Dictionary category: Definitions

• Total Internal reflection only takes place when the first medium (where the light originates) is denser than the second medium.
• The critical angle is the angle of incidence above which total internal reflection occurs.
• The critical angle is measured with respect to the normal.
  • The normal is an imaginary line drawn in a ray diagram perpendicular to, so at a right angle to (90⁰), to the boundary between two media.

• Transmittance meanwhile is a way to measure how well a material allows light or other forms of radiation to be transmitted through it. It is essentially the fraction of incoming radiation that gets transmitted through the material.
• A high transmittance value indicates the material allows most radiation to pass through, while a low transmittance indicates most radiation is absorbed or reflected.
• The opposite of transmission is absorption, where electromagnetic radiation is absorbed by a medium and converted into other forms of energy, such as heat.
• The degree of transmission or absorption of electromagnetic radiation through a material can depend on factors such as the wavelength of the radiation, the composition and thickness of the medium, and the angle of incidence of the radiation.
• The degree of transmission can also vary depending on the type of electromagnetic radiation. For example, materials that transmit visible light well may not transmit ultraviolet light or infrared radiation as effectively.

• A transverse wave is a type of  wave in which the particles of the medium oscillate (vibrate) perpendicular to the direction of wave propagation.
• Transverse waves can be observed in various phenomena, such as waves on a string, water ripples, and certain types of seismic waves.
• Note that light and other electromagnetic waves are transverse waves that can travel through a vacuum.
• Transverse waves exhibit specific properties, including wavelength, frequency, amplitude, and wave speed.
• The motion of a transverse wave can be represented graphically using a sine wave or cosine wave, illustrating the peaks and troughs of the wave.
• Transverse waves can be polarized, meaning the oscillations are confined to a particular plane or direction, which has important implications in optics and other fields.

• When a material is subjected to mechanical stress, it creates a separation of electric charges within the material. As the material reunites, the separated charges can recombine. This recombination releases energy in the form of a burst of visible light.
• Not fully understood: While the basic mechanism is understood, the exact process of charge separation and recombination isn’t fully established and varies depending on the material.

Examples

• Sugar crystals: When sugar crystals are crushed, they produce flashes of light due to triboluminescence.
• Adhesive tape: Quickly peeling adhesive tape in a dark environment can produce light.
• Quartz: Quartz minerals exhibit triboluminescence when they are hit or fractured.

Important Notes

• Triboluminescence is distinct from other forms of luminescence as it doesn’t rely on previous absorption of energy from light or heat.
• The intensity and colour of the light produced through triboluminescence depend on the specific material.

Trichromatic colour theory of human vision explores various aspects of trichromacy, including:

• The functions, differences, and connections between the three types of cone cells (and the one type of rod cell) and other types of neurons within the human retina.
• The sensitivity of the three types of cones to three overlapping ranges of wavelengths of light that make up the visible spectrum and enable trichromatic colour vision.
• The sensitivity and function of rod cells in low levels of lighting.
• The role of rods and cones in encoding colour information in anticipation of subsequent stages of visual processing.
• The details of the way in which colour information is produced across the entire surface of the retina of both eyes is encoded onto separate channels.

The foundation of the trichromatic colour theory lies in understanding the physiological basis for the subjective experience of colour. It seeks to explain how our eyes and brains work together to create the rich world of colour we see around us.

• Cone cells: Unlike rod cells, which primarily detect light and darkness, cone cells are responsible for colour vision. There are three different types of cone cells in the human retina:
  • S-cones: Most sensitive to short wavelengths (blue light)
  • M-cones: Most sensitive to medium wavelengths (green light)
  • L-cones: Most sensitive to long wavelengths (red light)
• Colour perception: When light enters the eye, it stimulates the cone cells based on its wavelength. The brain then receives signals from these cones and interprets their combination as specific colours.
  • If all three types of cone cells are stimulated in different proportions, the brain perceives a mixed colour. For example, a combination of strong red and green stimulation might be perceived as yellow.
  • If only one type of cone cell is stimulated, the brain perceives the corresponding primary colour (blue, green, or red). However, due to overlapping sensitivities of the cones, pure primary colours are rarely seen in real life.
• Variations in colour vision: While trichromatic vision is considered normal, there are variations in individual sensitivities and slight differences in the distribution of cone cells. This can lead to subtle differences in colour perception between people.
• Comparison to dichromatic vision: Individuals with dichromatic vision only have two types of functional cone cells, leading to difficulties distinguishing certain colours, particularly red and green.

Tristimulus colour values

• Tristimulus values are the backbone of colour measurement whether in terms of the physiological response of the human eye to light or within the world of colour matching or colour management.

LMS tristimulus colour values

• • LMS tristimulus colour values form the foundation for measuring and representing colour perceptions within the LMS colour space. The system is based on the premise that any colour can be described physiologically by measuring the response of L, M, and S cone cells in the human eye’s retina to different wavelengths of light.
  • LMS tristimulus colour values have a genuine association with the range of colours that fall within the observable visible spectrum of a typical human observer.
  • LMS tristimulus colour values have three components corresponding to the response of the L, M, and S cone types. Each response is measured on a scale with values between 0 and 1.

XYZ tristimulus colour values

• XYZ tristimulus colour values are equivalent to LMS colour values. The CIE (1931) XYZ colour space utilizes XYZ tristimulus colour values as the basis of the CIE colour system, which has become the global standard for conveying accurate colour information worldwide.
• XYZ tristimulus colour values have a virtual correspondence with observable colours, meaning that some colours are hypothetical and require adjustments to account for variations in brightness. For instance, fully saturated yellow, green, or cyan may appear much lighter than red or blue.
• XYZ tristimulus colour values correspond with the response of the L, M and S cone types.
• Tristimulus colour values are colour-matching functions insofar as they allow you to predict the corresponding colour experience when you know a tristimulus value.

LMS tristimulus colour values & the human eye

• The human eye with normal vision has three kinds of cone cells that sense light, having peaks of spectral sensitivity in:
  • Short wavelengths: S = 420 nm – 440 nm.
  • Middle wavelengths: M = 530 nm – 540 nm.
  • Long wavelengths: L = 560 nm – 580 nm.
• Every human colour sensation can be explained in terms of the stimulus each cone type receives.
• The LMS cone cells underlie human colour perception in conditions of medium and high brightness.
• However, in very dim light, colour vision diminishes, and the low-brightness, monochromatic “night vision” receptors, known as “rod cells,” become effective.
• The three parameters denoted as “S”, “M”, and “L” are represented in a 3-dimensional space known as the “LMS colour space,” which is one of many colour spaces designed to quantify human colour vision.
• The LMS colour space was the subject of intense scientific study during the 1920s because it established a direct link between the subjective human experience of colour and wavelengths of the visible spectrum.
• There were technical problems interpreting the LMS colour space, which led to the development of the CIE 1931 colour space. In the CIE 1931 colour space, LMS tristimulus values are denoted by X, Y, and Z tristimulus values.
• One of the most important innovations associated with the CIE 1931 colour space is the CIE xy chromaticity diagram.

• On a wave at sea, the trough is the lowest point in the wave cycle, where the water displacement is furthest down from its rest position. A crest, on the other hand, is the highest point where the displacement is furthest up.
• For electromagnetic waves, which have electric and magnetic fields, a trough on either axis represents the point where the field reaches its minimum value in the downward direction. A crest represents the point of maximum value in the upward direction.

• Wavelength refers to a complete wave-cycle from one crest to the next, or one trough to the next.
• Frequency refers to the number of wave cycles that pass a given point in a given amount of time.
• The amplitude of a wave is a measurement of the distance from the centre line (or the still position) to the top of a crest or to the bottom of a corresponding trough.
• Amplitude is related to the energy a wave carries. The energy a wave carries is related to frequency and amplitude. The higher the frequency, the more energy, and the higher the amplitude, the more energy.

• In the context of electromagnetic waves, velocity describes the rate of wave propagation, accounting for both magnitude (speed) and direction.
  • As a vector quantity, velocity provides a full description of the wave’s displacement over time, indicating how fast and in which direction it moves.
  • Velocity can be positive or negative, representing motion in different directions, and is measured in units such as meters per second (m/s), kilometres per hour (km/h), or miles per hour (mph), with an indication of direction.
• In contrast, speed refers only to the magnitude of motion—how fast the wave travels—without regard to direction.
  • It is the magnitude of the displacement of an object per unit of time.
  • Speed does not consider the direction of motion, only the rate at which an object moves.
  • Speed is always positive or zero, representing the magnitude of motion.
  • Speed is measured in units such as meters per second (m/s), kilometres per hour (km/h), or miles per hour (mph).
• Direction in the context of velocity is typically quantified by angles or coordinates relative to a reference point or axis.
  • For electromagnetic waves, this could be described using angles (e.g., degrees or radians) from a defined direction (such as the horizontal or vertical axis in a plane) or by using a coordinate system (such as Cartesian or polar coordinates) to specify the precise direction of wave propagation.
• Positive or negative values of velocity simply indicate direction along a defined axis, with positive values often representing motion in one direction and negative values representing the opposite direction.

Velocity (General)

Velocity is a vector quantity that refers to the rate of change of an object’s position over time. It combines both speed and direction. Any change in either speed or direction results in a change in velocity.

• Speed is a scalar quantity that describes how fast an object is moving, measured as the distance covered per unit of time. It does not include direction.
• Direction is typically quantified relative to a reference point (such as along an axis or by a specific angle).

• Virtual photons are thought to play a role in many different physical phenomena, including the electromagnetic force, the weak force, and the strong force.
• A photon is a particle that carries electromagnetic radiation. It is the fundamental unit of light.
• Unlike a real photon, which carries electromagnetic radiation, virtual photons are theorized to be exchanged between charged particles during these interactions. This exchange is believed to be the underlying mechanism behind the electromagnetic force, the weak force, and the strong force.
• Virtual photons are created when two charged particles interact with each other. For example, when two electrons interact with each other, they can exchange a virtual photon. This exchange of a virtual photon causes the electrons to repel each other. The electric force that we observe is thought to be due to the exchange of virtual photons between charged particles.
• Whether a virtual photon is real is a matter of debate among physicists. Some believe that virtual photons are simply a mathematical tool used to calculate the interactions of real photons. Other physicists believe that virtual photons are real particles that exist for a very short period of time.

• Visible light is one form of electromagnetic radiation. Other forms of electromagnetic radiation include radio waves, microwaves, infrared, ultraviolet, X-rays, and gamma rays. Visible light ranges from approximately 400 nanometres (nm) for violet to 700 nm for red.
• A human observer perceives visible light as a combination of all the spectral colours between red and violet, as well as a vast range of other colours produced from the blending of different wavelengths in varying proportions.

• A spectral colour is produced by a single wavelength of visible light.
• The complete range of colours that can be perceived by a human observer is called the visible spectrum.
• The range of wavelengths that generate visible light constitutes a small portion of the electromagnetic spectrum.

• Human beings don’t see wavelengths of visible light, but they do see the spectral colours that correspond with each wavelength and colours produced when different wavelengths are combined.
• The visible spectrum includes all the spectral colours between red and violet and each is produced by a single wavelength.
• The visible spectrum is often divided into named colours, though any division is somewhat arbitrary.
• Traditional colours in English include: red, orange, yellow, green, blue, and violet.
• The visible spectrum is continuous, and the human eye can distinguish many thousands of spectral colours.
• The fact that we see distinct bands of colour in a rainbow is an artefact of human colour vision.
• The visible spectrum is a small part of the electromagnetic spectrum.

• Vision begins when light emitted or reflected by an object or scene enters our eyes through the cornea, pupil, and lens.
• The cornea and the lens work together to concentrate and focus light onto the retina, which is the photosensitive layer of cells at the back of the eyeball.
• The iris, located between the cornea and the lens, regulates the amount of light reaching the retina. It also determines eye colour and controls the size of the pupil.
• The retina plays a vital role in converting differences in the wavelength and brightness of incoming light into electrical signals.
• The optic nerve, which exits at the back of the eye, carries these signals to the visual processing areas of the brain.

• Vision, as experienced by human beings, forms the foundation of visual perception.
• Visual perception is the human ability to see and understand our surroundings by virtue of the sensitivity of our eyes to wavelengths of light across the entire visible spectrum, from red to violet.
• Visual perception is linked to eyesight but also encompasses the brain’s capability to interpret and comprehend the information received from our eyes.
• Visual perception is the outcome of visual processing, the complex and dynamic process that involves interactions between various retinal cells, neural pathways, and brain regions, ultimately leading to conscious visual perception.

• Visual perception is a complex process that relies on the intricate interaction between our eyes, the brain, and the interpretation of light signals. It enables us to perceive various visual attributes such as shapes, sizes, textures, depths, motions, and spatial relationships, all of which contribute to our comprehensive understanding and interpretation of the visual world around us.

• These elements collectively contribute to our comprehensive understanding and interpretation of the visual world around us.
• Visual perception is associated with eyesight but also encompasses the brain’s capacity to interpret information received from our eyes.
• The interpretation of visual information depends on the attributes of visual perception.

Visual perception and the electromagnetic spectrum

• The human eye and vision are attuned and responsive to the visible portion of the electromagnetic spectrum.
• Light is typically composed of multiple wavelengths, and observers are usually exposed to a range of adjacent wavelengths or a combination of wavelengths from various parts of the spectrum.
• Colour is not an inherent property of electromagnetic radiation but rather a characteristic of an observer’s visual perception.
• Colour is what humans perceive when light is present.
• Objects appear to be different colours to an observer depending on the wavelength, frequency and intensity of light at the moment it strikes the retina at the back of the eye.

• All waves have shared characteristics such as height (amplitude), peaks (crests), direction of movement, rate of oscillation (frequency), and distance between peaks (wavelength).
• The speed of a wave depends on the medium it travels through. For example, sound waves travel slower through air than through water. In contrast, electromagnetic waves travel at a constant speed (the speed of light) in a vacuum.

• Electromagnetic waves are generally invisible to the human eye, except for 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.

Wavelength & frequency

• The wavelength of electromagnetic waves can vary greatly, from extremely long radio waves, sometimes measured in kilometres, to very short gamma rays, measured in picometers (there are a trillion picometers in a metre (10^12)).
• 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 10^15 hertz (10^15).

• 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 when 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.

• 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.
• While a 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 a straight line along the axis of the wave.
• Wavelength refers specifically to the horizontal distance between equivalent points in a single wave cycle, such as the distance between two consecutive crests or troughs.
• In contrast, a wave cycle encompasses the entire up-and-down movement of the wave, including the horizontal distance (wavelength) and the vertical displacement.