Dictionary tag: M

• 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 which carry electromagnetic radiation, are propagating, perpendicular oscillations of electric and magnetic fields.
• Whenever electric charges move, whether through a wire carrying a current, as charged particles traversing space, or electrons orbiting atoms, they generate magnetic fields around them.
• 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.
• Oscillating electromagnetic waves transfer energy through space. For example, in the case of light waves, oscillating electric and magnetic fields carry energy from a source (such as the Sun) to the observer.

Transfer of energy between electric and magnetic fields

• The energy carried by electromagnetic waves oscillates between electric and magnetic fields as light 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 is 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.
• This is because electromagnetic waves are mass-less particles that travel at the speed of light. Their mass-less nature accounts for their inability to be deflected by external fields.
• However, there are exceptions 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 whether in the presence of objects like galaxies or black holes.

• 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).

• 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 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 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.

• 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.

• 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.

• 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.

• Monochromatic colours are created by using variations of a single hue, incorporating both shades (by adding black) and tints (by adding white). For example, a monochromatic colour scheme could involve a range of blues or pinks.
• It’s important not to confuse monochrome with greyscale. Monochrome refers to variations of a single hue, which can include any colour. In contrast, greyscale consists solely of shades of grey, with no colour information.
• In physics, monochromatic light refers to visible light or other electromagnetic radiation that has a single wavelength or frequency, resulting in light of a single colour.
• A surface or material is considered monochromatic if it features only one hue or a combination of tints and shades of the same colour.