Dictionary tag: O

Object

An object is a physical entity and so a thing that has mass and occupies space.

  • An object, as a physical entity, can be defined by its mass and the space it occupies. Objects are characterized by properties such as size, shape, texture, and colour, all of which are perceptible through the senses.
  • The perception of an object arises from the brain’s interpretation of sensory information, primarily from the light reflected or transmitted by the object. At a fundamental level, all objects are composed of atoms or molecules, which form the basic structure of matter. The arrangement and behaviour of these atoms and molecules determine the object’s physical and chemical properties.
  • Light plays a critical role in how objects are seen. Different atoms and molecules interact with light based on their unique structures and how they combine to form compounds or mixtures. When light strikes an opaque object, its surface molecules largely determine how the light is absorbed, reflected, or scattered. By contrast, translucent and transparent objects allow light to pass through, leading to different optical effects.
  • Additionally, surface finish influences how light interacts with an object. Smooth, polished surfaces reflect light in a more uniform manner, while rough, textured, or rippled surfaces cause light to scatter in multiple directions.
Is a rainbow an object?
  • An atmospheric rainbow is not a physical object but an optical illusion caused by the refraction, reflection, and dispersion of light.
  • An atmospheric rainbow is created when sunlight is refracted, reflected and dispersed through water droplets in the air, forming a circular arc of colours in the sky that can be seen by an observer.
  • A rainbow is an optical effect, a trick of the light.
About the colour of objects
  • Objects are composed of atoms, which bond together to form molecules, elements, and compounds. These different combinations of atoms give objects their unique properties and determine how they react to light.
  • Objects appear to have different colours because they absorb some wavelengths of light and reflect or transmit others. The wavelengths that are reflected determine the colour of an object seen by an observer.
  • In physics, the absorption and reflection of light are explained as follows:
      • Atoms are composed of protons, neutrons and electrons. Protons and neutrons form the nucleus and electrons orbit the nucleus.
      • The orbit, or energy level, of an electron in an atom can change when the electron gains or loses energy.
      • When an electron absorbs a photon of light, it gains energy and moves to a higher energy level so a higher orbit.
      • The difference between the initial and final energy levels of the electron is equal to the energy of the absorbed photon.
      • When an electron moves to a higher energy level it is in an unstable state and eventually returns to its original lower energy level.
      • When the electron returns to its original lower energy level, it emits a photon of light with a frequency and wavelength corresponding to the energy difference between the two levels.
      • The difference between any two energy levels of an electron is specific to the type of atom and can be thought of as being equal to a “quantum” of energy,  where a quantum is understood to mean an indivisible unit of energy.
      • Every type of atom has a unique set of energy levels, and so it emits or absorbs photons of light at specific wavelengths or colours.
    • The colour of objects perceived by an observer can be affected by the lighting conditions in which they are viewed and so by the spectral power distribution of the light source.
    • The surface texture of objects affects how light interacts with them. Smooth and polished surfaces reflect light in a regular pattern, while rough and textured surfaces scatter light and colour in many directions.
    • Transparent objects allow much of the light that strikes them to pass through. The colour seen by an observer is affected by impurities or defects in the material and by the colour of the background against which they are viewed.
    • In the case of opaque objects, the surface of the object reflects, absorbs or scatters light, which determines what is seen by the observer.
Related diagrams

Each diagram below can be viewed on its own page with a full explanation.

References

Object

An object is a material thing that has mass and occupies space.

  • An object, as a physical entity, can be defined by its mass and the space it occupies. Objects are characterized by properties such as size, shape, texture, and colour, all of which are perceptible through the senses.
  • The perception of an object arises from the brain’s interpretation of sensory information, primarily from the light reflected or transmitted by the object. At a fundamental level, all objects are composed of atoms or molecules, which form the basic structure of matter. The arrangement and behaviour of these atoms and molecules determine the object’s physical and chemical properties.
  • Light plays a critical role in how objects are seen. Different atoms and molecules interact with light based on their unique structures and how they combine to form compounds or mixtures. When light strikes an opaque object, its surface molecules largely determine how the light is absorbed, reflected, or scattered. By contrast, translucent and transparent objects allow light to pass through, leading to different optical effects.
  • Additionally, surface finish influences how light interacts with an object. Smooth, polished surfaces reflect light in a more uniform manner, while rough, textured, or rippled surfaces cause light to scatter in multiple directions.

Observation of colour

About the observation of colour
  • The human eye is sensitive to the visible spectrum, which includes all the spectral colours ranging from approximately 400 to 700 nanometers.
  • The sensitivity of the eye to the visible spectrum enables us to perceive colours when light interacts with objects.
  • The visual perception of colour by an observer is associated with words such as red, blue, yellow, etc., which are commonly used to describe hue or dominant wavelength.
  • The colour an observer sees depends on:
    • The wavelengths of visible light present in the environment.
    • The wavelengths absorbed, transmitted, or reflected by an object or medium.
  • The perception of colour can be affected by factors such as brightness, contrast, and saturation, which are related to the amount of light present in a stimulus and its interaction with the eye and brain.
  • The observed colour of light is determined by its wavelength, not its frequency. However, as light travels from one medium to another, such as from air to glass, the colour seen by an observer may change due to refraction causing colours to disperse in different directions.

Observer

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 described in terms of wavelength, frequency and brightness.
  • Other properties of the world around us must be inferred from light patterns.
  • An observation can take many forms such as:
    • Watching an ocean sunset or the sky at night.
    • Studying a baby’s face.
    • Exploring something that can’t be seen by collecting data from an instrument or machine.
    • Experimenting in a laboratory setting.

 

Observer

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

  • Humans observe themselves, each other and the world around them.
  • The act of observation allows us to develop our understanding of ourselves and interact 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 a 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 setting.
  • In everyday life, an observer feels involved in the things they observe.
  • Observers can have biases and subjective interpretations, which may affect their observations.
  • A scientific observer is someone who avoids making unnecessary changes to the object of their observations.
Related diagrams

Each diagram below can be viewed on its own page with a full explanation.

References
  • A human observer is a person who engages in observation or watches something.
  • Humans observe themselves, each other and the world around them.
  • The act of observation allows us to develop our understanding of ourselves and interact 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 a 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 setting.

Observer effect

  • The observer effect is a principle of physics and states that any interaction between a particle and a measuring device will inevitably change the state of the particle. This is because the act of measurement itself imposes a disturbance on the particle’s wave function, which is the mathematical description of its state.
  • The concept of observation refers to the act of engaging with an electron or other particle, achieved through measuring its position or momentum.
  • In the context of quantum mechanics, observation isn’t a passive undertaking, observation actively alters a particle’s state.
  • This means that any kind of interaction with an atom, or with one of its constituent particles, that provides insight into its state results in a change to that state. The act of observation is always intrusive and will always change the state of the object being observed.
  • It can be challenging to reconcile this with our daily experience, where we believe we can observe things without inducing any change in them.

Observer effect

The observer effect is a principle of physics and states that any interaction between a particle and a measuring device will inevitably change the state of the particle. This is because the act of measurement itself imposes a disturbance on the particle’s wave function, which is the mathematical description of its state.

  • The concept of observation refers to the act of engaging with an electron or other particle, achieved through measuring its position or momentum.
  • In the context of quantum mechanics, observation isn’t a passive undertaking, observation actively alters a particle’s state.
  • This means that any kind of interaction with an atom, or with one of its constituent particles, that provides insight into its state results in a change to that state. The act of observation is always intrusive and will always change the state of the object being observed.
  • It can be challenging to reconcile this with our daily experience, where we believe we can observe things without inducing any change in them.
  • The observer effect implies that:
    • Measuring a particle’s position compels it to assume a specific location.
    • Measuring a particle’s momentum compels it to adopt a particular measurable value.
  • The observer effect applies to all quantum systems, including atoms, molecules, and photons.
  • The observer effect is not fully understood. There are many different theories about how it works, but no one theory is universally accepted.
Related diagrams

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References

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.

Opacity

Opacity refers to the extent to which an object or surface prevents light from passing through, thereby obstructing light from reaching objects or space beyond.

  • Opacity is influenced by factors such as absorption, reflection, and scattering of light. An entirely opaque substance reflects and absorbs all incident light, allowing no transmission or scattering.
  • When light strikes the boundary between two media, some of it is reflected, some is absorbed, and some is scattered. The remaining light is refracted and transmitted through the second medium. Opacity measures how effectively this second medium obstructs the passage of light.
  • An opaque object is neither transparent (which allows all light to pass through) nor translucent (which permits partial light transmission).
  • The opacity of certain materials can vary with the wavelength of light. For instance, some types of glass are transparent to visible light but opaque to ultraviolet radiation.

Opacity

Opacity refers to the extent to which an object or surface hinders or blocks light from passing through and so obstructs light from reaching objects or space beyond.

  • Opacity is influenced by factors such as absorption, reflection, and scattering of light. An entirely opaque substance reflects and absorbs all incident light, allowing no transmission or scattering.
  • When light strikes the boundary between two media, some of it is reflected, some is absorbed, and some is scattered. The remaining light is refracted and transmitted through the second medium. Opacity measures how effectively this second medium obstructs the passage of light.
  • An opaque object is neither transparent (which allows all light to pass through) nor translucent (which permits partial light transmission).
  • The opacity of certain materials can vary with the wavelength of light. For instance, some types of glass are transparent to visible light but opaque to ultraviolet radiation.
Related diagrams

Each diagram below can be viewed on its own page with a full explanation.

References

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 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 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.
Related diagrams

Each diagram below can be viewed on its own page with a full explanation.

References

Optic nerve

The optic nerve in the human eye is a cable-like bundle of nerve fibres composed of the axons of ganglion cells, responsible for transmitting visual information to the brain’s lateral geniculate nucleus.

  • This nerve contains about a million fibres that carry a constant stream of visual data, received from the eye’s photoreceptors—rods and cones—as well as intermediate neurons such as bipolar and amacrine cells.
  • The optic nerve functions like a parallel communication cable, with each fibre transmitting distinct information about light intensity and patterns from specific regions of the visual field, allowing the brain to construct a cohesive image of the surroundings.
  • The optic nerve exits the eye at a spot called the optic disc, where no photoreceptors are present, creating a natural “blind spot” in the visual field. The brain compensates for this by filling in the missing information.
  • Some fibres from the optic nerve cross over to the opposite side of the brain at the optic chiasm. This crossover allows visual information from both eyes to be processed in both hemispheres of the brain, which is crucial for depth perception and a unified field of 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.

Optic nerve

The optic nerve in the human eye is a cable-like bundle of nerve fibres composed of the axons of ganglion cells, responsible for transmitting visual information to the brain’s lateral geniculate nucleus.

  • This nerve contains about a million fibres that carry a constant stream of visual data, received from the eye’s photoreceptors—rods and cones—as well as intermediate neurons such as bipolar and amacrine cells.
  • The optic nerve functions like a parallel communication cable, with each fibre transmitting distinct information about light intensity and patterns from specific regions of the visual field, allowing the brain to construct a cohesive image of the surroundings.
  • The optic nerve exits the eye at a spot called the optic disc, where no photoreceptors are present, creating a natural “blind spot” in the visual field. The brain compensates for this by filling in the missing information.
  • Some fibres from the optic nerve cross over to the opposite side of the brain at the optic chiasm. This crossover allows visual information from both eyes to be processed in both hemispheres of the brain, which is crucial for depth perception and a unified field of vision.
Related diagrams

Each diagram below can be viewed on its own page with a full explanation.

References

Optic radiation

The optic radiation consists of neural tracts formed by the axons of neurons in the lateral geniculate nucleus (LGN), which project to the primary visual cortex.

  • There are two optic radiations, one on each side of the brain, each responsible for carrying visual information to the corresponding hemisphere.
  • The optic radiation is divided into two main pathways:
    • The upper division, which carries information from the lower visual field,
    • The lower division (also called Meyer’s loop), which carries information from the upper visual field.
  • These pathways ensure that visual information is accurately mapped to the primary visual cortex, where it is processed and interpreted.

Optic radiation

Optic radiation

The optic radiations are tracts formed from the axons of neurons located in the lateral geniculate nucleus and leading to areas within the primary visual cortex. There is an optic radiation on each side of the brain. They carry visual information through lower and upper divisions to their corresponding cerebral hemisphere.

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.
Related diagrams

Each diagram below can be viewed on its own page with a full explanation.

References

Optical density

Optical density is a measure of how much a material resists and slows the transmission of light.

  • The higher the optical density of a material, the slower light travels through it.
  • The lower the optical density of a material, the faster light travels through it.
  • A vacuum is not a medium and has zero optical density.
  • Light travels through a vacuum at the maximum possible speed of light which is 299,792 kilometres per second.
  • Optical density and refractive index are related properties.
  • In general, materials with higher optical density tend to have higher refractive indices and vice versa.
  • The greater the difference in refractive index between two materials, the more they will bend light when they come into contact.