Dictionary tag: W

• Waves spread through a medium as atoms and molecules collide with each other, creating a domino effect.
• Waves have shared characteristics such as height (amplitude), peaks (crests), direction of movement, rate of oscillation (frequency), and distance between peaks (wavelength).

• Electromagnetic waves are generally invisible to the human eye. An exception to this is the visible spectrum, with wavelengths between approximately 400 and 700 nanometres. Beyond this range, whether the wavelengths are longer (as in radio and microwaves) or shorter (as in ultraviolet, X-rays, and gamma rays), our eyes cannot detect them.
• Although we cannot see most electromagnetic waves, we can perceive some of them in other ways. For instance, infrared waves are felt as heat, and electric current (which produces electromagnetic waves) can cause a buzzing sensation in a wire or cause electrocution.
• The wavelength of electromagnetic waves can vary greatly, from extremely long radio waves, sometimes measured in kilometres, to very short gamma rays, measured in picometres (there are a trillion picometres in a metre (10¹²) .
• The frequency of electromagnetic waves can also range from extremely low (1 cycle per second, known as 1 hertz) to extremely high, such as a quadrillion cycles per second, which equals 1015 hertz(10¹⁵).

• A wave diagram provides a visual representation of how a wave behaves when it interacts with various different media or objects.
• The purpose of a wave diagram is to illustrate optical phenomena, including reflection, refraction, dispersion, and diffraction.
• Wave diagrams can be useful in both theoretical and practical applications, such as understanding the basics of the physics of light  or designing complex optical systems.
• Wave diagrams are not limited to light; they can also be used to represent other types of waves, such as sound or radio waves.

• A wave function provides information about the probabilities of various possible states the system might be in. It depends on the coordinates of the particles in the system (for example, position or momentum). It calculates the probability of finding the system in a particular state.
• Wave functions are used to determine the probability of various outcomes in quantum experiments.
• A wave function, in the context of quantum mechanics, must encapsulate a wealth of information about a quantum system, including its possible states, probabilities, and how it evolves over time:
  • Position and Momentum: The wave function must provide information about the possible positions and momenta of particles within the system. This information is crucial for predicting the outcomes of measurements.
  • Superposition: It should be able to represent the idea that a quantum system can exist in a superposition of multiple possible states. This means that the system can simultaneously occupy different states with certain probabilities until observed.
  • Probability Amplitudes: The wave function encodes probability amplitudes, which are complex numbers that determine the likelihood of finding the system in a particular state upon measurement.
  • Time Evolution: It should be able to evolve over time, allowing for the prediction of how the system’s state will change over the course of time.
  • Observable Properties: The wave function must account for the possible values of observable properties (such as energy, angular momentum, etc.) and their corresponding probabilities.
  • Normalization: It must satisfy the condition of normalization, meaning that the total probability of finding the system in any possible state must equal 1.
  • Boundary Conditions: For specific physical systems, the wave function must satisfy appropriate boundary conditions that reflect the constraints imposed on the system (e.g., within a finite box or in a specific potential field).
  • Interference and Entanglement: It should be capable of describing interference effects between different states and, in the case of multiple particles, account for entanglement, where the states of particles become correlated.
  • Wave Function Collapse: When a measurement is made, the wave function must be capable of undergoing a transition from a superposition of states to a single, definite state, in accordance with the process of wave function collapse.
  • Completeness and Orthogonality: In certain mathematical formulations of quantum mechanics, wave functions must form a complete and orthogonal set to be used as a basis for representing quantum states.

Wave Function Collapse

Wave function collapse is a phenomenon in quantum mechanics where the act of making a measurement on a quantum system causes it to transition from a superposition of multiple possible states to a single, definite state.

• Prior to measurement, a quantum system can exist in a superposition of states, meaning it simultaneously occupies multiple possible states with different probabilities – these are described by the wave. However, when a measurement is made, the wave function collapses, and the system assumes one of the possible states with certainty.
• Wave function collapse illustrates the profound influence that observation has on the behaviour of quantum systems.
• In the context of quantum mechanics, “observation” refers to the act of making a measurement or carrying out an experiment to gain information about a quantum system. When we observe a quantum system, we are attempting to determine one of its properties, such as position, momentum, energy, etc.
• The interpretation of wave function collapse is a subject of ongoing debate among physicists, with various interpretations positing different explanations for the phenomenon.

• Visualise a wave-cycle as a series of points plotted along the path of a wave from one crest to the subsequent crest.
• All electromagnetic waves have common characteristics like crests, troughs, vibrations, wavelength, frequency, amplitude, and propagation direction.
• As a wave vibrates, a wave-cycle can be seen as a sequence of individual vibrations, measured from one peak to the next, one trough to the next, or from the start of one wave-cycle to the start of the next.
• Whilst wave-cycle refers to the path from one point on a wave during a single oscillation to the same point on completion of that oscillation, wavelength is a measurement of the same phenomenon but in this case in a straight line along the axis of the wave.

• Wave-particle duality refers to the phenomenon where entities like light can exhibit characteristics of both waves and particles.
  •  Electromagnetic radiation, including light, is often described using wave properties. However, when it interacts with matter, it behaves like particles.
  •  A photon is a quantum of electromagnetic radiation and represents the smallest discrete amount of light energy.
  • When a photon is absorbed by matter, the energy becomes localized at specific points. This phenomenon is termed ‘wave function collapse.’ It describes the transition of a quantum system from a superposition of states to a definite state upon measurement.
  • Wave-particle duality is a fundamental aspect of quantum mechanics and applies to all particles, not just light. Particles like electrons also exhibit wave-like and particle-like behaviour.
• The double-slit experiment is an experiment in quantum physics that demonstrates the wave-like behaviour of particles, including photons and electrons, and is a key illustration of wave-particle duality.

Concepts used to describe light as waves and particles can be paired to illustrate the wave-particle character of light

Wave concept	Particle concept	Explanation
Frequency	Energy	Frequency is related to the energy of a photon. Higher frequency light corresponds to higher energy photons. This is described by Planck's equation E = hf, where E is energy, h is Planck's constant, and f is frequency.
Wavelength	Momentum	The wavelength of a wave is inversely proportional to its momentum. This is described by the de Broglie wavelength formula λ = h/p, where λ is wavelength, h is Planck's constant, and p is momentum.
Wave speed	Momentum	The speed of a wave is related to its momentum through its frequency and wavelength. For light, the speed of propagation (c, the speed of light) is equal to its frequency times wavelength (c = λf), which is also related to its momentum as described above.
Amplitude	Intensity	The amplitude of a wave determines its intensity, which is related to the brightness or energy density of light. Higher amplitude corresponds to higher intensity.
Period	Lifetime	The period of a wave is the time it takes for one complete cycle. In the context of light particles (photons), their "lifetime" refers to the duration of their existence before they are absorbed or undergo a different process.
Phase	Quantum state	Phase refers to the position of a point in a wave cycle, and it can be related to the quantum state of a particle. In quantum mechanics, a particle's state is described by a complex-valued wavefunction, and the phase of this wavefunction is significant in determining probabilities and interference patterns.

These pairings are all based on the wave-particle duality of matter, which states that all objects have both wave-like and particle-like properties.

The specific pairings in the table can be derived from the following equations:

• E = hf (Planck’s equation describes the relationship between the energy of a photon and its frequency.)
• p = h/λ (de Broglie wavelength equation describes the relationship between the momentum of a particle and its wavelength.)
• p = mv (momentum equation describes the relationship between the momentum of a particle and its mass and velocity.)
• I = 1/2 cε_0 E^2 (intensity equation describes the relationship between the intensity of an electromagnetic wave and its electric field.)

where:

• E is energy
• f is frequency
• p is momentum
• m is mass
• v is velocity
• λ is wavelength
• I is intensity
• c is the speed of light
• ε_0 is the permittivity of free space. The permittivity of free space, also known as the electric constant, is a fundamental constant of nature. It is a measure of how easily an electric field can penetrate a vacuum.

The equation E = hf tells us that the energy of a particle is proportional to its frequency. This means that the higher the frequency of a particle, the more energy it has. This is why high-frequency radiation, such as gamma rays and X-rays, is so dangerous.

The equation p = h/λ tells us that the momentum of a particle is inversely proportional to its wavelength. This means that the shorter the wavelength of a particle, the more momentum it has. This is why high-energy particles, such as protons and electrons, can be used to accelerate other particles to high speeds.

• Sources that emit light in all directions, known as point sources, generate spherical wavefronts.
• Lasers, which produce a narrow beam of parallel rays, create waves with flat wavefronts.
• An electromagnetic wave with a flat wavefront is known as a plane wave.
• In addition to plane waves and spherical waves, there are also cylindrical waves which are produced when a point source is extended along a straight line.
• The shape of the wavefront can be influenced by interaction with different mediums or obstacles, such as when:
  • Refraction causes the path of light to bend as it crosses the boundary between two transparent media.
  • Dispersion causes light to separate into its constituent wavelengths, each of which bends to a different degree as it crosses the boundary between two transparent media.
  • Diffraction causes light to bend around the edges of obstacles into regions that would otherwise be in shadow.
• The concept of wavefronts applies not only to light but also to other types of waves such as sound waves or water waves.

• While wavelength can be measured from any point on a wave, it is often simplest to measure from the peak of one wave to the peak of the next, or from the bottom of one trough to the bottom of the next, ensuring the measurement covers a whole wave cycle.
• The wavelength of an electromagnetic wave is usually given in metres.
• The wavelength of visible light is typically measured in nanometres, with 1,000,000,000 nanometres making up a metre.
• Each type of electromagnetic radiation – such as radio waves, visible light, and gamma waves – corresponds to a specific range of wavelengths on the electromagnetic spectrum.
• As energy increases, frequency also increases and wavelength decreases. Therefore, shorter wavelengths correspond to higher energy levels, and longer wavelengths correspond to lower energy levels.

Wavelength & the Visible Spectrum

• The visible part of the electromagnetic spectrum consists of a range of wavelengths that correspond with all the different colours we see in the world.
• The visible spectrum, which spans wavelengths from approximately 400 to 700 nanometres, is commonly described in terms of bands of colour—red, orange, yellow, green, blue, indigo, and violet. However, these divisions are somewhat arbitrary since the spectrum is continuous, and the exact wavelength boundaries between colours is subjective.
• Humans don’t see the wavelengths of visible light directly but do see the specific colour associated with each wavelength and the colour produced when different wavelengths mix.
• The visible spectrum encompasses all the colours from red to violet, with each colour corresponding to a single light wavelength.
• The concept of colour is a perceptual property, not a property of light itself. It’s how our brain interprets the wavelengths of light that reach our eyes.
• The perceived colour (hue) of a light stimulus is dependent on its wavelength.
• A colour produced by a single wavelength is known as a pure spectral colour.

Wavelength & Colour

• Light that we perceive as white when it is reflected towards an observer contains a mix of different wavelengths of light.
• Light that contains wavelengths corresponding with red, green and blue appear white when it is reflected off a neutral-coloured surface.
• Light is seldom made up of a single wavelength. It is typically a mix of many different wavelengths that together determine the colour an observer sees as they reflect off a surface.
• The greater the number of different wavelengths incident light contains, the lower the saturation of the reflected colour. So a light comprising mixed wavelengths tends to produce lighter (whiter) colours.
• Wavelengths matching the colours of the visible spectrum are typically measured in nanometres. Hence, there are 300 distinct colours between 400 nanometres (violet) and 700 nanometres (red). If picometres are the unit used to measure wavelength then there are 300,000 different wavelengths in the visible spectrum, each corresponding to a unique colour.

• The weak nuclear force was responsible for the creation of elements in the early universe including hydrogen, helium, and lithium. Today it plays a key role in the nuclear fusion reactions that power the sun and other stars.
• The weak nuclear force is responsible for the decay of radioactive isotopes, as well as for other nuclear reactions such as beta decay and neutrino interactions.
  • When unstable radioactive isotopes decay, they emit radiation and transform into a more stable elements.
  • In beta decay, a neutron in the nucleus of an atom decays into a proton, an electron, and an antineutrino.
  • neutrino interactions occur in nuclear reactors. Neutrinos are very light particles that rarely interact with matter, but they can interact with the nuclei of atoms through the weak nuclear force.
• The weak nuclear force is a very weak, but it has a long range. It is also very selective, interacting only with certain types of subatomic particles.
• The weak nuclear force is mediated by a family of particles called W and Z bosons.

• The sun emits white light because sunlight contains all the wavelengths of the visible spectrum in equal proportions.
• Light travelling through a vacuum or a medium is termed white light if it includes all wavelengths of visible light.
• Light travelling through a vacuum or air isn’t visible to our eyes unless it interacts with something.
• The term white light can have two meanings:
  • It can refer to a combination of all wavelengths of visible light travelling through space, regardless of observation.
  • What a person sees when all colours of the visible spectrum hit a white or neutral-coloured surface.
• The human eye sees white when the wavelengths of light associated with the three primary colours – red, green, and blue (RGB) – are projected onto an achromatic surface.
• White light appears as different colours to an observer when some wavelengths of light are reflected by an object’s surface while others are absorbed.
• Artificial light sources usually emit light with a varied distribution of wavelengths or intensities, so they don’t typically emit white light.
• While there isn’t a single, distinct definition for white light, it’s fair to say that in any given situation:
  • White is the lightest possible colour.
  • White is an achromatic colour, which means a colour without hue, but with maximum saturation and brightness.
• Colour constancy, the ability to perceive colours as relatively constant, even under changing lighting conditions, enables human observers to see very different whites as appearing the same.

Why do Light Bulbs Glow if Light is Invisible?

• Incandescent light bulbs pass an electrical current through a fine tungsten filament with high electrical resistance.
• The resistance causes the filament’s electrons to heat up, giving off a bright yellowish-white colour. This colour is produced by:
  • The appearance of the heated tungsten surface.
  • Refraction, reflection, and dispersion as light encounters microscopic water droplets and dust particles on its journey from the light bulb towards the observer’s eyes.
  • Refraction, reflection, and dispersion within the liquids and solids of an observer’s eyes before the light reaches the retina.
• Besides these observed properties, a filament, along with other types of light sources, may emit wavelengths of light:
  • That are outside the visible region of the electromagnetic spectrum.
  • Travel from source to the retina without encountering surfaces or objects (however large or small), so remain completely invisible to an observer.
• The light emitted by a tungsten bulb spans wavelengths between 200 and 3000 nanometres.