Dictionary tag: P-Q-R-S-T

The retinal pigment epithelium (RPE) is a layer of pigmented cells located between the retina and the choroid of the human eye that supports the photoreceptor cells (rods and cones).
<ul>
<li>The RPE plays a critical role in providing nutrients, removing waste products, and regenerating visual pigments needed for photoreceptor function.</li>
<li>The RPE is firmly attached to the underlying Bruch’s membrane of the choroid on one side, but less firmly connected to the photoreceptor cells of the retina on the other. This weaker attachment can contribute to retinal detachment.</li>
<li>The choroid is a vascular layer rich in blood vessels and connective tissue that lies between the retina and the sclera. It provides oxygen and nutrients to the outer layers of the retina and parts of the sclera, supporting the function of the retinal pigment epithelium and photoreceptors.</li>
</ul>

• 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).

• 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).

• In the context of light, polychromatic refers to light that contains multiple wavelengths, each corresponding to a different colour in the visible spectrum. For example, white light is polychromatic because it is a combination of all the colours (wavelengths) within the visible spectrum.
• Sunlight is a perfect example of polychromatic light, as it contains all the visible wavelengths that combine to form white light. When this light interacts with the atmosphere or objects, it can be scattered or reflected to produce a range of colours.
• In a rainbow, polychromatic sunlight is dispersed through water droplets, splitting into its constituent colours across the visible spectrum, producing the familiar bands of red, orange, yellow, green, blue, indigo, and violet.
• The opposite of polychromatic is monochromatic, which refers to light composed of only one wavelength, producing a single 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.

• Power measures how quickly energy is used or generated.
• In physics, power is defined as the rate at which work is done or the rate at which energy is transferred or converted. It quantifies how quickly energy is used or generated within a system.
• When work is done on an object, energy is transferred to or from it, and power measures how rapidly this transfer occurs.
• Essentially, power describes the efficiency or speed of energy conversion processes.
• The equation 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.

• 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 coloured lights or pigments 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.
• An important point to note is that while there are several different sets of primary colours, there is no universally agreed upon set of primary colours. Different colour models and industries use different sets of primary colours. The specific hue that correspond with each primary colour can also vary depending on the colour model , colour space and mediums concerned.

•  A primary rainbow appears when sunlight is refracted as it enters raindrops, reflects once off the opposite interior surface, is refracted again as it escapes back into the air, and then travels towards an observer.
• The colours in a primary rainbow are always arranged with red on the outside of the bow and violet on the inside.
• The outside (red) edge of a primary rainbow forms an angle of approx. 42.4⁰ from its centre, as seen from the point of view of the observer. The inside (violet) edge forms at an angle of approx. 40.7⁰.
• To get a sense of where the centre of a rainbow might be, imagine extending the curve of a rainbow to form a circle.
• If your shadow is visible as you look at a rainbow its centre is aligned with your head.
• A primary rainbow is only visible when the altitude of the sun is less than 42.4°.
• Primary bows appear much brighter than secondary bows and so are easier to see.
• The curtain of rain on which sunlight falls is not always large enough or in the right place to produce both primary and secondary bows.

Remember that:

• The centre of a rainbow is always on an imaginary straight line (the axis of the rainbow) that starts at the centre of the Sun behind you, passes through the back of your head, out through your eyes and extends in a straight line into the distance.
• The centre-point of a rainbow is sometimes called the anti-solar point. ‘Anti’, because it is opposite the Sun with respect to the observer.
• The axis of a rainbow is an imaginary line passing through the light source, the eyes of an observer and the centre-point of the bow.
• The space between a primary and secondary rainbow is called Alexander’s band.

• 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.