation is a periodic motion that repeats itself in a regular cycle. An oscillating movement is always around an equilibrium point or mean value. It is also known as a periodic motion.

Oscillation

An oscillation is a periodic motion that repeats itself in a regular cycle. An oscillating movement is always around a point of equilibrium.

An oscillation is a periodic motion that repeats itself in a regular cycle. An oscillating movement is always around an equilibrium point or mean value. It is also known as a periodic motion.

  • The term oscillation denotes something that moves in one direction, then moving back in a repeating pattern.
  • An electromagnetic wave describes an oscillatory motion as the electric field and then the magnetic field take turns to increase to their maximum values and then drop back to zero.

optical density

Optical density is a measurement of the degree to which a transparent medium slows the transmission of light.

  • The optical density of a medium is not the same as its physical density.
  • The more optically dense a medium, the slower light travels through it.
  • The less optically dense (or rare) a material is, the faster light travels through it.
  • A vacuum has the least optical density and so light travels through it at a maximum speed of 299,792 kilometres per second.
  • Optical density accounts for the variation in refractive indices of different media.

Opacity

Opacity refers to the degree to which an object, area or surface obscures objects or space beyond.

  • Different processes can lead to opacity including absorptionreflection, and scattering.
  • An entirely opaque substance transmits no light, and therefore reflects, scatters, or absorbs all of it.
  • When light strikes an interface between two media some light may be reflected, some absorbed, some scattered. The remainder undergoes refraction and is transmitted through the second medium.
  • So, opacity is the measure of impenetrability to electromagnetic radiation, especially visible light by a material.
  • An opaque object is neither transparent (allowing all light to pass through) nor translucent (allowing some light to pass through).
  • Both mirrors and carbon black are opaque.
  • Opacity depends on the wavelengths of the light being considered. For instance, some kinds of glass, while transparent in the visual range, are largely opaque to ultraviolet light.

optic radiation

The optic radiation are tracts formed from the axons of neurons located in the lateral geniculate nucleus and lead to areas within the primary visual cortex.

  • There is an optic radiation on each side of the brain. Each one carries visual information through two divisions called the upper and lower divisions to their corresponding cerebral hemisphere.

Optic chiasm

The optic chiasm is the part of the human brain where the optic nerves partially cross. The optic chiasm is located at the bottom of the brain immediately below the hypothalamus.

  • The cross-over of optic nerve fibres at the optic chiasm allows the visual cortex to receive the same hemispheric visual field from both eyes.
  • Superimposing and processing these monocular visual signals allow the visual cortex to generate binocular and stereoscopic vision.
  • For example, the right visual cortex receives the temporal visual field of the left eye, and the nasal visual field of the right eye, which results in the right visual cortex producing a binocular image of the left hemispheric visual field. The net result of optic nerves crossing over at the optic chiasm is for the right cerebral hemisphere to sense and process left hemispheric vision, and for the left cerebral hemisphere to sense and process right hemispheric vision.

Optic nerve

The optic nerve of the human eye is the cable–like grouping of nerve fibres formed from the axons of ganglion cells that transmit visual information towards the lateral geniculate nucleus.

  • The optic nerve contains around a million fibres that transport continuous stream of data which have been received from rods, cones and the intermediate neuron types, bipolar and amacrine cells.
  • The optic nerve is a parallel communication cable that enables every fibre to represent distinct information about the presence of light in each region of the visual field.

Object

An object is a material thing that can be seen and touched.

  • 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, whether a material, medium or substance, is made of matter of one kind or another.
  • The key differentiating factor between objects is the atoms, molecules and compounds they are constructed from.

An object is a material thing that can be seen and touched.

  • 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.
Is a rainbow an object?
The colour of objects
  • Objects are composed of atoms, molecules, elements and compounds that react to light in different ways because of their atomic structure and composition.
  • Objects appear to be different colours to an observer depending on the wavelengths, frequencies and intensity of light at the moment it strikes the retina at the back of the eye.
  • In the case of opaque objects, it is their surface that determines what happens when they are exposed to light.
  • Translucent and transparent objects behave differently because light can travel through them.
  • The surface finish of objects affects their appearance. Smooth and polished (specular) surfaces behave differently from those that are rough, textured or rippled.