Dictionary tag: I

• When a book is placed on a table, different levels of illuminance can be observed depending on whether the sky is overcast, the time of day, or whether the surface is indirectly lit.
• Illuminance is a measure of the amount of light that falls on a surface per unit area. It is determined by the intensity of the light source and the distance from the light source to the surface, but is independent of the characteristics of the surface it strikes, such as its colour or reflectivity.
• Illuminance is a measure of the quantity of light that falls on a surface, but it does not provide information about the spectral composition or other characteristics of the light. Other measures such as spectral power distribution, colour temperature, and colour rendering index can be used to describe other qualities of light beyond illuminance.
• The brightness of a light source does not necessarily correspond with its illuminance. For example, a 10-watt light bulb placed next to a person reading a book can provide enough illuminance, while a 1000-watt light bulb located far away may not provide enough light to read by.
• Illuminance is typically measured in units of lux (lx).

• Illumination can be provided through artificial light sources such as lamps and light fixtures, or natural illumination by capturing daylight.
• Daylighting, which involves the use of windows, skylights or light shelves, is sometimes used as the main source of light during daytime in buildings.
• Specialized forms of artificial lighting have been developed to suit every possible situation and purpose where natural light is not available, such as in underground spaces or during nighttime.
• The colour temperature of light can affect how colours appear to an observer. Lighting producing a colour temperature below 4000K produces the impression of warmer colours. Lighting producing a colour temperature above 5000K produces the impression of cooler colours.
• Colour temperature is measured in Kelvin (K) – the lower the number, the warmer the white light appears, while a higher Kelvin will appear cooler. Colour temperature generally ranges from 2700-3300K (warm) to 3300-5300 (cool) – and 6500K is daylight.

• Incandescent light is produced when electricity flows through a filament, typically made of tungsten, heating it to thousands of degrees Celsius. This intense heat excites the atoms in the filament, causing them to change their energy levels and behaviour:

Energy Levels and Electrons

• Atoms have distinct energy levels where electrons reside. Imagine these levels like steps on a ladder. The lowest energy level, called the ground state, is like the bottom step. Excitation occurs when atoms absorb energy, such as heat, causing their electrons to jump to higher energy levels (like climbing the ladder).

Energy Absorption and Excitation

• The heat energy that causes excitation comes in the form of electromagnetic radiation, at visible light or infrared wavelengths. When the radiation collides with atoms, they can transfer their energy to the electrons. If the transferred energy matches the difference between two energy levels in the atom, the electron absorbs it and “excites” to the higher level.

Instability and Return

• An excited atom is unstable and wants to return to its ground state. It does this by releasing the absorbed energy in different ways:
  • Light Emission: In many cases, the excited atom releases the energy as a photon (light particle) with a specific wavelength corresponding to the energy difference between the initial and final levels. This is how processes like incandescent light, neon signs, and some types of lasers work.
  • Collisions between atoms: The excited atom can transfer its energy to another atom through a collision. This can cause a chain reaction or lead to other physical or chemical reactions.

Colour and Efficiency

• The colour of incandescent light depends on the filament temperature. Hotter filaments emit more bluish light, while cooler ones glow yellow or orange. However, incandescent light sources are generally less efficient than other lighting technologies, converting a significant portion of their energy into heat rather than light.

Examples

• Incandescent light: In a hot gas, like the filament in an incandescent bulb, heating excites atoms, causing them to emit visible light, producing their characteristic glow.
• Fluorescent Light: In fluorescent lamps, UV radiation excites atoms in a gas, the excited atoms then transfer their energy to other atoms, which in turn emit visible light.
• Aurora: When sunlight excites atoms in the atmosphere, it causes them to emit specific wavelengths of light, resulting in phenomena like the aurora borealis.

Applications and Decline

• While largely replaced by more efficient options like LED bulbs, incandescent lighting is still used in some applications due to its familiar warm glow and dimming capabilities.
• However, its use is declining due to its lower energy efficiency and shorter lifespan.

• Incident light may come from the Sun, an artificial source or may have already been reflected off another surface, such as a mirror.
• When incident light strikes a surface or object, it may be absorbed, reflected, refracted, transmitted or undergo any combination of these optical effects.
• Incident light is typically represented on a ray diagram as a straight line with an arrow to indicate its direction of propagation.
• A ray diagram is a diagram that uses lines and arrows to represent the path of light, and labels to indicate the angles, directions, and other optical properties of the light.
• Incident light is an important concept in optics and is used in various fields, including photography, astronomy, and physics.

• Refractive index (or, index of refraction) is a measurement of how much the speed of light is reduced when it passes through a medium compared to the speed of light in a vacuum.
• The concept of refractive index applies to the full electromagnetic spectrum, from gamma-rays to radio waves.
• Refractive index can vary with the wavelength of the light being refracted. This phenomenon is called dispersion, and it is what causes white light to split into its constituent colours when it passes through a prism.
• The refractive index of a material can be affected by various factors such as temperature, pressure, and density.

• The refractive index of a medium is a numerical value and is represented by the symbol n.
• Because it is a ratio of the speed of light in a vacuum to the speed of light in a medium there is no unit for refractive index.
• The refractive index of water is 1.333, meaning that light travels at 2/3 the speed in water compared to a vacuum.
• If the refractive index of a medium is 1.5, for example, light travels at 2/3 the speed through glass compared to a vacuum.
• As light undergoes refraction, its wavelength changes, but its frequency remains the same.
• As light undergoes refraction its frequency remains the same.
• The energy transported by light is not affected by refraction or the refractive index of a medium, but the intensity of the light can be affected.

• Intensity measures the amount of energy carried by a light wave or stream of photons.
• When light is modelled as a wave, intensity is proportional to the square of the amplitude.
• When light is modelled as a particle, intensity is proportional to the number of photons present at any given point in time.
• The intensity of light falls off as the inverse square of the distance from a point light source increases.
• Light intensity at any given distance from a light source is directly related to the power of the light source and the distance from the source.
• The power of a light source describes the rate at which energy is emitted and is measured in watts.
• The intensity of light is measured in watts per square meter (W/m²) and is also commonly expressed in lux (lx).

About intensity and brightness

Intensity

• Intensity refers to the amount of light energy passing through a unit area perpendicular to the direction of light propagation. It measures the concentration of light energy per unit area. Intensity is typically quantified in units such as watts per square meter (W/m²) or lux (lx). In the case of a point source of light, intensity decreases with distance according to the inverse square law.

Brightness

• Brightness, on the other hand, refers to the subjective perception of how intense or luminous a light source appears to an observer. It is the quality of being bright or emitting or reflecting a lot of light. Brightness is influenced by factors such as the intensity of the light source, the surface area over which the light is distributed, the spectral composition of the light, and the sensitivity of the human eye. Unlike intensity, which is a physical quantity measured objectively, brightness is a perceptual attribute and can vary from person to person.

• A simple form of interference takes place when two plane waves of the same frequency meet at an angle and combine.
• Light interference is often observed as interference patterns, such as seen in supernumerary rainbows.
• Interference patterns are produced when the energy of waves combines constructively or destructively. For example, waves on a pond can create interference patterns.
  • Constructive interference occurs when the crest of one wave meets the crest of another wave of the same frequency at the same point. The resulting wave is the sum of the amplitudes of the original waves.
  • Destructive interference occurs when the trough of one wave meets the crest of another wave. The resulting wave is the difference between the amplitudes of the original waves.

• In the field of optics, a classical wave model based on the Huygens–Fresnel principle is commonly used to explain optical interference.
• In the field of quantum mechanics, another model called the Path Integral formulation, developed by Richard Feynman, is used to explain light interference.

• • • Internal reflection is a common phenomenon not only for visible light but for all types of electromagnetic radiation. For internal reflection to occur, the refractive index of the second medium must be lower than that of the first medium. This means internal reflection happens when light moves from a denser medium, such as water or glass, to a less dense medium, like air, but not when light moves from air to glass or water.
    • In everyday situations, light is typically both refracted and reflected at the boundary between water or glass and air, often due to irregularities on the surface. If the angle at which light strikes this boundary is less than the critical angle, the light is refracted as it crosses into the second medium.
    • When light strikes the boundary exactly at the critical angle, it neither fully reflects nor refracts but travels along the boundary between the two media. However, if the angle of incidence exceeds the critical angle, the light will undergo total internal reflection, meaning no light passes through, and all of it is reflected back into the original medium.
    • The critical angle is the specific angle of incidence, measured with respect to the normal (a line perpendicular to the boundary), above which total internal reflection occurs.
    • In ray diagrams, the normal is an imaginary line drawn perpendicular to the boundary between two media, and the angle of refraction is measured between the refracted ray and the normal. If the boundary is curved, the normal is drawn perpendicular to the curve at the point of incidence.
    • Here are the different outcomes that result from different angles of incidence:
      • At a 0⁰ degree angle of incidence, there is no internal reflection; the light passes straight through the boundary without deviation.
      • As the angle of incidence increases, more and more light is internally reflected and less and less light is refracted at the boundary. This means that less is refracted and so progressively less crosses the boundary into the medium with the lower refractive index.
      • At the critical angle, the light grazes the boundary, and all of it is internally reflected, resulting in no refraction into the second medium.
      • Beyond the critical angle, total internal reflection occurs, and the light is entirely reflected into the first medium.

Example

• • Here is an example. If light travels from water to air where the critical angle is about 48.6 degrees.
    • This means that if light reflects off a fish in a fish tank and then strikes the surface of the water at an angle of less than 48.6 degrees, the angle of incidence determines how much light is internally reflected.
    • If light reflects off a fish in a fish tank and then strikes the water’s surface at an angle of 48.6 degrees or greater, it will experience total internal reflection and no light will pass out of the water and into the air.
  • In reality, light is usually partially refracted and partially reflected because of irregularities in the surface at the boundary. This causes differences in the angle of incidence at different points across the boundary.
• Here are the different outcomes that result from different angles of incidence:
  • At a 0⁰ degree angle of incidence, there is no internal reflection; the light passes straight through the boundary without deviation.
  • As the angle of incidence increases, more and more light is internally reflected and less and less light is refracted at the boundary. This means that less is refracted and so progressively less crosses the boundary into the medium with the lower refractive index.
  • At the critical angle, the light grazes the boundary, and all of it is internally reflected, resulting in no refraction into the second medium.
  • Beyond the critical angle, total internal reflection occurs, and the light is entirely reflected into the first medium.

Example

• Here is an example. If light travels from water to air where the critical angle is about 48.6 degrees.
  • This means that if light reflects off a fish in a fish tank and then strikes the surface of the water at an angle of less than 48.6 degrees, the angle of incidence determines how much light is internally reflected.
  • If light reflects off a fish in a fish tank and then strikes the water’s surface at an angle of 48.6 degrees or greater, it will experience total internal reflection and no light will pass out of the water and into the air.
• In reality, light is usually partially refracted and partially reflected because of irregularities in the surface at the boundary. This causes differences in the angle of incidence at different points across the boundary.

• Interneurons can be classified into different types based on their functions, such as local circuit interneurons and relay interneurons.
  • Local circuit interneurons have short axons and form circuits with nearby neurons to analyse and process information locally.
  • Relay interneurons have long axons and connect circuits of neurons in different regions of the central nervous system, enabling communication and integration of information.
• Interneurons can be further classified into sub-classes based on their neurotransmitter type, morphology, and connectivity.
• Interneurons serve as nodes within neural circuits, enabling communication and integration of sensory and motor information between the peripheral nervous system and the central nervous system.
• The interaction between neurons and interneurons is essential for the proper functioning of the nervous system, enabling the brain to perform complex processes such as perception, cognition, and motor control.