## Magnetic field

A magnetic field is an aspect of the electromagnetic force. It is the interplay between electric and magnetic fields that gives rise to electromagnetic waves, such as visible light, radio waves, and X-rays. Electromagnetic waves travel through space as self-propagating packets of energy known as photons.

• According to Faraday’s law, a changing magnetic field induces an electric field, and conversely, a changing electric field induces a magnetic field.
• This mutual influence between electric and magnetic fields forms the basis of electromagnetic wave propagation, including visible light.
• Photons, the carriers of electromagnetic radiation, embody this duality by comprising self-reinforcing electric and magnetic fields oscillating perpendicular to each other as they propagate through space.
• Magnetic fields emerge in various scenarios within electromagnetism. Whenever electric charges move, whether through a wire carrying a current, charged particles traversing space, or electrons orbiting within atoms, they generate magnetic fields around them.
• Additionally, the acceleration of electric charges results in the creation of a changing electric field, which in turn induces a magnetic field, as described by Faraday’s law.
• To comprehensively understand and predict the behaviour of electric and magnetic fields, physicists employ Maxwell’s equations. These four fundamental equations elegantly encapsulate the principles governing electromagnetism, providing a framework for analysing and manipulating electromagnetic phenomena.
• The oscillation of electromagnetic waves propagating through space represent the transfer of energy. For example, in the case of light waves, the oscillating electric and magnetic fields carry energy from the source (such as the Sun) to the observer.
##### Transfer of energy between electric and magnetic fields
• The energy carried by electromagnetic waves can oscillate between electric and magnetic fields as the waves propagate through space. However, according to the principle of conservation of energy, the total energy within a closed system remains constant. This means that while energy can transfer between electric and magnetic fields, the overall amount of energy in the system remains unchanged.
• Light Absorption: When light waves encounter a material, such as a solar panel, some of the energy carried by the electromagnetic wave is absorbed. This absorption process involves the transfer of energy from the electromagnetic fields into the electrons of the material, causing them to move and generate an electric current. In this case, the energy initially stored in the electromagnetic fields is converted into electrical energy.
• Microwave Ovens: Microwave ovens utilize electromagnetic waves, specifically microwaves, to heat food. These microwaves contain energy in their electric and magnetic fields. As they pass through the food, the energy is absorbed by water molecules, causing them to vibrate and generate heat. In this process, the energy initially stored in the electromagnetic fields is transferred to the kinetic energy of the molecules, resulting in thermal energy.
• Radio Antennas: When a radio transmitter sends out electromagnetic waves, the energy initially resides in the electric field of the antenna. As the waves propagate through space, the electric and magnetic fields oscillate, carrying energy away from the antenna. At the receiving end, another antenna captures these waves, and the energy is transferred back into the electric field, which can then be converted into a signal by the receiver.
##### Photon generation
• According to Maxwell’s equations, which describe the behaviour of electric and magnetic fields in classical electromagnetism, accelerating charged particles produce changing electric and magnetic fields. When a charged particle accelerates, it generates a time-varying electric field around it, which in turn creates a time-varying magnetic field.
• These rippling changes in electric and magnetic fields propagate outward from the accelerating charged particle in the form of electromagnetic waves. The energy associated with these waves is quantized into discrete packets called photons, according to the principles of quantum mechanics.
• Electromagnetic radiation encompasses light, radio waves, microwaves, X-rays, and gamma rays.
• For example, in an incandescent light bulb, electrons are heated to a high temperature, leading them to accelerate and to emit photons of visible light.
• Electrons are the predominant type of charged particle that generates photons in light sources. They are found in numerous light sources, including the Sun, light bulbs, and even fireflies.
• The frequency of emitted photons by an electron depends on its energy level. Electrons possessing higher energy emit photons with greater frequencies.
• Photons can be generated by other means besides the acceleration of charged particles. For instance, photons can be produced through nuclear reactions and the decay of radioactive materials.
##### Photon behaviour
• As mentioned earlier, the acceleration of charged particles results in photons comprised of electric and magnetic fields.
• Both fields exhibit dynamic behaviour, meaning their strength oscillates between maximum and minimum values over time (time-varying fields), and in phase with one another. This creates an oscillating pattern.
• The oscillating wave motions of electric and magnetic fields are always perpendicular to each other. If one is horizontal, the other is vertical so electric and magnetic fields are always at right angles to each other, if one is horizontal then the other is vertical.
• Their wave-like motion are self-propagating disturbances in the electric and magnetic fields, capable of traveling through vacuum without the need for a medium..
• The frequency of the electric and magnetic waves is consistently identical and is determined by the photon’s energy. Higher-energy photons have higher frequencies
##### Deflection of electromagnetic waves
•  Once an electromagnetic wave propagates outward, it cannot be deflected by an external electric or magnetic field. Once an electromagnetic wave radiates outward, it remains unaffected by an external electric or magnetic field.
• This is because electromagnetic waves are massless particles that travel at the speed of light. Their massless nature accounts for their inability to be deflected by external fields.
• However, some exceptions exist to this rule.
• For instance, if an electromagnetic wave passes through an immensely strong magnetic field, it may experience slight deflection.
• Another exception is the deflection of electromagnetic waves by gravitational fields. However, the gravitational deflection of light is minuscule such as in the presence of objects like galaxies and black holes.

A magnetic field is created when electric current flows. The greater the current the stronger the magnetic field.

• Whilst an electric field is created by a change in voltage (charge), a magnetic field is created when electric current flows. The greater the current the stronger the magnetic field.
• An electromagnetic wave is the result of the interaction of an electric and magnetic field because an electric field induces a magnetic field and a magnetic field induces an electric field.
• An electromagnetic wave can be induced when either the charge of an electric field changes or when the current of a magnetic field changes or when they both change together.
• The waveform, wavelength and frequency of an electromagnetic wave result from the rapid periodic succession of transitions between the electrical and magnetic components and the forward propagation of the wave through space.
• When electric and magnetic fields come into contact to form electromagnetic waves they oscillate at right angles to one another.
• The direction of propagation of an electromagnetic wave is at right angles to the electric and magnetic fields.

## Magnetic field

A magnetic field is created when electric current flows. The greater the current the stronger the magnetic field.

• Whilst an electric field is created by a change in voltage (charge), a magnetic field is created when electric current flows. The greater the current the stronger the magnetic field.
• An electromagnetic wave is the result of the interaction of an electric and magnetic field because an electric field induces a magnetic field and a magnetic field induces an electric field.
• An electromagnetic wave can be induced when either the charge of an electric field changes or when the current of a magnetic field changes or when they both change together.
• The waveform, wavelength and frequency of an electromagnetic wave result from the rapid periodic succession of transitions between the electrical and magnetic components and the forward propagation of the wave through space.
• When electric and magnetic fields come into contact to form electromagnetic waves they oscillate at right angles to one another.
• The direction of propagation of an electromagnetic wave is at right angles to the electric and magnetic fields.

## Mass

Mass is a fundamental property of matter and is defined as the amount of matter present in an object and is independent of external factors such as location or the presence of gravitational fields.

• A large object made of a given material has greater mass than a small object made of the same material because it contains more matter.
• Mass is not the same as weight because weight varies with gravity while mass remains constant.
• Weight is the force exerted on an object due to gravity.
• An object of a known mass weighs more on earth than on the moon due to differences in gravity.
• The SI unit of mass is the kilogram (kg).
• Weight is the force exerted on an object due to gravity and is measured in newtons (N).
###### References
• Mass is a fundamental property of matter and is defined as the amount of matter present in an object and is independent of external factors such as location or the presence of gravitational fields.
• A large object made of a given material has greater mass than a small object made of the same material because it contains more matter.
• Mass is not the same as weight because weight varies with gravity while mass remains constant.
• Weight is the force exerted on an object due to gravity.
• An object of a known mass weighs more on earth than on the moon due to differences in gravity.

## Material

A material is the substances or matter that a thing is made of.

• Material is a broad term for a chemical substance or mixture of substances that constitute an object.
• Materials are composed of atoms and molecules arranged in various configurations, which determine their properties and behaviour.
• Materials can have natural origins, such as wood, stone, and metals, or synthetic origins, such as polymers and ceramics. Materials can also be classified based on whether they are organic or inorganic.
• The properties of a material depend on its structure at different length scales, from atomic to macroscopic scales.
• Materials can be classified based on physical and chemical properties such as mechanical, thermal, electrical, and magnetic properties.
• Materials are studied in materials science, a branch of engineering that focuses on structure, properties, and processing.
###### References
• A material is the substances or matter that a thing is made of.
• Material is a broad term for a chemical substance or mixture of substances that constitute an object.
• Materials are composed of atoms and molecules arranged in various configurations, which determine their properties and behaviour.
• Materials can have natural origins, such as wood, stone, and metals, or synthetic origins, such as polymers and ceramics. Materials can also be classified based on whether they are organic or inorganic.
• The properties of a material depend on its structure at different length scales, from atomic to macroscopic scales.
• Materials can be classified based on physical and chemical properties such as mechanical, thermal, electrical, and magnetic properties.
• Materials are studied in materials science, a branch of engineering that focuses on structure, properties, and processing.

## Material thing

A material thing is made up of matter, which includes all substances that have mass and occupy space. Matter is composed of atoms and molecules, and its properties include mass, volume, and density.

• Material things include objects, living organisms, and even intangible things such as sound or light, which are considered material because they are made up of particles.
• An attribute of an object is called a property if it can be measured or observed through the senses (e.g. its colour, size, weight, odour, taste, and location).
• Objects can be identified or characterized through their properties, which manifest themselves in various ways.
• These manifestations often exhibit consistent patterns, indicating that there is a underlying cause or mechanism that governs the properties.
• For example when different metals are mixed to form alloys, such as bronze or steel, the resulting material often exhibits a consistent relationship between its composition (the types and proportions of metals) and its density. So increasing the percentage of a denser metal in an alloy tends to increase its overall density.
###### References
• A material thing is made up of matter, which includes all substances that have mass and occupy space. Matter is composed of atoms and molecules, and its properties include mass, volume, and density.
• Material things include objects, living organisms, and even intangible things such as sound or light, which are considered material because they are made up of particles.
• An attribute of an object is called a property if it can be measured or observed through the senses (e.g. its colour, size, weight, odour, taste, and location).
• Objects can be identified or characterized through their properties, which manifest themselves in various ways.
• These manifestations often exhibit consistent patterns, indicating that there is a underlying cause or mechanism that governs the properties.
• For example when different metals are mixed to form alloys, such as bronze or steel, the resulting material often exhibits a consistent relationship between its composition (the types and proportions of metals) and its density. So increasing the percentage of a denser metal in an alloy tends to increase its overall density.

## Matter

Matter is anything that has mass and energy and occupies space by virtue of having volume.

• Matter is the substance that makes up all physical objects and substances in the universe, including solids, liquids, and gases.
• Matter is made up of atoms, which may combine to form molecules. Atoms in turn consist of subatomic particles such as protons, neutrons, and electrons.
• Subatomic particles have mass and may have energy.
• Matter can undergo physical and chemical changes, but the total amount of matter in a closed system remains constant (Law of Conservation of Matter). This means that matter cannot be created or destroyed in chemical reactions, only transformed into different forms.
• Light is a form of electromagnetic radiation, which does not have mass and is not considered matter.
• light interacts with matter (e.g., through absorption, reflection, and transmission), but is not composed of particles with mass.
• Einstein’s equation E=MC2 suggests that mass and energy are interchangeable and one can be converted to the other.
• Mass-energy equivalence means that mass can be converted into energy and vice versa, as demonstrated in nuclear reactions and particle interactions.
• Matter is anything that has mass and energy and occupies space by virtue of having volume.
• Matter is the substance that makes up all physical objects and substances in the universe, including solids, liquids, and gases.
• Matter is made up of atoms, which may combine to form molecules. Atoms in turn consist of subatomic particles such as protons, neutrons, and electrons.
• Subatomic particles have mass and may have energy.
• Matter can undergo physical and chemical changes, but the total amount of matter in a closed system remains constant (Law of Conservation of Matter). This means that matter cannot be created or destroyed in chemical reactions, only transformed into different forms.
• Light is a form of electromagnetic radiation, which does not have mass and is not considered matter.
• light interacts with matter (e.g., through absorption, reflection, and transmission), but is not composed of particles with mass.

## Medium

In physics and optics, a medium refers to any material (plural: media) through which light or other electromagnetic waves can travel. It’s essentially a substance that acts as a carrier for these waves.

• Light is a form of electromagnetic radiation, which travels in the form of waves. These waves consist of oscillating electric and magnetic fields.
• The properties of the medium, such as its density and composition, influence how light propagates through it.
• Different mediums can affect the speed, direction, and behaviour of light waves. For instance, light travels slower in water compared to a vacuum.
• Examples of Mediums:
• Transparent: Materials like air, glass, and water allow most light to pass through, with minimal absorption or scattering. These are good examples of mediums for light propagation.
• Translucent: Some materials, like frosted glass or thin paper, partially transmit light. They allow some light to pass through while diffusing or scattering the rest.
• Opaque: Materials like wood or metal block light completely. They don’t allow any light to travel through them.
• The permittivity (electrical response) and permeability (magnetic response) of a medium determine how light interacts with it. These properties influence factors like:
• Refraction: Bending of light as it travels from one medium to another with different densities.
• Reflection: Bouncing back of light when it encounters a boundary between mediums.
• Absorption: Light being captured and converted into other forms of energy (like heat) by the medium.
###### References
• In physics and optics, a medium refers to any material through which light or other electromagnetic waves can travel. It’s essentially a substance that acts as a carrier for these waves.
• Light is a form of electromagnetic radiation, which travels in the form of waves. These waves consist of oscillating electric and magnetic fields.
• The properties of the medium, such as its density and composition, influence how light propagates through it.
• Different mediums can affect the speed, direction, and behaviour of light waves. For instance, light travels slower in water compared to a vacuum.
• Examples of Mediums:
• Transparent: Materials like air, glass, and water allow most light to pass through, with minimal absorption or scattering. These are good examples of mediums for light propagation.
• Translucent: Some materials, like frosted glass or thin paper, partially transmit light. They allow some light to pass through while diffusing or scattering the rest.
• Opaque: Materials like wood or metal block light completely. They don’t allow any light to travel through them.

## Metameric

The term metameric refers to visually indistinguishable colour stimuli that appear the same but have different spectral compositions are called metameric.

• Metameric stimuli are colour stimuli that are indistinguishable from one another because they produce the same responses from the three types of cone cells in the human eye that are responsible for colour vision.
• Metameric matches can occur in different parts of the spectrum, which means that the spectral power distributions of different light sources can look similar, but not identical, to one another.
• A class of metameric stimuli can be specified by a set of tristimulus values, which represent the amounts of three reference colours, typically red, green, and blue, in a given trichromatic system, that are required to match the colour of the stimulus considered.
• The most important application of metameric stimuli is in the use of tristimulus values for additive colour mixing, such as in computer displays and TVs.
• The RGB colour model, for example, uses mixtures of red, green, and blue light to produce a wide range of colours visible to an observer.
###### References

The term metameric refers to visually indistinguishable colour stimuli that appear the same but have different spectral compositions are called metameric.

• Metameric stimuli are colour stimuli that are indistinguishable from one another because they produce the same responses from the three types of cone cells in the human eye that are responsible for colour vision.
• Metameric matches can occur in different parts of the spectrum, which means that the spectral power distributions of different light sources can look similar, but not identical, to one another.
• A class of metameric stimuli can be specified by a set of tristimulus values, which represent the amounts of three reference colours, typically red, green, and blue, in a given trichromatic system, that are required to match the colour of the stimulus considered.
• The most important application of metameric stimuli is in the use of tristimulus values for additive colour mixing, such as in computer displays and TVs.
• The RGB colour model, for example, uses mixtures of red, green, and blue light to produce a wide range of colours visible to an observer.

## Microscopic images of the Sun

When an observer looks up into the sky and sees an atmospheric rainbow they are looking at tiny images of the Sun mirrored in millions of individual raindrops. This is what produces the impression of arching bands of colour.

• It is the mirror-like surfaces on the inside of raindrops that reflect microscopic images of the Sun towards an observer.
• The images are tiny because raindrops are small, but also because the surface they reflect off is concave.
• At a micro-scale, each image of the Sun is different:
• Each and every image is a different colour and depends on the wavelength of light each raindrop is reflecting towards an observer’s eyes at any particular moment.
• For convenience sake, wavelength is usually measured in nanometres, but nanometres can be divided into picometres (or even smaller units). This means that an observer is looking at countless wavelengths of light and so countless colours.
• The images range in size and shape depending on the dimensions of the droplets and turbulence in the atmosphere. The size and roundness of raindrops also affect the appearance of a rainbow as a whole.
• The millions of microscopic images of the Sun that produce the impression of a rainbow is similar to the way pixels of light produce the images we see on digital displays.
###### Notice that:
• If all the rays of incident light that contribute to the formation of an observer’s rainbow are traced back from each raindrop towards the Sun it transpires that they are produced by parallel rays and that each incident ray is polarized as it passes through a droplet.
• If all the rays of incident light that travel through a single raindrop as it falls are compared, it transpires that they are all parallel with the axis of the rainbow.

## Minimum angle of deviation

The minimum angle of deviation of a ray of light of any specific wavelength passing through a raindrop is the smallest angle to which it must change course before it becomes visible within the arcs of a rainbow to an observer.

• Any ray of light (stream of photons) travelling through empty space, unaffected by gravitational forces, travels in a straight line forever.
• When light travels from a vacuum or from one transparent medium into another, it deviates from its original path (and changes speed).
• The more a ray changes direction the greater its angle of deviation.
• A ray reflected directly back on itself has an angle of deviation of 1800 – the maximum possible angle of deviation.
• It is the optical properties of air and raindrops that determines the angle of deviation of any ray of incident light.
• It is the optical properties of raindrops that prevent any ray of visible light within the visible spectrum from exiting a raindrop towards an observer at an angle of deviation less than 137.60.
• The angle of deviation and the angle of deflection are directly related to one another and together always add up to 1800.
• The angle of deviation and the viewing angle are always the same.
###### More about the minimum angle of deviation
• The optical properties of an idealised spherical raindrop mean that no light of any particular wavelength can deviate at less than its minimum angle of deviation.
• The minimum angle of deviation of visible light depends on its wavelength.
• The minimum angle of deviation for red light with a wavelength of approx. 720 nm is 137.60 but similar rays of the same wavelength but with other impact parameters can deviate up to a maximum of 1800.
• Different colours have different minimum angles of deviation because the refractive index of water changes with wavelength.
###### Impact parameter and minimum angle of deviation
• To form a primary rainbow, incident light must strike each raindrop above its horizontal axis.
• Rays of incident light of a single wavelength strike a raindrop at every possible point on the side of a raindrop facing the Sun.
• Only those that strike above the horizontal axis contribute to a primary rainbow.
• Points of impact of incident light striking a droplet can be measured on an impact parameter scale.
• It is the point of impact of rays of incident light of the same wavelength that is the variable factor that determines their subsequently different paths.
• Rays that strike nearest the horizontal axis, so with a value near 0.0 on an impact parameter scale have the largest angles of deviation.
• Rays that strike farthest away from the horizontal axis (near the topmost point on an impact parameter scale and so near 1.0) also have a large angle of deviation.

## Momentum

Momentum is a measure of how much mass an object has and how fast it is moving. It is calculated by multiplying the mass of the object by its velocity.

• Momentum is a vector quantity, which means that it has both magnitude and direction.
• Momentum = mass x velocity.
• Momentum is conserved, which means that the total momentum of a system remains constant unless an external force acts on the system.
• Momentum can only be transferred between objects, not created or destroyed.
• Examples of momentum:
• A bowling ball has more momentum than a baseball because it has more mass.
• A car moving at 60 mph has more momentum than a car moving at 30 mph.
• A rocket launching into space has a lot of momentum because it has a lot of mass and it is moving very fast.
##### Momentum & photons
• Photons are massless particles, but they can still exert a force on other objects. This is because photons have momentum and can interact with matter through the electromagnetic force:
• Photons have no rest mass, but they do have momentum.
• Photons can interact with matter through their momentum.
• Photons can also interact with matter through the electromagnetic force.
• The interaction of photons with matter can be used to explain a variety of phenomena, such as reflection, refraction, and absorption.
• The fact that photons have momentum and can interact with matter through the electromagnetic force means that they can exert a force on other objects.
• Here are some other examples of how photons can exert a force on other objects:
• Radiation pressure: Radiation pressure is the pressure exerted by photons on a surface. It is responsible for the tails of comets, which point away from the sun.
• Light pressure: Light pressure is the force exerted by photons on a particle. It is used in optical tweezers to trap and manipulate small particles.
• Photoelectric effect: The photoelectric effect is the emission of electrons from a metal surface when it is illuminated by light. It is caused by the transfer of momentum from the photons to the electrons.
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Summary

## Monochromatic

Monochromatic can refer to:

• Examples of monochromatic colour schemes include a range of blues or pinks.
• Monochrome and greyscale are sometimes confused. Monochrome refers to variations of a single hue, while greyscale refers specifically to shades of grey, with no colour information.
• Monochromatic colour schemes can be produced using various colour models, including HSB, RGB and CMYK.
• In the case of the HSB colour model, simply start with a single base hue and then adjust saturation and brightness to produce monochromatic variants.
• RGB and CMYK are less intuitive when creating monochromatic colour schemes.
• The easiest approach when working with the RGB and CMYK colour models in an application such as Adobe Illustrator is to use the Blend Tool, Colour Guide or Recolour Artwork options.
###### References
• Monochromatic can refer to:
• Examples of monochromatic colour schemes include a range of blues or pinks.
• Monochrome and greyscale are sometimes confused. Monochrome refers to variations of a single hue, while greyscale refers specifically to shades of grey, with no colour information.

## Müller cell

Müller glia, or Müller cells, are a type of retinal glial cells in the human eye that serve as support cells for the neurons, as other glial cells do.

• An important role of Müller cells is to funnel light to the rod and cone photoreceptors from the outer surface of the retina to where the photoreceptors are located.
• Other functions include maintaining the structural and functional stability of retinal cells. They regulate the extracellular environment, remove debris, provide electrical insulation of receptors and other neurons, and mechanical support of the neural retina.
• All glial cells (or simply glia), are non-neuronal cells in the central nervous system (brain and spinal cord) and the peripheral nervous system.
• Müller cells are the most common type of glial cell found in the retina. While their cell bodies are located in the inner nuclear layer of the retina, they span the entire retina.

## Müller cells

###### Müller cells

Müller glia, or Müller cells, are a type of retinal cell that serve as support cells for neurons, as other types of glial cells do.

An important role of Müller cells is to funnel light to the rod and cone photoreceptors from the outer surface of the retina to where the photoreceptors are located.

Other functions include maintaining the structural and functional stability of retinal cells. They regulate the extracellular environment, remove debris, provide electrical insulation of the photoreceptors and other neurons, and mechanical support for the fabric of the retina.

• All glial cells (or simply glia), are non-neuronal cells in the central nervous system (brain and spinal cord) and the peripheral nervous system.
• Müller cells are the most common type of glial cell found in the retina. While their cell bodies are located in the inner nuclear layer of the retina, they span the entire retina.