Dictionary tag: P

• Human perception of colour is based on the sensitivity of the eye to the electromagnetic spectrum, specifically the visible spectrum of light that includes spectral colours between red and violet.
• A set of primary colours is a set of pigments or coloured lights that can be combined in varying amounts to create a wide range of colours.
• Different sets of primary colours are used for additive colour mixing (of light) and subtractive colour mixing (of pigments).
• Colour models such as RGB, CMY and RYB use different sets of primary colours.
• The process of combining colours to produce other colours is used in applications such as electronic displays and colour printing to create a range of colours that can be perceived by humans.
• Additive and subtractive colour models can be used to predict how wavelengths of visible light or pigments interact with each other.
• RGB colour is a technology used to reproduce colour in ways that match human perception.
• The primary colours used in colour-spaces such as CIELAB, NCS, Adobe RGB (1998), and sRGB are determined by an extensive investigation of the relationship between visible light and human colour vision.

• Power measures how quickly energy is used or generated.
• The used to measure power is P = W/t, where P is power, W is work, and t is time.
• Energy is measured in joules, while power is measured in watts or joules per second.

Here is an example:

• If you lift a 10 kg object one meter in two seconds, the work done is W = Fd = mg*d = 10 kg * 9.81 m/s^2 * 1 m = 98.1 J, where F is the force applied, d is the distance lifted, m is the mass of the object, and g is the acceleration due to gravity.
• The power used to lift the object is then P = W/t = 98.1 J / 2 s = 49.05 W.
• This means that you are transferring energy to the object at a rate of 49.05 J/s, or 49.05 watts.

• Horsepower is another unit of power where one horsepower is equal to 745.7 watts
• One horsepower is equivalent to the power required to lift 550 pounds of weight at a rate of one foot per second.
• James Watt, a Scottish engineer, adopted the term in the late 18th century to compare the output of steam engines with the power of draft horses. It was later expanded to include the output power of other types of piston engines, turbines, electric motors, and other machinery.Please

• In the context of light, polychromatic refers to light that contains multiple wavelengths or colours
• White light which is a combination of all colours in the visible spectrum is polychromatic.
• The vibrant colours of a sunset or the many shades of green in a forest are polychromatic.
• The opposite of polychromatic is monochromatic, which refers to something that is composed of only one colour or hue.

• Potential energy can be converted into other forms of energy, such as kinetic energy, which is the energy of motion.
• Potential energy is not currently being used, but it has the potential to do work in the future.
• Potential energy comes in different forms such as:
  • Chemical potential energy is the energy stored in the bonds between atoms and molecules in a substance, such as the energy stored in food.
  • Elastic potential energy is the energy stored in an object when it is compressed or stretched, such as a spring.
  • Electric potential energy is the energy stored in an electric field due to the position of charged particles, such as the energy stored in a battery.
  • Gravitational potential energy is the energy an object has due to its position in a gravitational field, such as a ball held up in the air.

• The Planck constant is a measure of the smallest possible amount of energy that can be carried by a single quantum of electromagnetic radiation (a photon).
• The Planck constant is also related to the wavelength of a photon by the equation E = hf, where E is the energy of the photon, f is its frequency, and h is the Planck constant.
• The equation, energy (E) = Planck constant (h) x frequency (f), allows the quantity of energy associated with electromagnetic radiation to be calculated if the frequency is known.
• The Planck constant is used extensively in modern physics, particularly in the fields of quantum mechanics, atomic physics, and condensed matter physics.
• It plays a crucial role in determining the energy levels of atoms and molecules, as well as the behaviour of subatomic particles such as electrons and photons.
• The value of the Planck constant is approximately 6.626 x 10^-34 joule-seconds (Js).

• The higher the photon’s frequency, the higher its energy. Equivalently, the shorter the photon’s wavelength, the higher its energy.
• Photon energy is determined solely by the photon’s frequency and wavelength.
• Other factors, such as the intensity of the radiation, do not affect the energy of individual photons. In other words, two photons of light with the same frequency have the same energy, regardless of their source. even if one was emitted from a wax candle and the other from the Sun.
• The electronvolt (eV) and the joule are the units commonly used to express the energy of photons.
• The energy of a photon can also be expressed:
  • In terms of its wavelength or frequency using Planck’s constant (h). E = hν = hc/λ, where E is the energy, ν is the frequency, λ is the wavelength, and c is the speed of light.
  • As a quantum of electromagnetic radiation.

• The standard photopic curve used in the CIE 1931 colour space is based on the photopic luminosity function, which describes the average sensitivity of the human eye to different wavelengths of light under normal, bright lighting conditions.
• A photopic luminosity function is a mathematical function used to derive the photopic curve from the CIE 1931 colour space.
• The CIE 1931 colour space is a standardized system for describing colours based on human colour perception. It was developed by the International Commission on Illumination (CIE) in 1931 and is still widely used today.
• In low light conditions, the sensitivity of the human eye to light changes, and the scotopic curve is used to describe the response of the eye to light.

• Measuring human visual responses to light is not straightforward because the eye is a complex and intricate organ.
• An internationally recognized system of measurements, known as the CIE system, was established in 1931 by the Commission Internationale de l’Eclairage (CIE).
• The Commission established the typical spectral responsiveness of the human eye to wavelengths across the visible spectrum and compiled the data into a photopic curve.
• The CIE’s photopic curve shows that, in bright light, the strongest response of the human eye is to the colour green with less sensitivity towards the spectral extremes, red and violet.
• A second set of measurements of the typical responsiveness of the human eye to wavelengths across the visible spectrum at low levels of light, (where determining colour differences is difficult), resulted in data compiled into the scotopic curve.
• Having defined the spectral response of the human eye, the CIE sought a standard light source to measure luminous intensity.
• Luminous intensity is a measure of how bright a light source appears to the human eye, and it is typically used to describe the brightness of light sources such as light bulbs, lamps, and LEDs.
• The first source, which led to the development of the terms footcandle and candlepower, was a specific type of candle commonly used in the 18th and 19th centuries before standardised artificial light sources were developed.
• In 1948, the standard was redefined for better repeatability. It was named the International Candle and defined as the amount of light emitted from a given quantity of melting platinum.

• Electromagnetic radiation propagates through space, carrying electromagnetic energy in the form of electromagnetic waves.
• The propagation of electromagnetic radiation through space is sometimes described in terms of photons rather than waves.
• Photons are particles that are sometimes used to explain the behaviour of electromagnetic waves.
• Propagation of electromagnetic waves can occur in a vacuum as well as through different media. Other wave types such as sound waves cannot propagate through a vacuum and require a transmission medium.
• All forms of electromagnetic radiation propagate in similar ways whether they are radio waves, microwaves, infrared, (visible) light, ultraviolet, X-rays, and gamma rays.
• Light waves across the electromagnetic spectrum behave in similar ways. When a light wave encounters an object, it may be transmitted, reflected, absorbed, refracted, polarized, diffracted, or scattered depending on the composition of the object and the wavelength of the light.
• The speed of electromagnetic waves as they propagate through a vacuum is constant. This constant speed is a fundamental principle of physics.

• A pixel, also known as a picture element, is a physical point in a digital image and the smallest addressable element of a display device.
• In the editing process, a pixel is the smallest controllable element of a digital image.
• Many digital displays, including LCD screens, contain LEDs arranged in a grid pattern and emit light when an electrical current is passed through them, allowing them to display different colours and brightness levels.
• OLED displays use a different technology that uses organic compounds that emit light when an electrical current is passed through them.
• The RGB colour model is commonly used for still images displayed on digital screens, such as computer monitors and televisions.
• In the RGB colour model, each pixel is composed of three subpixels that control the red, green, and blue colour channels.
• By varying the light emitted by an LED, every pixel can display a wide range of colours and shades, allowing for the creation of highly detailed and vibrant images on screen.
• The resolution of a digital screen, or the number of pixels it can display, is an important factor in determining its overall image quality and sharpness.
• Higher-resolution screens can display more pixels per inch (PPI), resulting in smoother, more detailed images with less visible pixelation.
• Newer display technologies may use variations of the RGB colour model to display still images, such as RGBW (Red, Green, Blue, White) or RGBY (Red, Green, Blue, Yellow).

• A photon is an elementary particle, a quantum of light , so a photon is the smallest quantity (quantum, plural quanta) into which light can be divided.
• Light can exhibit both wave-like and particle-like behaviour. So at times light behaves as a wave, at others, it behaves like both waves and photons, and at others just as particles.
• Photons are the force carriers of radiant energy (electromagnetic radiation).
• Photons can be emitted or absorbed by charged particles such as electrons and protons.
• Photons carry energy and momentum, which are proportional to their frequency and wavelength respectively.
• Photons can interact with matter through processes such as scattering, absorption, and emission.
• Photons can be polarized, meaning their electric and magnetic fields oscillate in a particular direction.

• Photons do not have mass and propagate at the speed of light in a vacuum. We can break this statement down as follows:
  • According to the theory of relativity, any object that has mass requires energy to accelerate.
  • The amount of energy required to accelerate an object increases as the object’s mass increases.
  • Photons are unique in that they have zero rest mass, which means they do not require any energy to be accelerated.
  • As a result, they always move at the speed of light in a vacuum.
  • So photons always travel at the speed of light and never slow down or come to a stop.

• The fact that photons have zero rest mass has important implications for the behaviour of light and electromagnetic radiation in general. It means that:
  • The energy of a photon is directly proportional to its frequency (as described by Planck’s law).
  • The momentum of a photon is proportional to its frequency and direction of travel (as described by Einstein’s theory of relativity).

• Visual information coming from the eyes goes through the lateral geniculate nucleus within the thalamus and then continues towards the point where it enters the brain. At the point where the visual cortex receives sensory inputs is also a point where there is a vast expansion of the number of neurons
• Both cerebral hemispheres contain a visual cortex. The visual cortex in the left hemisphere receives signals from the right visual field, and the visual cortex in the right hemisphere receives signals from the left visual field.