Dictionary category: Definitions

• Spacetime and light are closely related insofar as the speed of light is constant in all frames of reference. This means that the speed of light is the same for all observers, regardless of the speed and direction in which each observer is moving.
• This constancy of the speed of light as it travels through spacetime means:
  • The speed of light in a vacuum is 299,792,458 meters per second (m/s). This is believed to be true for all observers.
  • There is no absolute reference frame for space or time, in other words, everything is in motion relative to everything else and so regardless of the place or speed at the moment of measurement, the speed of light always appears the same. As a result,  light travels at the same speed, regardless of whether an observer is moving towards or away from the light source.
  • If the speed of light is a constant then it must be spacetime that is curved. The idea that spacetime is curved refers to the idea that if the speed of light is a constant then spacetime must be dynamic. In practice,  the path of light through space is affected by gravity, and gravity causes spacetime to bend. For example, the curvature of spacetime around a massive object, such as a star, will cause light rays to bend. This is known as gravitational lensing.
  • The curvature of spacetime also affects the motion of objects. Objects will always follow the shortest path through spacetime, which is called a geodesic. In curved spacetime, geodesics are not necessarily straight lines. As a result, gravity affects the paths of objects, such as planets and asteroids.
  • The fact that the speed of light is constant is one of the key pieces of evidence that supports Einstein’s theory of general relativity. If spacetime were not curved, then the speed of light would vary depending on the gravitational field. However, observations have shown that the speed of light is the same in all gravitational fields.

Spacetime and quantum field theory

• The concept in quantum field theory is that the fabric of spacetime is not a smooth, continuous medium, but is instead made up of discrete quantum fields, and one such field is the spacetime field.
• The idea that spacetime is made up of quantum fields has several implications including that gravity is not a fundamental force but is instead a property of spacetime. This means that gravity is not something that is exerted by one object on another but is instead a property of the fabric of spacetime itself.
  • The gravitational field: This field is responsible for the curvature of spacetime.
  • The Higgs field: This field gives elementary particles their mass.
  • The electroweak field: This field is responsible for the electromagnetic force and the weak nuclear force.
  • The strong nuclear field: This field is responsible for the strong nuclear force. The quantum fields that form spacetime are still a matter of research, but some possible candidates include:
  • It is also possible that spacetime is made up of some combination of these fields, or even of other fields that have not yet been discovered.
• One way to reconcile the idea that gravity is a property of spacetime with the idea that gravity is a fundamental force is to view gravity as an emergent force. An emergent force is a force that arises from the interactions of more fundamental particles or fields.
• From this viewpoint, gravity could be seen as an emergent force that arises from the interactions of the quantum fields that make up spacetime.
• Physicists are still working to develop a unified theory of physics that can explain the inter-relationship between all of the fundamental forces, including gravity. Until such a theory is developed, it is possible that we will not have a complete understanding of how gravity relates to the other fundamental forces – the electromagnetic, weak and strong forces.

• • The spectral colour model is typically represented as a continuous strip, with red at one end (longest wavelength) and violet at the other (shortest wavelength).
  • Wavelengths and Colour Perception: In the spectral colour model, each wavelength corresponds to a distinct colour, ranging from red (with the longest wavelength, around 700 nanometres) to violet (with the shortest wavelength, around 400 nanometres). The human eye perceives these colours as pure because they are not the result of mixing other wavelengths.
  • Pure Colours: Spectral colours are considered “pure” because they are made up of only one wavelength. This is in contrast to colours produced by mixing light (like in the RGB colour model) or pigments (in the CMY model), where a combination of wavelengths leads to different colours.
  • Applications: The spectral colour model is useful in understanding natural light phenomena like rainbows, where each visible colour represents a different part of the light spectrum. It is also applied in fields like optics to describe how the eye responds to light in a precise, measurable way.

Spectral colour concepts

• • • It helps explain concepts such as:
    • The visible portion of the electromagnetic spectrum, which ranges from approximately 400 to 700 nanometres.
    • The relationship between wavelengths of light and how we perceive colour.
    • Natural phenomena like rainbows and other effects involving chromatic dispersion, where light is split into its component spectral colours.

Beyond spectral colour

• • • The spectral colour model helps us to understand the connection between the physics of light and the perception of colour but other models are needed to help to make sense of how the eye works and to manage colour in different practical situations:
    • The trichromatic colour model deals with the physiological aspects of colour.
    • CMY colour model deals with how coloured inks behave in the world of digital printing.
    • RGB colour model deals with the display and management of colour in digital environments.
    • HSB colour model provides an intuitive way to select and edit colours in digital workflows.

Spectral colour

• A spectral colour is a hue perceived by human vision when exposed to a single wavelength of visible light. In normal (trichromatic) vision, each wavelength evokes a distinct spectral colour, ranging from red to violet.
• The human eye is finely tuned to the visible spectrum, where each wavelength corresponds to the perception of a different spectral colour.
• A spectral colour is considered monochromatic, meaning it contains just one wavelength and produces a hue different from any other wavelength.
• All light sources, such as sunlight, artificial lights, lightning, candles, and even glow worms, emit a mix of wavelengths. Each individual wavelength within this mix corresponds to a specific spectral colour.
• The colours we perceive in the world arise from different combinations of wavelengths of light reflecting off surfaces and objects. When light reaches the light-sensitive cells in our eyes, this combination produces the colours we experience.
• While the Sun emits a broad range of wavelengths that include all the colours in the visible spectrum, an LED bulb might only emit light at a specific wavelength, corresponding to one spectral colour.
• Although we cannot distinguish all wavelengths individually, we see the full range of spectral colours when observing a rainbow or when light is dispersed through a prism.
• In everyday experience, what we perceive as a spectral colour often includes a narrow band of adjacent wavelengths.
• Spectral colours are sometimes referred to as pure hues or monochromatic hues because they consist of one wavelength, unblended with others.
• While sunlight traveling through the air appears invisible, if it strikes a neutral-coloured surface and all wavelengths are reflected equally, the light is perceived as white.
• Spectral colours are commonly represented as a continuous strip, with red at one end and violet at the other, illustrating the range of hues corresponding to different wavelengths.

• Spectral power distribution is usually measured with a spectroscope. These instruments break down the light into its constituent wavelengths, allowing for precise analysis of the light’s spectral composition. This helps with understanding the exact colour of a light source or how it interacts with materials.
• SPD is critical in defining colour perception. The way the human eye perceives colour is heavily influenced by the distribution of power at various wavelengths, as different combinations of wavelengths will stimulate the cones in our retinas to varying degrees, resulting in a specific colour experience.
• SPD helps in identifying and comparing light sources. Different light sources, such as sunlight, LED lamps, or incandescent bulbs, have distinct SPDs. For instance, sunlight has a broad, continuous spectrum, while LEDs or fluorescent lights often have spikes at certain wavelengths, affecting how we perceive colour under these lights.
• SPD plays a key role in material appearance. When light reflects off a surface, the spectral power distribution of the reflected light reveals how different wavelengths are absorbed or reflected by the material, influencing the material’s colour and brightness.

• The continuous distribution of colours in the visible spectrum consists of a range of wavelengths, rather than distinct, separate colours.
• A diagram of the visible spectrum is a linear scale displaying the range of colours our eyes can perceive, arranged by their wavelengths with red at one end and violet at the other.
• This spectrum is naturally produced when light refracts through a prism or raindrops.
• The continuous distribution of colours in a spectrum arises because visible light consists of a range of wavelengths, rather than distinct, separate colours.
• Diagrams of spectral colours are usually presented as spectra: elongated linear bands with red at one end and violet at the other, allowing viewers to see as many colour gradations as possible.
• The process of separating light into its constituent colours is called dispersion.

• All objects obey the law of reflection on a microscopic level.
• If the irregularities on the surface of an object are smaller than the wavelengths of incident light then reflected light travels away from the surface at consistent angles.
• When an observer looks at specular reflections (regular reflections), they see mirror-like images on the surface of an object.

About diffuse reflection

• If the irregularities on the surface of an object are larger than the wavelengths of the incident light, light reflects in all directions and produces diffuse reflections.
• A diffuse reflection is easily distinguished from the mirror-like qualities of a specular reflection.
• Diffuse reflection is responsible for the way we perceive the colours and textures of objects.

• The combination of specular and diffuse reflections determines the overall appearance of objects in terms of shininess, dullness, or roughness.

• The speed of light is typically measured in metres per second (m/s).
• In a vacuum, light travels at approximately 300,000 kilometres per second, or more precisely, 299,792,458 metres per second.
• Light moves at slower speeds when passing through different media.
• A vacuum is a region of space that contains no matter.
• Matter refers to anything that has mass and occupies space (i.e., it has volume).
• When discussing electromagnetic radiation, the term medium (plural: media) refers to anything through which light propagates, including empty space and any material such as a solid, liquid, or gas.
• In other contexts, empty space is not considered a medium because it lacks matter.

• Light itself can be described as a wave that carries energy.
• When light interacts with matter, it can behave like particles called photons.
• These photons are massless particles that travel at the speed of light. They carry energy and momentum in packets, not as continuous values.
• In science, this concept is called quantization, which means certain properties can only exist in specific amounts.
• For photons, this means their energy and momentum come in distinct, separate packets rather than a continuous range.

• The Standard Model is a quantum field theory, which means that it uses the principles of quantum mechanics to describe the behaviour of matter and energy at the atomic and subatomic levels.
• The Standard Model is based on two fundamental theories:
  • Quantum mechanics provides a description of the physical properties of nature as interactions between fields of energy at the scale of atoms and subatomic particles. It is the foundation of all quantum physics.
  • Special relativity is a theory of space and time developed by Albert Einstein in 1905. It states that:
    • The laws of physics are invariant (i.e., identical) in all inertial frames of reference.
    • The speed of light in a vacuum is the same for all observers, regardless of the motion of the light source or observer.
• The Standard Model describes three of the four fundamental forces:
  • Electromagnetic force: The force that attracts or repels charged particles.
  • Strong nuclear force: The force that binds quarks together into protons and neutrons.
  • Weak nuclear force: The force that is responsible for radioactive decay.
• Despite its success, the Standard Model has a major limitation: it doesn’t describe gravity. This is because gravity is not compatible with the principles of quantum mechanics at very small scales. Scientists are still searching for a more comprehensive theory, sometimes called a “Theory of Everything,” that can unify the Standard Model with gravity.

• The Standard Model describes 17 fundamental particles

  • Quarks: Up, down, charm, strange, top, and bottom.
  • Leptons: Electron, muon, tau, and their 3 corresponding neutrinos.
  • Bosons: Photon, gluon, W and Z bosons, and the Higgs boson.
• Arguably, the Standard Model stands as the most successful theory created so far by human beings in our attempts to comprehend the totality of existence – the Universe, the world around us, and every aspect of our own physical being.
• Whilst the model has been able to explain the presence of three of the the four fundamental forces, the behaviour of subatomic particles, and the structure of the atom, it remains a work in progress, with physicists continuously seeking to refine it.
• The following scientists have received a Nobel Prize for their work on the Standard Model:
  • Richard Feynman, Julian Schwinger, and Yoichiro Nambu (1965 Nobel Prize in Physics) for their work on quantum electrodynamics.
  • Murray Gell-Mann (1969 Nobel Prize in Physics) for his classification of elementary particles.
  • Steven Weinberg, Sheldon Glashow, and Abdus Salam (1979 Nobel Prize in Physics) for their unification of the weak and electromagnetic forces.
  • Peter Higgs and François Englert (2013 Nobel Prize in Physics) for their discovery of the Higgs boson.

• Unlike traditional sources of light on Earth, stars ignite with a far more powerful process – nuclear fusion.
• Deep within their incredibly dense and hot cores, immense pressure and temperatures fuel nuclear fusion.
• This process forces hydrogen atoms to merge into heavier elements, primarily helium, releasing tremendous energy.
• A fraction of this energy escapes the star as the radiant light we call sunlight and starlight.

Nuclear Fusion Process

• Fuel and Fusion: Nuclear fusion (thermonuclear fusion) primarily occurs in a star’s core, consuming hydrogen and generating energy and light.
• Immense Conditions: The immense pressure and temperature within the core are crucial for hydrogen atoms to overcome their natural repulsion and fuse.
• Energy Release: This fusion process releases vast amounts of energy, including the light we see from stars. Fusion reactions release millions of times more energy than traditional chemical reactions, like burning fossil fuels.
• Temperature and Colour: Hotter stars, fusing at a faster rate, emit more energy and appear blue or white. Cooler stars with slower fusion emit less and appear red or orange.
• Lifespan and Evolution: Massive stars burn hotter and brighter, but as their hydrogen reserves deplete and their fusion processes change, they produce a dimmer glow.

From Core to Surface

• The light generated in the core of stars doesn’t immediately escape.
• It interacts with surrounding layers of hot gas (plasma) within the star.
• These layers absorb and re-emit light at different wavelengths, ultimately shaping the final spectrum we detect.
• Finally, the light escaping a star’s surface embarks on its interstellar journey, eventually reaching Earth, our telescopes and eyes.
• Sunlight originates from the Sun’s core and reaches Earth in about 8 minutes after travelling through its outer layers.

Sunlight and stellar light

• Light emitted by the Sun and stars is not just a single colour but a spectrum of colours.
• The electromagnetic spectrum extends far beyond the visible light we perceive.
• The Sun and stars emit radiation across the whole electromagnetic spectrum, although a limited range of wavelengths make it through the Earth’s atmosphere.
• Sun and stellar light on Earth include wavelengths stretching into the ultraviolet and infrared.

• The strong nuclear force is the strongest of the four fundamental forces of nature but only acts over very small distances, about the size of an atom’s nucleus. This short-range force is about 100 times stronger than the electromagnetic force, 106 times stronger than the weak nuclear force, and 1038 times stronger than gravity.
• The strong nuclear force is the fundamental force that binds matter together and is responsible for holding together protons and neutrons which are the subatomic particles within the atomic nucleus.
• The strong nuclear force counteracts the electrical repulsion between protons, which would otherwise push the positively charged protons apart.
• The strong nuclear force plays a crucial role in nuclear reactions, allowing the release of tremendous energy in processes like nuclear power generation and nuclear weapons.

• One kind of substance is a chemical substance. A chemical substance is a specific type of matter with molecules that share the same structure and composition.
• These molecules are held together by chemical bonds. Substances cannot be separated into their component parts (elements or compounds) without breaking these chemical bonds.
• Substances can be generally classified into two main categories:
  • Elements: The simplest form of a substance, elements are made up of only one type of atom.
  • Compounds: These substances are formed when two or more elements chemically bond together.

• The properties of a substance, such as melting point, boiling point, and density, are unique and can be used to identify the substance and distinguish it from other substances. Chemical reactions involve the formation and breaking of these chemical bonds, transforming one or more substances into different substances with new properties.

• Widely used subtractive colour models include:
  • CMY colour model: This model is a theoretical foundation for understanding how cyan, magenta, and yellow inks combine to create a wide range of colours.
  • CMYK colour model: This is a practical application of CMY, used in printing. It adds black ink (K) to the CMY combination for better contrast and richer blacks, especially when printing on highly reflective surfaces.
  • RYB colour model: This model is a historical approach that uses red, yellow, and blue pigments to teach colour-mixing concepts.

Subtractive colour with opaque pigments

• All subtractive colour models rely on the principle that the colour of an opaque object or surface is determined by the wavelengths of light it absorbs and the wavelengths it reflects.
• When light hits an object, some wavelengths are absorbed by the material, and the remaining reflected wavelengths are perceived by our eyes as colour.
• Mixing opaque pigments creates a subtractive effect because each pigment absorbs a specific range of wavelengths.
• As more pigments are combined, they absorb even more wavelengths of light.
• This reduces the amount of light reflected to our eyes, resulting in a new colour perception.
• Additionally, mixing contrasting colours like red and green (which absorb opposite ends of the light spectrum) leads to a darker result because they absorb a wider range of wavelengths combined.

Subtractive colour with translucent pigments

• For translucent inks and dyes applied to a material like paper, the observer perceives a mixture of two things:
  • Reflected wavelengths of light: Similar to opaque pigments, translucent materials reflect certain wavelengths of light that determine the perceived colour.
  • Transmitted light reflected by the paper: Some light passes through the translucent pigment layer and reflects from the surface beneath, such as the paper. This adds another layer of reflected light to the overall colour perception.
• The colour an observer sees ultimately depends on both the properties of the translucent ink or dye and the properties of the underlying material (like paper). Here are some factors that can influence the final colour:
  • Surface finish of the paper: A smooth paper surface might cause more light reflection compared to a textured surface.
  • The angle of incoming light: The angle at which light hits the translucent material can affect how much light passes through and how much reflects.
  • Viewing angle of the observer: Depending on the angle from which you look at the coloured material, the interplay of reflected and transmitted light can cause the colour to appear slightly different.

Here are some basic facts about the Sun:

• Age: 4.6 Billion Years.
• Star type: Yellow Dwarf (G2V).
• Diameter: 1,392,684 km (109 x Earth).
• Mass: 333,060 x Earth.
• Surface temperature: 5500 °C.
• Internal temperature: 15 million °C.
• Composition: Hydrogen (72%), and helium (26%).
• Energy generation: Thermo-nuclear fusion using hydrogen as fuel.
• Energy production: Equal to 100 billion tons of dynamite per second.
• Energy output: Electromagnetic radiation.
• Electromagnetic radiation emitted by the Sun = Solar energy or solar radiation.
• Visible solar radiation = Sunlight.
• Sunlight: Takes 8 minutes to reach Earth (150 million km).
• Wavelengths between red and violet are visible to the human eye.

• Sunlight is only one form of electromagnetic radiation emitted by the Sun.
• Sunlight is a form of electromagnetic radiation that our eyes are sensitive to.
• We are sensitive to other types of electromagnetic radiation, such as infrared radiation that we feel as heat, and ultraviolet radiation which can cause sunburn but is invisible to us.
• The electromagnetic spectrum includes all possible wavelengths of electromagnetic radiation, ranging from low-energy radio waves through visible light up to high-energy gamma rays.
• Sunlight is a tiny portion of a much larger spectrum called the electromagnetic spectrum.
• This spectrum encompasses all possible wavelengths of electromagnetic radiation, ranging from low-energy radio waves to high-energy gamma rays. Visible light, the portion we perceive as sunlight, occupies a small band within this spectrum.
• The human eye is sensitive to this specific range of wavelengths, allowing us to distinguish colours from red to violet.

ABOUT SUNLIGHT & NUCLEAR FUSION

The Sun generates electromagnetic waves primarily through nuclear fusion. Here’s a step-by-step explanation:

Nuclear fusion

• At the Sun’s core, extremely high temperatures and pressure allow for the fusion of hydrogen nuclei (protons) into helium.
  • This process is also known as thermonuclear fusion.
  • During this reaction, a small portion of the mass of the hydrogen atoms is converted into energy according to Einstein’s mass-energy equivalence principle (E=mc^2).

Photon production

• The energy produced by nuclear fusion is initially in the form of high-energy gamma photons.

Photon’s journey to the surface

• Gamma photons then embark on a zig-zag journey to the surface of the Sun, being absorbed and re-emitted by atoms in the Sun’s interior and gradually losing energy in the process.

Surface emission

• Once photons reach the Sun’s surface (photosphere), they escape and radiate into space. While the majority of this energy is in the form of visible light, it also emits significant amounts of energy in the ultraviolet and infrared parts of the spectrum, as well as smaller amounts in the X-ray, gamma ray, and radio wave parts of the spectrum.

Solar radiation

• The emitted electromagnetic waves, known collectively as solar radiation or sunlight, then travel through space and can interact with objects they encounter, such as planets. For Earth, these interactions provide light and heat essential to life.

Magnetic field

• It’s also worth noting that the Sun’s magnetic field can contribute to the generation of some forms of electromagnetic radiation, like solar flares or coronal mass ejections, which can emit radio waves and X-rays.

• There are two main contexts to consider:
  • Circles: A tangent to a circle is a straight line that touches the circle at exactly one point, like a line just brushing against a ball. There’s also a special property – the radius drawn from the centre of the circle to the point of touch is always perpendicular (at a 90-degree angle) to the tangent line.
  • General Curves: A tangent line can also be applied to any smooth, curved shape. Here, the concept gets a bit more mathematical. We can define a tangent as a straight line that intersects the curve at exactly one point, but if we could zoom in infinitely close to that point, the curve would begin to resemble a straight line, and the tangent line would become indistinguishable from the curve itself.

• • Hot objects: Emit more electromagnetic radiation at shorter wavelengths. Imagine a hot fire burning bright with blue hues. Similarly, hot objects emit a higher proportion of their energy at shorter wavelengths, which often appear bluish.
  • Cold objects: Emit more electromagnetic radiation at longer wavelengths. Think of a dimmer fire glowing red. Colder objects emit more radiation at longer wavelengths, which tend to be perceived as redder.
• The relationship between temperature and the peak wavelength of an object’s radiation is described by Wien’s displacement law. This law states that the product of an object’s temperature and the peak wavelength of the radiation it emits is a constant.
• Black Bodies: Wien’s law applies to a theoretical concept called a black body. A black body is an idealized object that absorbs all incoming radiation and emits radiation at all wavelengths. Real objects aren’t perfect black bodies, but they still emit electromagnetic radiation based on their temperature.

Explanation of Thermal Radiation

• All matter consists of atoms and molecules in constant motion. This motion has kinetic energy, which is associated with the temperature of an object. As temperature increases, the motion of the particles becomes more agitated. This causes charged particles within the matter (like protons and electrons) to accelerate and change their energy states.

Emission of Energy

• When charged particles change energy states, they release energy in the form of electromagnetic waves.
• The frequency and intensity of this radiation depend directly on the object’s temperature.

The Spectrum of Thermal Radiation

• Thermal radiation covers a wide range of the electromagnetic spectrum. However, much of it falls within the infrared region, which we experience as heat. Hotter objects emit more thermal radiation and a higher proportion of radiation in the visible light spectrum. This is why very hot objects can start to glow red or white.

Examples

• The Sun: A primary source of thermal radiation. Its high surface temperature causes it to emit a broad spectrum of electromagnetic radiation, including infrared, visible light, and ultraviolet radiation.
• A Radiator: Designed to emit heat through thermal radiation, warming a room.
• The Human Body: Emits infrared radiation, which is why thermal imaging cameras can detect us in the dark.
• The Earth: Absorbs solar radiation and then emits thermal radiation back out into space.

Key Points

• A Constant Process: As long as an object has some internal heat, it emits thermal radiation.
• Heat Transfer Thermal radiation is one of the three main forms of heat transfer (alongside conduction and convection).
• Universal Phenomenon: Thermal radiation occurs throughout the universe, from stars to everyday objects.

• The First Law of Thermodynamics:
  • This law is a statement of the principle of conservation of energy. It states that energy can neither be created nor destroyed but only transferred from one form to another.
  • The total amount of energy in a closed system (one that does not exchange energy with its surroundings) remains constant.
• The Second Law of Thermodynamics:
  • This law deals with the concept of entropy, a measure of disorder in a system.
  • A system with high entropy is more disordered than a system with low entropy. The second law states that in an isolated system (one that does not exchange matter or energy with its surroundings), entropy always increases over time.
  • This means that usable energy tends to disperse over time into less usable forms, leading to a gradual increase in disorder.
  • Entropy can be understood as a measure of how spread out or disorganized the energy in a system is. Over time, energy tends to disperse from concentrated usable forms to more spread-out unusable forms, increasing the overall disorder.
• These two laws of thermodynamics have been extensively tested and verified through experiments.

Major contributors

• Major contributors to the laws of thermodynamics were Nicolas Léonard Sadi Carnot, James Prescott Joule and Lord Kelvin all of whom were at work during the 19^th century.
  • Nicolas Léonard Sadi Carnot (1796-1832) was a French physicist and engineer. He is best known for his work on thermodynamics, particularly his development of the Carnot cycle, a theoretical thermodynamic cycle that describes the maximum efficiency of a heat engine.
  • James Prescott Joule (1818-1889) was an English physicist and brewer. He is best known for his work on the relationship between heat and work, which led to the development of the first law of thermodynamics.
  • William Thomson, 1st Baron Kelvin (1824-1907) was a Scottish mathematician, physicist and engineer. He is best known for his work on thermodynamics, particularly his development of the Kelvin scale of temperature.

Examples of thermodynamics

• The First Law in Action:
  • Imagine throwing a ball up in the air. As the ball rises, its kinetic energy of motion is converted into potential energy due to its height.
  • When the ball falls back down, the potential energy is converted back into kinetic energy.
  • Even though the form of the energy changes (kinetic to potential and back), the total amount of energy in the ball remains constant.
  • This exemplifies the principle of energy conservation as described by the first law of thermodynamics.
• The Second Law in Action:
  • Consider a light bulb. When you turn it on, the electrical energy from the outlet is transformed into light energy and thermal energy (heat).
  • The total amount of energy is still conserved, following the first law.
  • However, the light bulb’s heat dissipates into the surroundings, making it less concentrated and usable.
  • Heat energy, in this sense, is more spread out and less usable than electrical energy, making the system more disordered according to the second law of thermodynamics. Entropy, a measure of disorder, therefore increases in this process.

• Exposure to Radiation: When a material (usually a crystalline solid) is exposed to ionizing radiation (like X-rays, gamma rays, or cosmic rays), some electrons within the material get trapped in imperfections within the crystal structure.
• Heating and Light Emission: When the material is heated, these trapped electrons gain enough energy to escape their traps. As they return to their original energy state, they release energy in the form of visible light.
• Measuring Radiation: The intensity of light emitted during thermoluminescence is proportional to the amount of radiation the material was previously exposed to. This makes it a useful technique for fields like archaeology and radiation safety.

There are two forms of thermonuclear fusion (nuclear fusion):

• Uncontrolled Fusion: This is the process where atomic nuclei merge spontaneously and release a tremendous amount of energy in an uncontrollable manner.
• It is the natural process happening within stars and the principle behind thermonuclear weapons.
• Controlled Fusion: Scientists are actively researching ways to achieve controlled fusion, where atomic nuclei are combined in a controlled environment.
• This would allow us to harness the immense energy released for constructive purposes like generating clean and sustainable power, reducing reliance on fossil fuels, and potentially powering future space exploration endeavours.

Challenges of Fusion

• Achieving controlled fusion is a significant scientific and engineering challenge. Fusion requires incredibly high temperatures and pressure to overcome the natural repulsion between atomic nuclei and force them to fuse.

Energy Release during Fusion

• Nuclear fusion reactions release energy in various forms. The primary form is in the form of high-energy photons called gamma rays. These gamma rays interact with the surrounding matter in a star, including hydrogen atoms. These interactions create other forms of light, ultimately resulting in the visible light we see radiating from stars. So, the light we perceive from stars is a product of the fusion processes happening within their cores.
• In stars, fusion primarily occurs at their core, where immense gravitational pressure and scorching temperatures provide the perfect environment for hydrogen atoms to fuse into helium. This fusion process releases the energy that powers the star and ultimately reaches us as the light we see in the night sky.

• In the context of additive colour models such as RGB or HSB, a darker tone of a hue is produced by reducing its colour brightness. The result is a desaturated, muted version of the original colour.
• In the context of subtractive colour models such as CMY and RYB, A darker tone (or shade) of a colour is achieved by adding black or a darker colour to it. The result is a desaturated, muted version of the original colour.
• In photography, tone refers to the different shades of grey that can be produced, ranging from pure white to pure black.
  • Here, tone describes the relative darkness or lightness of a specific shade of grey.
  • A greyscale image is created by discarding hue information from a range of colours. The resulting shades of grey reflect the original colours’ luminance (light intensity) but may not perfectly match their perceived brightness.
  • Whilst yellow appears to have a very light tone when converted to greyscale, blue appears to have a very dark tone.
• In the context of a greyscale, tone is used to describe the relative darkness or lightness of a specific shade of grey.
  • A greyscale is the result of removing hue from a range of colours leaving their saturation and brightness unaffected.
  • More sophisticated methods of producing a greyscale use specific algorithms to create images that better represent the perceived brightness of the original colours.
• Tone and value are closely related concepts. Tone describes the perceived lightness or darkness within a colour context, while value refers to the objective amount of light reflected or emitted, independent of colour.
  • To clarify this difference, think of value as a light meter reading that measures the amount of light reflecting off a surface. This reading would be a numerical value, independent of any colour information.