Emission occurs when an element or compound releases energy as either particles (such as electrons or ions) or electromagnetic radiation (such as photons). This process often results from energy changes within atoms or molecules, including electron transitions between energy levels or atomic/molecular vibrations. Emission can occur across a range of wavelengths, including visible light, and is typically triggered by heating or other forms of excitation.
Energy Changes in Atoms/Molecules: Atoms and molecules absorb and release energy, causing changes in electron positions or molecular vibrations. This energy is often emitted as electromagnetic radiation.
Electron Transitions: Electrons jump between specific energy levels within an atom. When they absorb energy, they move to higher levels; when they return to lower levels, they release energy as photons.
Molecular Vibrations: In molecules, atoms vibrate within chemical bonds. When energy is absorbed, these vibrations increase, and the energy can be emitted as electromagnetic radiation, often in the infrared spectrum.
Atomic vibrations refer to the back-and-forth movements of individual atoms around their fixed positions. This is often discussed in solid-state physics, where atoms in a solid are arranged in a regular pattern and can oscillate slightly while remaining in place overall.
An elementary particle ( fundamental particle) is the most basic unit of matter that is not composed of smaller particles. These particles are considered the building blocks of everything in the universe.
In the context of electromagnetism, there is only one fundamental particle, the photon, which acts as the force carrier, transmitting the electromagnetic force and carrying energy and momentum.
One way photons are created and destroyed is through subatomic processes within atoms and molecules.
These processes often involve interactions between fundamental particles governed by the strong nuclear force, which binds the building blocks of atoms (protons and neutrons) together.
Remember that when photons are created within atoms and molecules through interactions like electron transitions and interactions with the strong nuclear force they produce light.
Other light-producing processes (light sources) include blackbody radiation (incandescent light bulb), nuclear fusion (sunlight), annihilation (gamma rays) and high-energy phenomena (supernovae).
Electron spin is an intrinsic property of electrons, along with their mass and charge. Spin is not a classical rotation. It’s a quantum property and shouldn’t be interpreted literally as spinning. It is quantized, meaning it can only have certain discrete values.
Quantized Nature: The spin of an electron is always quantized, meaning it can only take on specific values. Electrons have a spin quantum number s=1/2, meaning they can exist in one of two possible spin states:
“Spin-up” (+1/2+1/2+1/2)
“Spin-down” (−1/2-1/2−1/2)
Magnetic Moment: Due to its spin, the electron generates a tiny magnetic field (magnetic moment), which interacts with external magnetic fields. This property is exploited in techniques like MRI and electron spin resonance (ESR).
Pauli Exclusion Principle: Electron spin is crucial for the Pauli Exclusion Principle, which states that no two electrons in an atom can have the same set of quantum numbers. This is why two electrons in the same atomic orbital must have opposite spins.
Intrinsic Property: Unlike orbital angular momentum, which depends on the electron’s motion around the nucleus, spin is intrinsic to the electron itself. It’s as if the electron has an inherent angular momentum that’s always present.
Electron mass is a fundamental property of electrons, representing the intrinsic amount of matter they possess. It is a measure of the electron’s resistance to acceleration.
Atomic Structure: The mass of electrons contributes to the overall mass of atoms and influences the behaviour of electrons within the atom.
The arrangement of electrons in orbitals and their energy levels are partly determined by their mass.
Energy levels are partly determined by their mass due to the interaction between the electrons and the positively charged nucleus of the atom.
The mass of an electron contributes to its inertia, which is its resistance to changes in motion.
This inertia affects the electron’s ability to respond to the electrostatic attraction from the nucleus and influences its distribution within the atom.
An electron-electron interaction that is mediated by a photon is a process in which two electrons interact with each other through the exchange of a photon. The process is common and is responsible for many of the properties of matter.
Imagine electrons as tiny magnets with an electric field surrounding them. This electric field affects the space around them.
When two electrons are close, their electric fields push against each other because they have the same charge (like poles of magnets repel).
According to the theory of quantum electrodynamics (QED), electrons can’t directly interact with each other. Instead, they exchange energy in the form of a photon, the carrier of the electromagnetic force.
One electron emits a photon due to a fluctuation in its electric field. This photon carries some energy and momentum.
The other electron, influenced by the changing electric field of the first electron (carried by the photon), absorbs the photon. This changes the second electron’s energy and momentum.
So, even though the electrons never directly touch, the exchanged photon acts as a messenger, causing a repulsive force due to the way their electric fields interact.
An electron is a subatomic particle, considered to be an elementary particle, as it doesn’t have any known parts or structure within it.
In an atom, electrons are arranged in orbitals, which are regions in which there is a high probability of them being found. These orbitals are grouped into shells based on their energy levels.
Electrons are the primary carriers of a negative electric charge in an atom, and through their interactions with other particles and fields play an essential role in electromagnetism, electricity, magnetism, chemistry, heat transfer, gravitational and weak interactions.
Electrons significantly contribute to electromagnetism because they have an electric field around them, and when moving produce a magnetic field.
When accelerated electrons radiate or absorb energy in the form of photons.
Electrons can collide with other particles and can be diffracted in a similar way to light.
Electrons exhibit both particle and wave properties. This wave-particle duality is a concept in quantum mechanics and is essential for understanding the behaviour of electrons.
Ectromagnetic energy refers to the energy transported by electromagnetic waves.
The amount of energy carried by an electromagnetic wave is directly proportional to its frequency and inversely proportional to its wavelength.
Radio waves have the longest wavelength and lowest frequency so carry the least energy among electromagnetic waves, while gamma rays have the shortest wavelength and highest frequency in the electromagnetic spectrum and carry the most energy.
Electromagnetic waves are composed of oscillating electric and magnetic fields that travel through space at the speed of light.
Electroluminescence (EL) refers to the phenomenon where light is emitted as a direct result of an applied electric field. Unlike other luminescence mechanisms that rely on chemical reactions or light absorption, electroluminescence is driven solely by the electrical energy itself.
Energy source: An electric field directly interacts with electrons, exciting them to higher energy levels.
Electron transitions: Excited electrons return to their ground state, releasing excess energy as light (determining the colour).
Materials: Certain materials like semiconductors and phosphors are susceptible to electroluminescence under electric fields.
An electrically charged particle is a particle that has a positive or negative charge.
Electrically charged particles are associated with the electromagnetic force, which is one of the four fundamental forces of nature.
The electromagnetic force is responsible for the attraction and repulsion between electrically charged particles, as well as for the propagation of light and other electromagnetic radiation.
All electrically charged particles interact with each other through the electromagnetic force. The strength of the interaction depends on the magnitude of the charges and the distance between the particles.
All particles, except for those with no charge, have a positive or negative charge.
Particles with opposite charges (+ and -) are attracted towards one another by the electric force.
Particles with the same charge (+ and +, or – and -) are repelled away from one another by the electric force.
The charge of a particle is a property that is intrinsic to the particle. It cannot be created, destroyed, or changed.
An electric field line is a component of a diagram representing an electric field. It is a line that corresponds with the electric field at every point along its length.
The direction of the electric field line shows the direction of the electric field.
Drawing field lines to show an electric field is a way of visualizing the direction and strength of the field at given points in space.
Electric field lines point away from their origin at a positively charged object and towards their termination at a negatively charged object.
At a sub-atomic scale, the electric field around a positively charged particle (such as a proton) is directed away from the charge, and the electric field around a negative charge particle (such as an electron) is directed towards the charge.
The density of field lines is used to represent the strength of the electric field, with more lines indicating a stronger field.
An electron orbital is a region of space around the nucleus of an atom where an electron is most likely to be found. Orbitals are not well-defined paths. They represent regions of space where the probability of finding an electron is high.
The arrangement of electrons in shells and orbitals around the nucleus of an atom is governed by the Pauli exclusion principle. The Pauli exclusion principle applies to electrons, protons, neutrons, (and neutrinos). It ensures that no two electrons can occupy the same quantum state simultaneously, leading to the formation of distinct shells and sublevels within each shell, known as orbitals.
The Pauli exclusion principle states that within an atom, each electron occupies a unique quantum state defined by its four quantum numbers: the principal quantum number (n), the azimuthal quantum number (l), the magnetic quantum number (m_l), and the spin quantum number (s).
Four quantum numbers together describe the quantum state of an electron and govern its specific orbital position.
Electron excitation is a general term for any interaction between a photon (particle of light) and an electron. It refers to the process where an electron in an atom or molecule gains energy and jumps to a higher energy level.
Electrons can be excited by various sources, such as:
Photoexcitation: This specifically refers to the absorption of a photon by an electron, leading to the electron’s transition to a higher energy level. This excited state can have various consequences, depending on the system involved. For example, it can trigger chemical reactions, cause fluorescence, or lead to the emission of other photons.
Collisional excitation: This occurs when an energetic particle (another electron, ion, etc.) collides with an electron, transferring some of its kinetic energy to the electron and promoting it to an excited state.
Thermal excitation: When atoms or molecules vibrate due to heat, this energy can be transferred to their electrons, exciting them to higher energy levels. This plays a crucial role in many chemical reactions and physical phenomena.
Chemical excitation: During chemical reactions, the rearrangement of electrons creates and breaks bonds, often resulting in excited states within the participating molecules.
Impact excitation: In certain materials like semiconductors, bombarding the material with high-energy particles (e.g., electrons, ions) can directly excite electrons.
Electrostatic excitation: Applying a strong electric field can create an external force on electrons, potentially pushing them to higher energy levels.
Field ionization: In very strong electric fields, electrons can be ripped out of their atomic or molecular orbitals altogether, resulting in a highly excited state before reaching the vacuum level.
The concept of electron clouds is a part of the quantum mechanical model of the atom, which describes the behaviour of electrons in terms of probability distributions rather than fixed orbits. In this model, electrons are not confined to specific orbits as proposed by the Bohr model but instead are described by electron clouds or probability distributions.
Electrons in an atom are organized into energy levels or shells. These shells are designated by principal quantum numbers (n = 1, 2, 3, …), with higher values of n corresponding to higher energy levels. Electrons in higher energy levels are farther from the nucleus.
Each energy level is divided into subshells or orbitals. These are designated by letters (s, p, d, f). The number of subshells in an energy level is equal to its principal quantum number (n). For example, the first energy level (n = 1) has one subshell (s), the second energy level (n = 2) has two subshells (s and p), and so on.
An orbital is a region in space where there is a high probability of finding an electron. Each orbital can hold a maximum of two electrons with opposite spins. The shape of the orbital depends on its type (s, p, d, f).
The electron cloud is a visual representation of the probability distribution of finding an electron in a particular region around the nucleus. It is not a physical boundary but rather a region where the electron is likely to be found.
The s orbital is spherical and is found in all energy levels. The p orbitals are dumbbell-shaped and are present in the second energy level and higher. The d and f orbitals have more complex shapes and are found in higher energy levels.
The electromagnetic force is one of the four fundamental forces in nature, responsible for various phenomena including electricity, magnetism, and light. It governs the interaction between electrically charged particles, such as electrons and protons. The other forces are the strong nuclear force, the weak nuclear forces and gravity.
The electromagnetic force is a fundamental force, its effects manifest as the push and pull interactions between charged particles.
This means this force is not derived from anything else. It cannot be further broken down or explained by simpler forces.
Even though the nature of the electromagnetic force is not fully understood, classical physics and quantum mechanics can provide a precise understanding of its emergence from charged particles, its behaviour, and the interactions it governs.
It is one of the four most basic and essential forces in the universe. These four forces exist independently of each other, although they can interact and influence each other in certain situations.
Electromagnetism is the fundamental force that governs the behaviour of electric and magnetic fields. It encompasses the generation, interaction, and propagation of these fields as electromagnetic waves, and includes the principles and phenomena related to these interactions.
In its broadest sense, electromagnetism refers to the entire realm of phenomena arising from the fundamental electromagnetic force. This includes:
The electromagnetic force itself: The interaction between electrically charged particles, causing attraction or repulsion.
Electromagnetic fields: Invisible fields associated with charged particles and currents, exerting forces on other charged particles.
Electromagnetic radiation: Energy travelling in the form of waves or particles (photons), such as light, radio waves, and X-rays.
Colour is something we see every moment of our lives if we are conscious and exposed to light. Some people have limited colour vision and so rely more heavily on other senses – touch, hearing, taste and smell.
Colour is always there whether we are aware and pay attention to it or not. Colour is what human beings experience in the presence of light. It is important to be clear about this. Unless light strikes something, whether it is air, a substance like water, a physical object or the retina at the back of our eyes, light, as it travels through space, is invisible and so has no colour whatsoever. colour is an artefact of human vision, something that only exists for living things like ourselves. Seeing is a sensation that allows us to be aware of light and takes the form of colour.
The experience of colour is unmediated. This means that it is simply what we see and how the world appears. In normal circumstances, we feel little or nothing of what is going on as light enters our eyes. We have no awareness whatsoever of the chemical processes going on within photosensitive neurons or of electrical signals beginning their journey towards the brain. We know nothing of what goes on within our visual cortex when we register a yellow ball or a red house. The reality is, we rarely even notice when the world disappears as we blink! In terms of immediate present perception, colour is simply something that is here and now, it is that aspect of the world we see as life unfolds before us and is augmented by our other senses, as well as by words, thoughts and feelings etc.
It takes about 0.15 seconds from the moment light enters the human eye to conscious recognition of basic objects. What happens during this time is related to the visual pathway that can be traced from the inner surface of the eyeball to the brain and then into our conscious experience. The route is formed from cellular tissue including chains of neurons some of which are photosensitive, with others tuned to fulfil related functions.
So, let’s start at the beginning!
Before light enters the eye and stimulates the visual system of a human observer it is often reflected off the surfaces of objects within our field of view. When this happens, unless the surface is mirror-like, it scatters in all directions and only a small proportion travels directly towards our eyes. Some of the scattered light may illuminate the body or face of an observer or miss them completely. Some is reflected back off the iris enabling others to see the colour of their eyes. Sometimes light is also reflected off the inside of our eyeballs – think of red-eye in flash photography.
Cross-section of the human eyeball
The fraction of light that really counts passes straight through the pupil and lens and strikes the retina at the back of the eyeball. From the point of view of an observer, this leads to two experiences:
Things an observer sees right in the centre of their field of vision, which is to say, whatever they are looking at.
Things an observer sees in their peripheral vision and so fill the remainder of a scene.
If we think of light in terms of rays, then the centre of the field of vision is formed from rays that enter our eyes perpendicular to the curvature of the cornea, pass right through the centre of the pupil and lens and then continue in a straight line through the vitreous humour until they strike the retina. Because these rays are perfectly aligned with our eyeballs they do not bend as they pass through the lens and so form an axis around which everything else is arranged. The point where this axis strikes the retina is called the macula and at its centre is the fovea centralis where the resulting image appears at its sharpest.
Peripheral vision is formed from rays that are not directly aligned with the central axis of the cornea and pupil, and do not pass through the very centre of the lens. All the rays of light around this central axis of vision change direction slightly because of refraction.
It deserves mention at this point that the lenses in each eye focus in unison to accommodate the fact those things we scrutinise most carefully may be anywhere from right in front of our noses to distant horizons.
We must also not forget that the optical properties of our lenses mean that the image that forms on the retina is both upside down and the wrong way round.
Energy is a property of matter and fields, which can be transferred between systems or transformed into different forms but cannot be created or destroyed.
Everything contains energy including all forms of matter and so all objects.
Energy is evident in all forms of movement, interactions between, and changes to the forms and properties of matter.
At an atomic level, energy is evident in the movement of electrons around the nucleus of an atom. Energy is stored in the nucleus of atoms as a result of the forces that bind protons and neutrons together.
Energy can be transferred between objects, and converted from one form to another, but cannot be created or destroyed.
Everything in the universe uses energy in one form or another.
When it comes down to it, matter is energy.
Light has energy but no mass so does not occupy space and has no volume.
Energy is often described as either being potential energy or kinetic energy.
An electronvolt (eV) is a unit of energy commonly used in atomic, nuclear, and particle physics to measure the energy carried by individual particles and electromagnetic radiation. It’s a convenient unit because the energies involved in these fields are much smaller than those we encounter in everyday life.
Electronvolts can be used to measure the energy of elementary particles, including photons, which are the smallest units of electromagnetic radiation (quanta of the electromagnetic field).
One electronvolt is equal to the energy gained by a single electron when it is accelerated across a potential difference of 1 volt.
Photons (quanta of light) travelling through this same potential difference would also gain 1 eV of energy.
The electronvolt is not part of the SI unit system. The SI unit for energy is the joule (J). However, joules are too large for many particle-level interactions.
1 electronvolt (eV) is equivalent to approximately 1.602 x 10^-19 joules (J).
Electric fields are a property of photons. These dynamic fields, along with corresponding magnetic fields, are responsible for the transmission of electromagnetic energy, such as visible light.
Photons are massless particles that carry electromagnetic energy, with each photon representing a quantum of light. The electric fields produced by photons oscillate, meaning their strength varies cyclically between maximum and minimum values over time.
The frequency of the electric field determines the frequency of the photon. A higher photon frequency corresponds to a shorter wavelength, as frequency and wavelength are inversely related.
Electric and magnetic fields are two interrelated aspects of the electromagnetic force, a fundamental force of nature. This force governs the attraction and repulsion between electrically charged particles, and its fields are responsible for the propagation of light and other forms of electromagnetic radiation.
The connection between force and field can be summarized as:
A force pushes or pulls an object. A field is a region of space in which a force acts on objects.
In the case of the electromagnetic force, the electric field is the region of space in which electric forces act on charged particles, and the magnetic field is the region of space in which magnetic forces act on moving charged particles.
In other words, electric and magnetic fields are the regions in space where the electromagnetic force pushes and pulls microscopic objects such as electrons and macroscopic objects such as magnets or metal filings.
An electromagnetic wave carries electromagnetic radiation.
An electromagnetic wave is formed as electromagnetic radiation propagates from a light source, travels through space and encounters different materials.
Electromagnetic waves can be imagined as synchronised oscillations of electric and magnetic fields that propagate at the speed of light in a vacuum.
Electromagnetic waves are similar to other types of waves in so far as they can be measured in terms of wavelength, frequency and amplitude.
We can feel electromagnetic waves release their energy when sunlight warms our skin.
Remember that electromagnetic radiation can be described either as an oscillating wave or as a stream of particles, called photons, which also travel in a wave-like pattern.
The notion of waves is often used to describe phenomena such as refraction or reflection whilst the particle analogy is used when dealing with phenomena such as diffraction and interference.
Electromagnetic radiation is a type of energy more commonly simply called light. Detached from its source, it is transported by electromagnetic waves (or their quanta, photons) and propagates through space at the speed of light.
Electromagnetic radiation (EM radiation or EMR) includes radio waves, microwaves, infrared, (visible) light, ultraviolet, X-rays, and gamma rays.
Man-made technologies that produce electromagnetic radiation include radio and TV transmitters, radar, MRI scanners, microwave ovens, computer screens, mobile phones, all types of lights and lamps, electric blankets, electric bar heaters, lasers and x-ray machines.
At the quantum scale of electromagnetism, electromagnetic radiation is described in terms of photons rather than waves. Photons are elementary particles responsible for all electromagnetic phenomena.
The term quantum refers to the smallest quantity into which something can be divided. A quantum of a thing is indivisible into smaller units so they have no sub-structure. A photon is a quantum of electromagnetic radiation.
A single photon with a wavelength corresponding with gamma rays might carry 100,000 times the energy of a single photon of visible light.
The electromagnetic spectrum includes electromagnetic waves with all possible wavelengths of electromagnetic radiation, ranging from low-energy radio waves through visible light to high-energy gamma rays.
There are no precisely defined boundaries between the bands of electromagnetic radiation in the electromagnetic spectrum.
The electromagnetic spectrum includes, in order of increasing frequency and decreasing wavelength: radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X-rays and gamma rays.
Visible light is only a very small part of the electromagnetic spectrum.
An electromagnetic field is a physical field that describes the behaviour of electrically charged particles and their interactions. It is a region of space where electric and magnetic forces are present. These fields are created by the presence or movement of electrically charged particles.
An electromagnetic field results from the coupling of an electric and magnetic field.
When an electromagnetic field experiences a change in voltage or current its reconfiguration into an electromagnetic wave can be described in terms of wavelength, frequency and energy.
An electromagnetic wave can be thought to come into existence when a static electric field experiences a change in voltage or a static magnetic field experiences a change in current producing radiating oscillations of electromagnetic energy that propagate through space.
The difference between an electromagnetic field and an electromagnetic wave is that the wave has a non-zero frequency component which is the source of the energy it transports.
Electromagnetic radiation is essentially the result of an oscillating electromagnetic field propagating through space.
Cross-section of the human eyeball