Dictionary tag: F-G-H-I-J

• Light travels through a vacuum at 299,792 kilometres per second.
• Light travels at slower speeds through other media compared to a vacuum. The exact speed of light in a medium depends on its optical properties, particularly its refractive index.
• While a vacuum is devoid of matter, in the context of optics, a vacuum is considered a reference point for comparing the speeds of light in other media. It’s not contradictory to refer to a vacuum as a “medium” in this context; rather, it’s a baseline for comparison.
• Within the Earth’s atmosphere, light can travel at speeds near the speed of light. However, when the air is full of water droplets or dust, light travels at significantly lower speeds.
• Understanding whether a medium is considered “fast” or “slow” is valuable in predicting the behaviour of light when it crosses the boundary between different media. As such:
  • When light crosses the boundary from a fast medium to a slower medium, it will bend towards the normal.
  • When light crosses the boundary from a slow medium to a faster medium, the light ray will bend away from the normal.
• In optics, the “normal” is a line drawn in a ray diagram that is perpendicular, or at a right angle (90 degrees), to the boundary between two media.

• The phenomenon of light bending when it crosses the boundary between different media is known as refraction.
• The speed of light in a medium is determined by its refractive index, which is a measure of how much the medium slows down light compared to its speed in a vacuum.

• In the context of refraction, Fermat’s Principle accounts for why light follows the specific path it does when bending at the interface between two media and helps to explain why the bending follows Snell’s Law.
• Fermat’s Principle states that light travels between two points along the path that requires the least time. In other words, when light transitions from one medium to another (e.g., air to glass), it bends in such a way that minimizes the time taken for its journey.

Light and Refraction

• Light travels at different speeds in different media. For example, light travels slower in water than in air. When light encounters an interface between two media, it bends due to the change in speed.
• Although Fermat’s Principle doesn’t rely on the concept of conservation of energy, it’s important to note that the total energy of the light wave remains constant before and after refraction.

Fermat’s Principle & wavefronts

Fermat’s Principle and refraction are related to the shape of the wavefront at the leading edge of a wave.

• Wavefronts are typically used to describe the behaviour of waves in classical physics, where waves exhibit properties such as interference, diffraction, and refraction.
• While wavefronts are often associated with the behaviour of light, which can exhibit both wave-like and particle-like properties, they are a description of the wave nature of light rather than individual photons.
• At the quantum level, the behaviour of individual photons is described differently, often using concepts such as wave functions in quantum mechanics.
• The shape of a wavefront depends on the light source, the medium through which light is propagating and the obstacles it encounters.

• Fields are fundamental in physics, playing key roles in areas like electromagnetism, quantum mechanics, and general relativity.
• Fields can be represented by lines showing the direction of the force experienced by objects within the field.
• Fields are created by a source object and influence other objects within their range.
• Electromagnetic fields combine electric and magnetic components, interconnected through electromagnetic waves.
• Electric fields are associated with positive or negative charges and exert forces on charged objects.
• Magnetic fields are generated by moving electric charges, such as currents in wires, and can affect magnetic materials and charged particles.
• Electric fields and  magnetic fields together make up the electromagnetic field, which governs interactions between charged particles.
• According to quantum field theory, all particles and forces in the universe arise from interactions between underlying fields, which give rise to the properties of matter and energy.

Key features of fluorescence

• Fluorescence takes place when a substance absorbs light of a specific energy level, gets excited to a higher energy state, and then quickly emits light of a lower energy (longer wavelength) as it returns to its ground state. This emission typically happens within a very short time frame, ranging from nanoseconds to milliseconds. Fluorescence involves:
  • Light absorption: The substance absorbs light of a specific wavelength, exciting an electron within the molecule to a higher energy level.
  • Excited state: The excited electron wants to return to its ground state.
  • Energy emission: Instead of dropping back down, the excited molecule releases some absorbed energy as light of a lower energy (longer wavelength). This difference reflects the “lost” energy used for excitation.
• Rapid process: This emission happens very quickly, from nanoseconds to milliseconds.

Examples of fluorescence

• Fluorescent dyes: Used in highlighters, clothing, and biological experiments. These dyes absorb ultraviolet light and emit visible light, making them appear bright.
• Minerals: Some minerals fluoresce under ultraviolet light, used in identification and dating techniques.
• Chlorophyll: The green pigment in plants fluoresces under certain wavelengths, contributing to photosynthesis.

Distinguishing fluorescence from bioluminescence

• Fluorescence differs from bioluminescence as it doesn’t require complex biological reactions. It’s a purely physical process triggered by light absorption.
• While some bioluminescent systems might exhibit weak fluorescence, the primary light emission mechanism involves enzymatic reactions and doesn’t follow the principles of fluorescence.

The sub-atomic process

The subatomic process involved in fluorescence can be broken down into several key steps:

• Light Absorption: The process starts with a molecule (the fluorophore) absorbing a photon of light with a specific energy level.
  • This energy excites an electron within the molecule, promoting it from its ground state to a higher energy level (often a singlet excited state).
  • The energy of the absorbed photon and the specific electron transition determine the wavelength of the absorbed light.
• Internal Relaxation: In some cases, the excited electron might undergo non-radiative transitions within the molecule. This involves losing some energy through processes like vibrations or collisions with other molecules, without emitting light.
  • This internal relaxation typically happens within picoseconds (trillionths of a second) and doesn’t directly contribute to the observed fluorescence.
  • Radiative Emission: Eventually, the excited electron returns to its ground state, releasing energy in the form of a photon.
  • This emitted photon usually has a lower energy (longer wavelength) than the absorbed photon due to the internal energy losses mentioned above.
  • The specific energy difference between the absorbed and emitted light determines the colour of the emitted fluorescence.
• Excited State Lifetime: The time it takes for the excited electron to emit a photon and return to its ground state is known as the excited state lifetime. This typically ranges from nanoseconds to nanoseconds in fluorescent molecules.
  • A shorter lifetime indicates a faster emission rate, influencing the intensity and overall efficiency of the fluorescence process.
• Additional details: The specific energy levels involved, electron transitions, and excited state lifetimes depend on the structure and characteristics of the fluorescent molecule.
  • Fluorescence is sensitive to factors like temperature and the surrounding environment, which can affect the internal relaxation processes and emission properties.
  • While the basic principles remain the same, there are different types of fluorescence and specialized fluorophores used in various applications.

• Forces can be either contact forces or non-contact forces.
  • Contact forces: These happen when two objects touch, like friction when you rub your hands together, or the push you give the ball. Other examples of contact forces include tension, air resistance and the force exerted by springs.
  • Non-contact forces: These act even when objects aren’t touching, like gravity pulling you down, or a magnet attracting a paperclip. Other examples of non-contact forces include gravity, electromagnetism, and the strong nuclear force.
• Forces can make things move faster (accelerate), slower (decelerate), or change direction altogether.
• Objects, bodies, matter, particles, radiation, and space-time are all in motion.
• On a cosmological-scale, concentrated matter in planets, stars, and galaxies leads to significant push-pull interactions.
• Motion signifies a change in the position of the elements of a physical system including translational motion, rotational motion, vibrational motion, and oscillatory motion.

Push-pull interactions

• Forces can bind objects together or push them apart. Force can therefore be described intuitively as a push or a pull with magnitude and direction, making it a vector quantity.
• Whenever there is a push-pull interaction between two objects, forces are exerted on both. Once the interaction ceases, the forces no longer act, and the momentum of the objects continues unchanged.

• The push-pull interactions between things are explained by the interplay of forces.
  • The existence of forces explains how objects interact with each other throughout the entirety of the natural world.
  • Forces generate motion and can cause changes in velocity for objects possessing mass.
  • Changes in velocity encompass various scenarios, such as initiating motion from a state of rest, accelerating, or decelerating.

Fundamental forces

• Four fundamental forces account for all the forms of pulling and pushing between things in the Universe.
  • The electromagnetic force is responsible for interactions between charged particles, such as electrons and protons, and is fundamental to electrical and magnetic phenomena.
  • The weak nuclear force is involved in processes like radioactive decay and plays a role in the interactions of subatomic particles.
  • The strong nuclear force binds atomic nuclei together and is responsible for the stability of matter. It is the strongest of the four fundamental forces, but it has the shortest range.
  • Gravity governs the interactions between massive objects and is responsible for phenomena like planetary motion and the attraction of objects towards Earth.

• Read more about the four fundamental forces here.

• Take light as an example of a force carrier for electromagnetic radiation.
  • Light is a form of energy that travels as waves, but it also behaves like a stream of tiny particles called photons.
  • These photons are the force carriers for the electromagnetic force, one of the fundamental forces in the universe.
  • The electromagnetic force is responsible for a variety of phenomena, including the attraction between oppositely charged particles and the repulsion between like charges.
  • Photons can also interact with individual electrons in atoms, causing them to move or change energy levels.

Force carriers for the fundamental forces

• Each fundamental force in the universe is transmitted by its own set of particles called force carriers.
• Force carriers act like messengers, mediating the interaction between other particles.
• The force carriers for the four fundamental forces are as follows:
  • Electromagnetic Force: The force carrier for electromagnetism is the photon, a massless particle that travels at the speed of light. Photons are responsible for various phenomena like light, electricity, and magnetism.
  • Strong Force: The strong force, responsible for binding protons and neutrons within an atomic nucleus, uses gluons as force carriers. Gluons are massless particles that only interact with other particles called quarks, the fundamental building blocks of protons and neutrons.
  • Weak Force: The weak force, involved in radioactive decay and some nuclear fusion processes, utilizes W and Z bosons as force carriers. These massive particles are unlike photons and mediate the transformation of one type of subatomic particle into another during weak interactions.
  • Gravity: The force that attracts objects with mass, remains a bit of a mystery. Scientists believe a theorized particle called the graviton might be the force carrier for gravity. However, the graviton has not yet been directly detected. It’s predicted to be massless and travel at the speed of light.

• The eyes continuously rotate in their sockets to focus objects of interest as precisely as possible onto the fovea centralis.
• These rapid movements, called saccades, position objects of interest on the fovea, allowing for detailed inspection of the environment.
• Although the entire surface of the retina contains nerve cells, the fovea  is the small region (about 0.25 mm in diameter )at the centre of the macula which has the highest concentration of cones, making it ideal for capturing fine detail.
• While cones are concentrated in the fovea for detecting fine detail and colour, rods, which are more sensitive to light but not colour, are spread throughout the rest of the retina and are essential for peripheral and low-light vision.

• Normally, electrons exist in energy levels (orbitals) around an atom’s nucleus. A bound electron needs to absorb enough energy to overcome the attractive force holding it to the nucleus. This energy can come from various sources:

  • Heat: Adding heat to a material increases the vibration of atoms, which can bump electrons loose. This is why metals conduct electricity better when heated.
  • Light: Light, particularly high-energy light like ultraviolet (UV) radiation, can impart enough energy to eject electrons. This is the principle behind the photoelectric effect.
  • Electric field: A strong enough electric field can accelerate electrons and provide the energy needed to escape the atom’s attraction. This is important in devices like cathode ray tubes.
  • Collisions: In high-energy collisions between atoms or molecules, electrons can be knocked free. This is relevant in processes like particle accelerators.

• Electrons in the outermost energy level (valence electrons) are generally more loosely bound and have a higher probability of being freed by these energy sources.

Free electrons play a role in many electromagnetic phenomena:

• Photoelectric Effect: Free electrons are involved in the photoelectric effect, where photons (light particles) strike a material and transfer energy to electrons. If the energy from the light is sufficient, it can release electrons from their bound state, creating free electrons. This phenomenon is fundamental to the operation of devices like solar cells and photodetectors.
• Interaction with Light: Free electrons can scatter light. When light interacts with a material, free electrons can absorb and re-emit photons, contributing to effects like reflection, refraction, and the generation of certain colours in materials.
• Plasma and Light: In a plasma state, which consists of free electrons and ions, light behaves differently compared to its behaviour in neutral gases. Free electrons can reflect and absorb electromagnetic radiation, influencing how light propagates through plasma.
• Electrical Conductivity and Light Emission: In conductors, free electrons facilitate electrical currents, and when these electrons transition between energy levels, they can emit light, as seen in incandescent bulbs or LED technology.

• Frequency is measured in Hertz (Hz) and signifies the number of wave-cycles per second. Sub-units of Hertz enable measurements involving a higher count of wave-cycles within a single second.
• The frequency of electromagnetic radiation spans a broad range, from radio waves with low frequencies to gamma rays with high frequencies.
• The wavelength and frequency of light are closely linked. Specifically, as the wavelength becomes shorter, the frequency increases correspondingly.
• It is important not to confuse the frequency of a wave with the speed at which the wave travels or the distance it covers.
• The energy carried by a light wave intensifies as its oscillations increase in number and its wavelength shortens.

Radio waves

• Radio waves have the lowest frequencies among these types of electromagnetic radiation. They typically range from a few kilohertz (kHz) to hundreds of gigahertz (GHz). Radio waves are commonly used for communication purposes, such as radio broadcasting and wireless communication.

Microwaves

• Microwaves have frequencies higher than radio waves, typically ranging from several gigahertz (GHz) to hundreds of gigahertz (GHz). They are commonly used in microwave ovens, satellite communication, and radar technology.

Infrared radiation

• Infrared radiation (IR) has frequencies higher than microwaves, ranging from several hundred gigahertz (GHz) to several hundred terahertz (THz). Infrared radiation is associated with heat and is used in various applications, including thermal imaging, remote controls, and infrared spectroscopy.

Visible light

• Visible light is the range of frequencies that can be detected by the human eye, approximately ranging from 430 terahertz (THz) for red light to 750 terahertz (THz) for violet light. Visible light enables us to perceive colours and is responsible for our sense of vision.

Ultraviolet

• Ultraviolet (UV) radiation has frequencies higher than visible light, typically ranging from several hundred terahertz (THz) to several petahertz (PHz). UV radiation is known for its effects on the skin and can be harmful in excessive exposure. It is used in applications like sterilization, fluorescence analysis, and tanning beds.

X-rays

• X-rays have higher frequencies than UV radiation, typically ranging from several petahertz (PHz) to several exahertz (EHz). X-rays have shorter wavelengths and are commonly used in medical imaging, security screening, and industrial inspections.

Gamma rays

• Gamma rays have the highest frequencies among these types of electromagnetic radiation, typically exceeding several exahertz (EHz). They have the shortest wavelengths and are associated with high-energy phenomena, such as radioactive decay and nuclear reactions. Gamma rays are used in medical treatments, scientific research, and industrial applications.