Nanometre

A nanometre is a metric unit of measurement, which is equal to one billionth of a meter (1 nm = 10^-9 m).

  • A nanometre is a metric unit of measurement, which is equal to one billionth of a meter (1 nm = 10^-9 m).
  • A nanometre is a unit of measurement of the wavelength of electromagnetic radiation.
  • A nanometre (nm) is a unit of length used to express the size of objects on a nanoscale, including the wavelength of electromagnetic radiation.
  • Nanometres are particularly useful when specifying the wavelength of electromagnetic waves in the visible region of the electromagnetic spectrum.
  • The visible spectrum ranges from around 380 to 750 nm.
  • When discussing the size of nanoparticles or DNA molecules,  nanometres can help readers visualize their minute scale.

Natural light source

A natural light source is any source of light that occurs in nature without human intervention or creation. These sources can be celestial objects, atmospheric phenomena, or living organisms.

Celestial Objects
  • The Sun: Our primary source of natural light, providing warmth, driving photosynthesis, and allowing us to see.
  • Other Stars: Distant stars are inherently sources of light, though they appear far less bright to us due to their vast distances.
  • The Moon: It doesn’t produce its own light but reflects sunlight, providing a source of natural illumination at night.
Atmospheric Phenomena
  • Lightning: Electrical discharges in the atmosphere create bright flashes of natural light.
  • Auroras (Borealis and Australis): Caused by charged particles from the sun interacting with the Earth’s magnetic field, creating vibrant displays of light in the sky.
Living Organisms (Bioluminescence)
  • Fireflies: Use chemical reactions to generate light for attracting mates or prey.
  • Jellyfish: Some species emit light as a defence mechanism or method of communication.
  • Deep-sea creatures: Many creatures in the depths of the ocean produce light to navigate, lure prey, or find mates in a completely dark environment.
Key Points about Natural Light
  • Essential: Natural light is crucial for life on Earth, influencing plant growth, animal behaviour, and even human well-being.
  • Spectrum: Natural light sources often emit a broad spectrum of wavelengths, including colours visible to the human eye.
  • Unpredictable (sometimes): The availability and intensity of some natural light sources can be affected by factors such as weather, time of day, or season.
Light sources
Emission mechanism DescriptionExamples
LIGHT-EMITTING PROCESS
LuminescenceLight emission due to the excitation of electrons in a material.Electrons within a material gain energy and then release light as they return to a lower energy state.Bioelectroluminescence
Electroluminescence
Photoluminescence
- Fluorescence
- Phosphorescence
Sonoluminescence
Thermoluminescence
Blackbody radiation (Type of thermal radiation)Electromagnetic radiation (including visible light) emitted by any object with a temperature above absolute zero.Electromagnetic radiation (including visible light) emitted by any object with a temperature above absolute zero.All objects above temperature of absolute zero.
ChemiluminescenceLight from natural and artificial chemical reactions.Light from natural and artificial chemical reactions.Bioluminescence
Chemiluminescent reactions:
- Luminol reactions
- Ruthenium chemiluminescence
Nuclear reactionLight emission as a byproduct of nuclear reactions (fusion or fission).Light emitted as a byproduct of nuclear reactions.Nuclear reactors
Stars undergoing fusion
Thermal radiationLight emission due to the thermal excitation of atoms and molecules at high temperatures.Light emission due to the thermal excitation of atoms and molecules.Sun
Stars
Incandescent light bulbs
TriboluminescenceLight emission due to mechanical stress applied to a material.Light emission due to the mechanical stress applied to a material, causing the movement of electric charges and subsequent light emission.Sugar crystals cracking
Adhesive tape peeling
Quartz crystals fracturing.
Natural light source
Fireflies
Deep-sea creatures
Glowing mushrooms
Bioluminescence Light emission from biological organisms.Involves the luciferase enzyme.
Sun
Stars
Nuclear FusionLight emission as a byproduct of nuclear fusion reactions in stars.Electromagnetic spectrum (visible light, infrared, ultraviolet).
Fire
Candles
Thermal radiationLight emission due to the thermal excitation of atoms and molecules during the combustion of a fuel source.Burning of a fuel source, releasing heat and light.
Artificial light source
Fluorescent lights Highlighters
Safety vests
Chemiluminescence Light emission from chemical reactions.Fluorescence (absorption and re-emission of light).
Glow sticks
Emergency signs
ChemiluminescenceLight emission due to phosphorescence - a type of chemiluminescence.A type of chemiluminescence where light emission is delayed after the initial excitation.
Glow sticks
Light sticks
Chemiluminescence Chemiluminescence Light emission from a chemical reaction that does not involve combustion.
Tungsten light bulbs
Toasters
Thermal radiationHeated filament radiates light and heat.Light emission from a hot filament.
Fluorescent lamps
LED lights
ElectroluminescenceExcitation of atoms by electric current.Light emission when electric current excites atoms in a material.
Neon signsElectrical DischargeDischarge of electricity through gas.Light emission when electricity flows through a gas.
Sugar crystals cracking
Pressure-sensitive adhesives
TriboluminescenceLight emission from friction or pressure.Light emission due to mechanical forces.
Fluorescent paint Highlighters
Safety vests
PhotoluminescenceAbsorption and subsequent re-emission of light at a lower energy.Absorption and re-emission of light.

Light Sources: Mechanism, examples, and everyday applications

Footnote: Cerenkov radiation and Synchrotron radiation are not included in the table because they are not conventionally classified as light sources.

    • A natural light source is any source of light that occurs in nature without human intervention or creation. These sources can be celestial objects, atmospheric phenomena, or living organisms.
Key Points about Natural Light
      • Essential: Natural light is crucial for life on Earth, influencing plant growth, animal behaviour, and even human well-being.
      • Spectrum: Natural light sources often emit a broad spectrum of wavelengths, including colours visible to the human eye.
      • Unpredictable (sometimes): The availability and intensity of some natural light sources can be affected by factors such as weather, time of day, or season.

Nature

Nature, in the broadest sense, refers to the physical universe including all living and non-living thing.

  • In a limited sense, nature refers to interconnected living organisms including plants, insects, and animals, but excluding non-living phenomena such as oceans, continents, and climate.
  • It is worth noting that even non-living phenomena can be considered part of nature, as they are integral to the natural world and ecosystems.
  • The concept of nature is therefore complex and multifaceted one, with varying definitions depending on context and discipline.
    • For example, although humans are part of nature, human activities such as cities, agriculture, and industries are sometimes seen as separate from other natural phenomena.
    • Again, many indigenous cultures have rich understandings of and relationships with the natural world that differ from Western perspectives.
  • Nature, in the broadest sense, refers to the physical universe including all living and non-living thing.
  • In a limited sense, nature refers to interconnected living organisms including plants, insects, and animals, but excluding non-living phenomena such as oceans, continents, and climate.
  • It is worth noting that even non-living phenomena can be considered part of nature, as they are integral to the natural world and ecosystems.
  • The concept of nature is therefore complex and multifaceted one, with varying definitions depending on context and discipline.
    • For example, although humans are part of nature, human activities such as cities, agriculture, and industries are sometimes seen as separate from other natural phenomena.

Neuron

Neurons are the cells that transmit electrical impulses around the brain and the other parts of the central nervous system.

  • Neurons are the electrically excitable cells that are the fundamental building blocks of the central nervous system of human beings.
  • Neurons interconnect the systems and organs that maintain the body’s essential functions.
  • Neurons send and receive signals that allow us to sense the external world, move, think, form memories and much more.
  • Neurons are of three principal types: motor neurons, sensory neurons and interneurons.
  • Neurons connect together via specialized filaments called synapses.
  • In the neocortex (making up about 80% of the human brain), approximately 70-80% of nervous tissue is in the form of neurons whilst the remainder is composed of interneurons.
About the anatomy of neurons
  • A typical neuron consists of a cell body (soma), dendrites, and a single axon.
  • Dendrites and axons form filament-like extensions of the soma.
  • Dendrites typically form into a profusion of branches as they extend from the soma.
  • An axon can be as long as a metre in length.
  • At the farthest tip of the axon’s branches are axon terminals, where the neuron can transmit a signal across a synapse to another cell.
About interneurons
  • Interneurons are also referred to as relay neurons, connector neurons, intermediate neurons and local circuit neurons each of which helps to explain their function.
  • Interneurons form nodes within neural circuits, enabling communication between sensory or motor neurons and the central nervous system.
  • Interneurons can be further broken down into two groups: local interneurons and relay interneurons.
    • Local interneurons have short axons and form circuits with nearby neurons to analyse small pieces of information.
    • Relay interneurons have long axons and connect circuits of neurons in one region of the brain with those in other regions.
  • The interaction between interneurons allows the brain to perform complex functions such as sense-making.
About neurons and the human retina

Neuron anatomy

About the anatomy of neurons
  • Neurons are the building blocks of the nervous system.
  • A typical neuron consists of a cell body (soma), dendrites, and a single axon.
  • Dendrites and axons form filamentous extensions of the soma.
  • Dendrites typically branch profusely as they extend from the soma.
  • An axon can be as long as a metre in length.
  • At the farthest tip of the axon’s branches are axon terminals, where the neuron can transmit a signal across a synapse to another cell.

Neurons & the human retina

About neurons and the human retina

Newtonian mechanics

Newtonian mechanics is a branch of physics that describes the motion of objects under the influence of forces. It is based on the three laws of motion that were developed by Isaac Newton in the 17th century.

The three laws of motion are:

  1. An object at rest will remain at rest, or if in motion, will remain in motion at a constant speed and in a straight line, unless acted upon by an external force.
  2. The acceleration of an object is directly proportional to the net force acting on it, and inversely proportional to its mass.
  3. For every action, there is an equal and opposite reaction.

These laws can be used to describe a wide range of phenomena, from the motion of planets to the behavior of fluids to the propagation of waves. They are also used in many different fields, including engineering, medicine, and astronomy.

Newtonian mechanics is a very successful theory, and it has been used to predict the motion of objects with a high degree of accuracy. However, it has some limitations. For example, it cannot explain the behavior of light at the atomic and subatomic level. This is because light behaves both like a wave and a particle, which is something that Newtonian mechanics cannot describe.

Despite its limitations, Newtonian mechanics is still a very important and useful theory. It is used in many different fields, and it has helped us to understand the universe in a much deeper way.

Here are some examples of Newtonian mechanics in action:

  • When you throw a ball, the ball accelerates due to the force of gravity.
  • When you ride a bike, you need to pedal to keep moving forward because of the force of friction.
  • When you sit in a chair, the chair exerts an upward force on you that balances the downward force of gravity.
  • When you jump off a cliff, you accelerate due to the force of gravity until you hit the water.

Newtonian mechanics is a fundamental theory of physics, and it is essential for understanding how the world works.

Summary

About sections (temp)

References

Non-spectral colour

A non-spectral colour is a colour that is not present in the visible spectrum and cannot be produced by a single wavelength or narrow band of wavelengths of light.

  • While spectral colours are evoked by a single wavelength of light in the visible spectrum, non-spectral colours are produced by a combination of spectral colours from different parts of the spectrum.
  • Colours evoked by a single wavelength of light are often described as being produced by monochromatic light.
  • Magenta, pink, cyan and brown are examples of non-spectral colours produced by combining different wavelengths of light:
    • Blue and red = magenta
    • Red and purple = pink
    • Blue and green = cyan
    • Red, yellow and blue = brown
  • When we look around us, the colours of things we see rarely include pure spectral colours but are more likely composed of narrow bands of contiguous wavelengths.
  • Since both the RGB and CMY colour models mix primary colours from different parts of the visible spectrum, digital screens and digital printers produce non-spectral colours.
    • The RGB colour model generates a complete range of colours on TVs, computers and phones by blending the primary colours (red, green and blue) in varying proportions.
    • The CMY colour model produces a full spectrum of colours by blending the primary colours of cyan, magenta, and yellow in varying proportions.
How spectral colours produce non-spectral colours
  • When spectral colours from different parts of the visible spectrum are combined, they stimulate multiple types of cone cells in the human eye, leading to the perception of intermediate hues that do not correspond to any single wavelength.
  • For example, when red light (long wavelengths) and blue light (short wavelengths) are combined, they stimulate both the red-sensitive and blue-sensitive cones, resulting in the perception of magenta, a colour that is not present in the spectrum.
  • Similarly, combining green and blue light stimulates both green-sensitive and blue-sensitive cones, resulting in the perception of cyan. The mixing of different spectral colours in varying proportions leads to the creation of a wide range of non-spectral colours, each with its own unique appearance.
  • A non-spectral colour is a colour that is not present in the visible spectrum and cannot be produced by a single wavelength or narrow band of wavelengths of light.
  • While spectral colours are evoked by a single wavelength of light in the visible spectrum, non-spectral colours are produced by a combination of spectral colours from different parts of the spectrum.
  • Colours evoked by a single wavelength of light are often described as being produced by monochromatic light.
  • Magenta, pink, cyan and brown are examples of non-spectral colours produced by combining different wavelengths of light:
    • Blue and red = magenta
    • Red and purple = pink
    • Blue and green = cyan
    • Red, yellow and blue = brown

Normal

If one line is normal to another, then it is at right angles to it.

In geometry, normal (a or the normal) refers to a line drawn perpendicular to a given line, plane or surface.

  • How a normal appears in a geometric drawing depends on the circumstances:
    • When light strikes a flat surface or plane, or the boundary between two surfaces, the normal is drawn perpendicular to the surface, forming a right angle (90°) with it.
    • Expressed more formally, in optics, the normal is a geometric construct, a line drawn perpendicular to the interface between two media at the point of contact. This conceptually defined reference line is crucial for characterizing various light-matter interactions, such as reflection, refraction, and absorption.
    • When dealing with curved surfaces, such as those found on spheres or other three-dimensional objects, determining the normal requires a slightly different approach. Instead of simply drawing a line perpendicular to the surface as with a flat plane, draw the normal straight up from the point where light hits the surface.
    • Similarly, when considering the centre of a sphere, the normal line passes through the centre of the sphere. This is because, regardless of where light enters or exits the sphere, the normal represents the direction perpendicular to the surface at that point.
Remember that:
  • If one line is normal to another, then it is at right angles to it.
  • In geometry, normal (a or the normal) refers to a line drawn perpendicular to a given line, plane or surface.
  • How a normal appears in a geometric drawing depends on the circumstances:
    • When light strikes a flat surface or plane, or the boundary between two surfaces, the normal is drawn perpendicular to the surface, forming a right angle (90°) with it.
    • Expressed more formally, in optics, the normal is a geometric construct, a line drawn perpendicular to the interface between two media at the point of contact. This conceptually defined reference line is crucial for characterizing various light-matter interactions, such as reflection, refraction, and absorption.
    • When dealing with curved surfaces, such as those found on spheres or other three-dimensional objects, determining the normal requires a slightly different approach. Instead of simply drawing a line perpendicular to the surface as with a flat plane, draw the normal straight up from the point where light hits the surface.
    • Similarly, when considering the centre of a sphere, the normal line passes through the centre of the sphere. This is because, regardless of where light enters or exits the sphere, the normal represents the direction perpendicular to the surface at that point.

Normal

If one line is normal to another, then it is at right angles. So in geometry, the normal is a line drawn perpendicular to and intersecting another line.

In optics, the normal is an imaginary line drawn on a ray diagram perpendicular to, so at a right angle to (900), to the boundary between two media.

  • Expressed more formally, in optics, the normal is a geometric construct, a line drawn perpendicular to the interface between two media at the point of contact. This conceptually defined reference line is crucial for characterizing various light-matter interactions, such as reflection, refraction, and absorption.
  • Light travels in a straight line through a vacuum or a transparent medium such as air, glass, or still water.
  • If light encounters a force, an obstacle or interacts with an object, a variety of optical phenomena may take place including absorption, dispersion, diffraction, polarization, reflection, refraction, scattering or transmission.
  • Optics treats light as a collection of rays that travel in straight lines and calculates the way in which they change direction (deviate) when encountering different optical phenomena.
  • When the normal is drawn on a ray diagram, it provides a reference against which the amount of deviation of the ray can be shown.
  • The normal is always drawn at right angles to a ray of incident light at the point where it arrives at the boundary with a transparent medium.
  • Expressed more formally, in optics, the normal is a geometric construct, a line drawn perpendicular to the interface between two media at the point of contact. This conceptually defined reference line is crucial for characterizing various light-matter interactions, such as reflection, refraction, and absorption.

Nuclear reaction

A nuclear reaction involves changes within the nucleus of an atom, leading to the release of energy and often the emission of various particles, including electromagnetic radiation. This radiation can encompass a range of the electromagnetic spectrum, with gamma rays being a particularly common form.

Here’s a breakdown of how nuclear reactions can be sources of electromagnetic radiation:

  • Nuclear Fission: When the nucleus of a heavy atom splits into smaller nuclei, it releases a significant amount of energy. A significant portion of this energy is emitted as gamma rays, which are high-energy photons within the electromagnetic spectrum. Nuclear power plants and atomic bombs harness fission reactions.
  • Nuclear Fusion: When the nuclei of lighter atoms combine to form a heavier nucleus, it also releases energy. In stars like our Sun, nuclear fusion releases large amounts of energy, including a range of electromagnetic radiation from infrared light to ultraviolet light, and even gamma rays.
  • Radioactive Decay: Unstable atomic nuclei undergo decay and change their composition to reach a more stable state. During this process, they can release charged particles (like alpha or beta particles), neutrinos, and often gamma rays.
Key Points
  • Spectrum of Radiation: Nuclear reactions can produce electromagnetic radiation across a wide range of frequencies, but gamma rays are particularly common and known for their high penetrating power.
  • Energy Release: The high amount of energy released during nuclear reactions translates to high-frequency, highly energetic electromagnetic radiation.
  • Natural and Artificial Sources: Nuclear reactions naturally occur in stars and through radioactive decay processes. They can also be artificially induced in nuclear reactors and nuclear weapons.
Light sources
Emission mechanism DescriptionExamples
LIGHT-EMITTING PROCESS
LuminescenceLight emission due to the excitation of electrons in a material.Electrons within a material gain energy and then release light as they return to a lower energy state.Bioelectroluminescence
Electroluminescence
Photoluminescence
- Fluorescence
- Phosphorescence
Sonoluminescence
Thermoluminescence
Blackbody radiation (Type of thermal radiation)Electromagnetic radiation (including visible light) emitted by any object with a temperature above absolute zero.Electromagnetic radiation (including visible light) emitted by any object with a temperature above absolute zero.All objects above temperature of absolute zero.
ChemiluminescenceLight from natural and artificial chemical reactions.Light from natural and artificial chemical reactions.Bioluminescence
Chemiluminescent reactions:
- Luminol reactions
- Ruthenium chemiluminescence
Nuclear reactionLight emission as a byproduct of nuclear reactions (fusion or fission).Light emitted as a byproduct of nuclear reactions.Nuclear reactors
Stars undergoing fusion
Thermal radiationLight emission due to the thermal excitation of atoms and molecules at high temperatures.Light emission due to the thermal excitation of atoms and molecules.Sun
Stars
Incandescent light bulbs
TriboluminescenceLight emission due to mechanical stress applied to a material.Light emission due to the mechanical stress applied to a material, causing the movement of electric charges and subsequent light emission.Sugar crystals cracking
Adhesive tape peeling
Quartz crystals fracturing.
Natural light source
Fireflies
Deep-sea creatures
Glowing mushrooms
Bioluminescence Light emission from biological organisms.Involves the luciferase enzyme.
Sun
Stars
Nuclear FusionLight emission as a byproduct of nuclear fusion reactions in stars.Electromagnetic spectrum (visible light, infrared, ultraviolet).
Fire
Candles
Thermal radiationLight emission due to the thermal excitation of atoms and molecules during the combustion of a fuel source.Burning of a fuel source, releasing heat and light.
Artificial light source
Fluorescent lights Highlighters
Safety vests
Chemiluminescence Light emission from chemical reactions.Fluorescence (absorption and re-emission of light).
Glow sticks
Emergency signs
ChemiluminescenceLight emission due to phosphorescence - a type of chemiluminescence.A type of chemiluminescence where light emission is delayed after the initial excitation.
Glow sticks
Light sticks
Chemiluminescence Chemiluminescence Light emission from a chemical reaction that does not involve combustion.
Tungsten light bulbs
Toasters
Thermal radiationHeated filament radiates light and heat.Light emission from a hot filament.
Fluorescent lamps
LED lights
ElectroluminescenceExcitation of atoms by electric current.Light emission when electric current excites atoms in a material.
Neon signsElectrical DischargeDischarge of electricity through gas.Light emission when electricity flows through a gas.
Sugar crystals cracking
Pressure-sensitive adhesives
TriboluminescenceLight emission from friction or pressure.Light emission due to mechanical forces.
Fluorescent paint Highlighters
Safety vests
PhotoluminescenceAbsorption and subsequent re-emission of light at a lower energy.Absorption and re-emission of light.

Light Sources: Mechanism, examples, and everyday applications

Footnote: Cerenkov radiation and Synchrotron radiation are not included in the table because they are not conventionally classified as light sources.

  • A nuclear reaction involves changes within the nucleus of an atom, leading to the release of energy and often the emission of various particles, including electromagnetic radiation. This radiation can encompass a range of the electromagnetic spectrum, with gamma rays being a particularly common form.
  • Here’s a breakdown of how nuclear reactions can be sources of electromagnetic radiation:
    • Nuclear Fission: When the nucleus of a heavy atom splits into smaller nuclei, it releases a significant amount of energy. A significant portion of this energy is emitted as gamma rays, which are high-energy photons within the electromagnetic spectrum. Nuclear power plants and atomic bombs harness fission reactions.
    • Nuclear Fusion: When the nuclei of lighter atoms combine to form a heavier nucleus, it also releases energy. In stars like our Sun, nuclear fusion releases large amounts of energy, including a range of electromagnetic radiation from infrared light to ultraviolet light, and even gamma rays.
    • Radioactive Decay: Unstable atomic nuclei undergo decay and change their composition to reach a more stable state. During this process, they can release charged particles (like alpha or beta particles), neutrinos, and often gamma rays.

The normal, angles of incidence, reflection & refraction

About the normal, angles of incidence, reflection & refraction

In geometry, normal (a or the normal) refers to a line drawn perpendicular to a given line, plane or surface.

    • How a normal appears in a geometric drawing depends on the circumstances:
        • When light strikes a flat surface or plane, or the boundary between two surfaces, the normal is drawn perpendicular to the surface, forming a right angle (90°) with it.
        • When light hits a curved surface, the normal line is drawn straight up from the point where the light hits the surface.
        • If light travels directly through the centre of a sphere, the normal line also passes through the centre of the sphere.
        • When a normal is drawn on a ray-tracing diagram, it provides a reference perpendicular to the surface against which changes in direction of light can be measured.