Dictionary tag: K-L-M-N-O

• Light waves are made up of tiny packets of energy called photons.
• Normal light emission happens when atoms or molecules release photons when they transition from higher energy states to lower ones. These emitted photons have random directions and energies, creating a diffuse light.
• The concept that makes lasers unique is stimulated emission. This occurs when an incoming photon interacts with an excited atom in the laser material. The photon’s energy triggers the excited atom in the material to emit a new photon with identical characteristics. The new photon has the same wavelength and so colour, phase and direction as the original.
• Laser material refers to the medium that is used to generate the laser light.
• This phenomenon creates a cascade effect. The newly emitted photon can itself stimulate another excited atom, leading to two identical photons travelling in the same direction. This process repeats, rapidly amplifying the initial light within the laser cavity.
• The cavity comprises two mirrors strategically positioned at the opposite ends of the laser material. One mirror is fully reflective, while the other partially reflects.
• As the amplified light bounces between the mirrors, it continues to stimulate more emissions, resulting in an intense beam of identical photons. The partially reflective mirror allows a portion of this intense light to escape as the laser beam, while the rest continues to contribute to the amplification within the cavity.

How lasers work

• Here’s how a laser works in simplified terms:
  • Energy Source: A laser needs an energy source to “pump” the material it will interact with. This can be electricity, chemical reactions, or even sunlight.
  • Material: The type of material varies depending on the desired properties and application. Examples include:
    • Gas: Examples include helium-neon (HeNe) lasers used commonly in laboratories or carbon dioxide (CO2) lasers used for industrial cutting and welding.
    • Solid: Solid-state lasers are becoming increasingly common, with materials like neodymium-doped yttrium aluminium garnet used in various applications like medical procedures and material processing.
    • Liquid: Dye lasers, where the gain medium is a liquid solution containing organic dyes, offer tunable wavelengths making them suitable for research and spectroscopy.
    • Semiconductor: Diode lasers, also known as semiconductor lasers, like the ones used in CD/DVD players and laser pointers, utilize a p-n junction in a semiconductor material to create light.
  • Laser cavity: The cavity sometimes includes lenses or prisms to control the light path and manipulate its properties, such as focusing and directing the beam. They are also chosen to be as transparent as possible at the laser’s operating wavelength to minimize light absorption within the cavity.
  • Mirrors: Two mirrors at the ends of the laser cavity reflect the light back and forth, further amplifying it before one mirror allows a small portion to escape as the laser beam.
  • Excited State: The energy source excites the atoms or molecules in the laser material, bringing them to a higher energy level.
  • Stimulated Emission: A photon enters the material and “stimulates” an excited atom to release its energy as another photon.
  • Amplification: This new photon can then stimulate other excited atoms to release photons as well, creating a chain reaction that amplifies the light.

Properties of laser light

• This process leads to several unique properties of laser light:
  • Coherence: All the photons in a laser beam have the same wavelength and travel in sync, creating a very pure and concentrated light source.
  • Monochromaticity: Laser light has a single, monochromatic, pure colour (wavelength), unlike normal light, which is a mixture of many colours.
  • Collimation: The light is focused into a very narrow beam, giving it high intensity and precision.

• The thalamus which houses the lateral geniculate nucleus is a small structure within the brain, located just above the brain stem between the cerebral cortex and the midbrain and has extensive nerve connections to both.
• The LGN specializes in processing visual information from both eyes. It resolves relationships between different visual inputs, helping us understand the sequence of events and the location of objects in our field of view.
• Some of this processing involves signals from one eye, while others deal with information from both eyes to create a three-dimensional perception of the world.The LGN acts as a central connection for the optic nerve to the primary visual cortex in the occipital lobe. Both the left and right hemispheres of the brain have a lateral geniculate nucleus.
• There are three major cell types in the LGN, each connecting to different types of ganglion cells and playing specific roles in vision:
  • P cells: Process information about colour and fine detail.
  • M cells: Respond to motion.
  • K cells: Involved in low-resolution processing.

• Refractive index is influenced by the wavelength of light, the material’s density, and its temperature.
• The law of refraction is useful because it allows us to predict how light will bend at the boundary between two materials.
• We can use this law to calculate the angle of refraction if we know the angle of incidence and the refractive indices of the two materials. This is important for understanding how lenses and prisms work.
• For example, if we know the angle at which light strikes a lens (angle of incidence) and the refractive indices of air and the lens material, we can use Snell’s law to calculate the angle at which the light exits the lens (angle of refraction).
• Transparent media all have different refractive indices (index of refraction) that measure how much the speed and direction of light changes as it passes out of one and into another.
• Factors that affect the refractive index of a medium include the wavelength of light passing through it, its optical density and its temperature.

The formula

• So the law of refraction explains the relationship between the angle of incidence as light approaches the boundary of one medium with a particular refractive index and the angle of refraction as it enters the second with a different refractive index.
• It derives a formula from the fact that when light of a particular frequency crosses the boundary between any pair of media, the ratio of the sines of the angles of incidence and the sines of the angles of refraction is a constant in every case.
• The formula is: n1 sin θ1 = n2 sin θ2, where n1 and n2 are the refractive indices of the two media, and θ1 and θ2 are the angles of incidence and refraction, respectively.
• Because there are only four terms in the law of refraction if three are known then the fourth can be calculated.
• So, for example, it is possible to calculate the angle of refraction associated with the use of a lens or prism if the angle of  incidence and the refractive indices of the first medium (air) and the second (optical glass) are known.

• Refractive indices can be measured experimentally using techniques like refractometry, and they are unique to each medium and can be used to identify unknown substances.

• Here at lightcolourvision.org, we use the term ‘light’ as shorthand for ‘light of all wavelengths’ within the electromagnetic spectrum and so all forms of electromagnetic radiation. This includes:
  • Radio waves (longest wavelength)
  • Microwaves
  • Infrared radiation
  • Visible light
  • Ultraviolet radiation
  • X-rays
  • Gamma rays (shortest wavelength)

light in classical physics

• Classical physics thinks of light as continuous waves. This means that a wave of light in the vacuum of space has a constant wavelength (so colour), frequency and brightness and that it propagates through space without any reduction in force or energy.
• This Classical view is applied in the sub-field of optics when dealing with things like the reflection, refraction and polarisation of light and when analysing the behaviour of lenses, mirrors, and lasers. Visible light can be described in terms of:
  • Electromagnetic waves with wave-like properties including wavelength, frequency, amplitude, and phase.
  • Particles called photons have both wave-like and particle-like properties.
• The electromagnetic wave theory of light is a key component of our understanding of visible light and its interactions with matter. It helps to explain phenomena such as reflection, refraction, and diffraction of light and plays a crucial role in technologies such as wireless communication, remote sensing and medical imaging.

light in quantum mechanics

• In the field of quantum mechanics, light is described as a stream of particles called photons, which are the quanta of the electromagnetic field. According to this theory, photons are massless particles of light that have no electric charge but have momentum and each photon constitutes a single packet of electromagnetic energy.
• One of the most famous experiments that demonstrate the particle-like nature of light is the photoelectric effect, in which electrons are emitted from a metal surface when exposed to certain wavelengths of light. The photoelectric effect can not be explained by the wave theory of light but is explained by Einstein’s theory of the photoelectric effect, which proposes that photons transfer their energy to electrons in the metal.

• The wave model and the quantum mechanical model of light are not mutually exclusive and can be used to develop different perspectives on the same phenomena.
  • The wave model is useful for understanding light in situations where it behaves like a wave but largely ignores the way it interacts with matter at a sub-atomic scale.
  • The quantum mechanical model of light is useful in understanding interactions between light and matter at a sub-atomic scale, particularly interactions involving single photons and other quantum particles such as electrons.

Semiconductors and Light Emission

• Semiconductors, typically made from gallium nitride, are solid-state materials with unique properties that allow them to emit light at specific wavelengths, determining the perceived colour.

LED Colours

• LEDs typically emit one colour with a narrow range of wavelengths.
• Multicoloured LEDs combine three diodes emitting the RGB primary colours – red, green, and blue light.
• By adjusting the relative brightness of the primary colours, a vast array of colours can be created.
• Combining the three primary colours in equal proportions produces white light.

Role of Electrons in LEDs

• Applied Voltage: When connected to a power source, current flows through the LED exciting electrons: As electrons move across a junction within the semiconductor, they encounter empty energy levels called “holes”. Due to the attractive force between opposite charges, these electrons are drawn towards the holes and fill them. However, occupying a higher energy level places them in an unstable state.
• Energy Release: To reach a more stable state, each excited electron releases energy in the form of a photon (light packet). The energy of the emitted photon is equal to the energy difference between the higher energy level the electron occupied before and the lower energy level (the hole) it fills.
• Colour Control: The energy gap between the electron and the hole determines the photon’s energy, which translates to the perceived colour of the emitted light.

Holes in Semiconductors

• In semiconductor materials, “holes” represent the absence of an electron in a specific region where one could be.
• It’s not a physical hole or space but rather a way to understand the movement of electrons. These “holes” behave like positively charged particles because they represent a missing negative charge (electron).

• A light source is a natural or man-made object that emits one or more wavelengths of light.
  • Natural light sources encompass a vast spectrum of electromagnetic radiation, ranging from long, low-energy radio waves to powerful, high-energy bursts of gamma rays.
  • An artificial light source is an object or device that creates visible light without relying on natural processes such as sunlight, bioluminescence, or atmospheric phenomena. It utilizes various mechanisms to convert other forms of energy into visible light.

NATURAL SOURCES OF LIGHT

A list of natural light sources might include:

• Solar Radiation: This is the most abundant natural light source, originating from the sun’s nuclear fusion process. This light travels to Earth in various wavelengths, including visible light, ultraviolet (UV) light, and infrared (IR) radiation.
• Bioluminescence: Bioluminescence involves living organisms such as fireflies, deep-sea creatures, and some fungi emitting light through chemical reactions.
• Triboluminescence: This less common form of light emission occurs when certain materials, like sugar or quartz, are subjected to friction or stress, causing them to glow briefly.
• Chemiluminescence: This chemical reaction produces light as a byproduct. It is seen in glow sticks, fireflies, and certain biological processes. The chemical reaction excites the electrons within these molecules, causing them to jump to higher energy levels. When they return to their ground state, they release energy in the form of light.
• Atmospheric phenomena: Lightning, auroras, and airglow (faint nighttime glow) are examples of light produced by interactions between atmospheric gases, charged particles, and electromagnetic fields. These processes involve various mechanisms like electron excitation, recombination, and collisions.

ARTIFICIAL SOURCES OF LIGHT

A list of natural artificial sources might include:

• Incandescent light: This traditional lighting method relies on heating a filament (usually tungsten) to high temperatures until it glows, emitting a warm, yellowish light. Incandescent bulbs are less efficient than many other options.
• Fluorescent light: These tube-shaped lamps use fluorescent coatings that convert ultraviolet radiation from a mercury vapour discharge into visible light. They come in various colour temperatures and offer higher efficiency than incandescent bulbs.
• LED lighting: Light-emitting diodes (LEDs) are popular due to their high efficiency and long lifespan. They convert electrical energy directly into light with minimal heat generation, and their colour temperature can be tailored to various applications.
• Gas-discharge lamps: These encompass high-intensity discharge (HID) lamps, sodium-vapour lamps, and metal halide lamps. They use electrical discharges in different gases to create high-intensity lighting with colour spectra that suit applications such as street lighting and stadiums.

ABOUT NATURAL & ARTIFICIAL LIGHT: SUB-ATOMIC PROCESSES

Here is a list of key sub-atomic processes involved in the generation of both natural and artificial light, categorized by the nature of the process:

Electron Transitions

• Atomic emission: Excited electrons in atoms lose energy by dropping to lower energy levels, emitting photons (light) with specific wavelengths. (Natural: Stellar light, Auroras; Artificial: Fluorescent lamps, Gas discharge lamps)
• Recombination: Electrons recombine with positive ions (missing electrons), releasing energy as photons. (Natural: Nebulae; Artificial: LEDs, Semiconductor lasers)

Radiative Decay

• Nuclear decay: Certain radioactive decay processes (alpha decay, gamma decay) involve the direct emission of photons from the nucleus. (Natural: Radioactive materials; Artificial: Nuclear reactors, Radioisotope tracers)
• Annihilation: Particle-antiparticle collisions result in complete mass conversion to energy, often releasing high-energy photons (gamma rays). (Natural: Cosmic ray interactions; Artificial: Positron emission tomography (PET) scans)

Collisional Processes

• Bremsstrahlung: Charged particles interacting with other charged particles or strong electric fields decelerate, emitting photons. (Natural: Stellar interiors, Black hole accretion disks; Artificial: X-ray machines)
• Synchrotron radiation: Charged particles moving at high speeds in magnetic fields experience acceleration and emit characteristic light spectra. (Natural: Pulsars, Magnetars; Artificial: Synchrotrons, Particle accelerators)

Other Processes

• Cherenkov radiation: Charged particles travelling faster than light in a medium emit a faint bluish glow due to this specific radiation. (Natural: Cosmic rays in water; Artificial: Cherenkov detectors)
• Piezoluminescence: Certain materials emit light under pressure due to electron transitions induced by mechanical stress. (Natural: Quartz crystals; Artificial: Ultrasonic cleaning devices)
• Sonoluminescence: Cavitation bubbles collapsing in liquids can produce brief flashes of light due to various mechanisms (exact processes still debated). (Natural: Shrimp snapping claws; Artificial: Sonoluminescence research)

Additionally

• Chemical reactions: Some chemical reactions directly convert chemical energy into light through specific mechanisms (chemiluminescence) involving excited molecules. (Natural: Fireflies, Deep-sea creatures; Artificial: Glow sticks)
• Plasma: Hot, ionized gases (plasmas) exhibit various processes leading to light emission, including recombination, Bremsstrahlung, and interactions with charged particles. (Natural: Stellar atmospheres, Lightning; Artificial: Plasma torches, Fusion reactors)

• Different organisms respond differently to light stimuli, depending on the presence or absence of specialized light-sensitive cells or photoreceptors.
• Light that enters the human eye and stimulates the visual system is called a visual stimulus.
• The term colour stimulus is used because the light stimulus produces the perception of colour for an observer.
• Every light stimulus can be described in terms of the composition and intensity of wavelengths of light that enter the eye.
• A light stimulus may consist of a combination of red, orange, yellow, green, blue, and violet wavelengths of light. The colour perceived by an observer is influenced not only by the mixture of wavelengths but also by the intensity of light at each wavelength.
• In many cases, the intensity of light varies across a range of wavelengths, and this variation can be described by the spectral power distribution of the stimulus.