Light waves are another name for electromagnetic radiation. They consist of self-propagating waves of electric and magnetic fields that travel through space. This wave motion transports energy but doesn’t involve the movement of physical matter itself.
- The distance between peaks in a wave is known as the wavelength. Different wavelengths correspond to specific portions of the electromagnetic spectrum.
- The visible spectrum, which our eyes can detect, occupies a limited range within this spectrum.
- Within the visible spectrum, red light has a longer wavelength than violet light.
- Light exhibits wave-particle duality. This means that light can demonstrate properties of both waves and particles (photons) depending on the experimental setup.
- Light waves interact with matter through various mechanisms. These include reflection (e.g., bouncing off a mirror), absorption (e.g., conversion of light energy to heat by dark clothing), and refraction (e.g., bending of light as it passes through a prism).
A light-emitting diode (LED) is a semiconductor device that emits light when an electric current flows through it. Electroluminescence is the process where this happens: voltage applied to the semiconductor makes electrons flow across a junction, releasing energy as light.
- Semiconductors, typically made from gallium nitride, are solid-state materials with unique properties that allow them to emit light at specific wavelengths, determining the perceived colour.
- LEDs typically emit one colour with a narrow range of wavelengths.
- Multicoloured LEDs combine three diodes emitting the RGB primary colours – red, green, and blue light.
- By adjusting the relative brightness of the primary colours, a vast array of colours can be created.
- Combining the three primary colours in equal proportions produces white light.
The LMS colour space is a practical implementation of trichromatic colour theory that enables the full range of human observable colours to be specified by measuring the responsiveness of the L, M and S cones to each wavelength of light within the visible spectrum.
- The LMS colour space was one of the first systematic demonstrations of trichromatic colour theory.
- LMS describes how the three types of cone photoreceptors (L, M and S cone types) in a human eye respond given any particular light stimuli.
- The method used in the development of the LMS colour space produced a generalized representation of human colour perception.
- The underlying principle was that any colour can be described in physiological terms by measuring the response of the L, M and S cone cells in the human eye’s retina to different wavelengths of light.
- The initial source of data for the LMS colour space was taken from experiments that compared the spectral sensitivity of subjects with normal sensitivity with other subjects experiencing forms of colour blindness.
- A more recent technique used to collect data for LMS belongs to the field of visual psychophysics and is known as heterochromatic flicker photometry. It provides extensive and accurate spectral sensitivity data obtained from cellular material removed from the eye.
- The LMS colour space describes human observable colours using three parameters, known as tristimulus colour values, each component of which corresponds with the response of the L, M and S cone types.
There are several particularly noticeable things to see when looking closely at rainbows:
- The arcs of spectral colours curving across the sky with red on the outside and violet on the inside, this is a primary rainbow. The arcs appear between the angles of approx. 40.7° and 42.4° from the centre (anti-solar point) as seen from the point of view of an observer.
- There may be another rainbow, just outside the primary bow with violet on the outside and red on the inside, this is the secondary rainbow. The arcs appear between the angles of approx. 50.4° and 53.4° from its centre as seen from the point of view of an observer.
- Faint supernumerary bows often appear just inside a primary rainbow and form shimmering arcs of purples and cyan-greens. These bands appear at an angle of approx. 39° to 40° from the centre so just inside the violet arc of the primary bow.
- The remaining area inside a rainbow from its centre out to approx. 39° often appears lighter or brighter in comparison to the sky outside the rainbow. There are three main causes:
- Light strikes multiple droplets in succession and randomly scatters in all directions.
- Small amounts of light of all wavelengths are deflected towards the centre and combine to produce the appearance of weak white light.
- Almost no light is deflected to the area outside a rainbow.
- When a secondary rainbow appears, the area between the two often appears to be darker in tone than any other area of the sky. This is called Alexander’s band. The effect is the result of rays being deflected away from this area as primary and secondary bows form.
The weather, season and time of day are all important if you hope to see an atmospheric rainbow.
- The best rainbows appear in the morning and evening when the Sun is strong but low in the sky.
- Northern and southern latitudes away from the equator are good for rainbows because the Sun is lower at its zenith.
- Mountains and coastal areas can create ideal conditions because as air sweeps over them, it cools, condenses and falls as rain.
- Rainbows are rare in areas with little or no rainfall such as dry, desert conditions with few clouds.
- Too much cloud is not good because it blocks direct sunlight.
- Winter is not necessarily the best season because the light is weaker and there can be excessive cloud.
- Rainbows are less common around midday because the higher the Sun is in the sky the lower the rainbow.
- If the Sun is too high, then by the time the raindrops are in the right position to form part of a rainbow they are lost in the landscape.
Luminance is a measure of the perceived brightness of light reaching the human eye, considering both the amount of light emitted, transmitted, or reflected from a surface and the human eye’s sensitivity to different wavelengths of light. In simpler terms, luminance quantifies how bright a surface appears to the human eye under given conditions.
- Luminance focuses on luminous intensity as experienced by an observer. While luminous intensity refers to the amount of light emitted in a specific direction by a source, luminance measures how bright a surface appears based on both the light it reflects or emits and the observer’s perspective.
- For example, imagine a lamp shining in a dark room. While the lamp emits a certain amount of total light (luminous flux), the actual brightness (luminance) of a wall the light falls on depends on several factors:
- Reflectivity: The wall reflects only a portion of the light that hits it. The amount of light reflected affects the overall luminance the human eye perceives.
- Spectral Sensitivity: Human eyes are more sensitive to green light than to blue or red light. Therefore, even if two surfaces receive the same light, a green surface will appear brighter than a blue one because of the eye’s higher sensitivity to green wavelengths.
- Measuring luminance helps us understand real-world scenarios:
- Moonlight: While not very luminous (doesn’t emit much light), moonlight creates a certain luminance on sand in a desert, allowing us to see our surroundings.
- Road safety: Streetlights need specific luminance levels to ensure safe visibility for drivers, considering both the total light emitted and road reflectivity.
- Book reading: The luminance of a book under a lamp determines how comfortable and clear the text appears to your eyes.
Luminosity refers to the total amount of light being given off by a source, regardless of the direction.
- The luminosity of a light source depends on the total amount of power it consumes and the efficiency with which it converts that power into visible light.
- Luminosity is a measurable quantity and is often used as an objective measure of the total amount of light being emitted by a source.
- So luminosity refers to the total amount of light emitted by a source per unit time and is often measured in units like watts or lumens. It indicates the raw power of the light source, regardless of its colour or direction.
- The maximum luminosity of a display device corresponds with the brightest white it can reproduce and is called the white point.
- The black point corresponds with the minimum luminosity of a device, so corresponds with the device being turned off.
- The contrast ratio of the maximum and minimum luminosity of a television or computer screen is typically more than 280:1.
Mass is a fundamental property of matter and is defined as the amount of matter present in an object and is independent of external factors such as location or the presence of gravitational fields.
- A large object made of a given material has greater mass than a small object made of the same material because it contains more matter.
- Mass is not the same as weight because weight varies with gravity while mass remains constant.
- Weight is the force exerted on an object due to gravity:
- An object of a known mass weighs more on earth than on the moon due to differences in gravity.
A material is the substances or matter that a thing is made of.
- Material is a broad term for a chemical substance or mixture of substances that constitute an object.
- Materials are composed of atoms and molecules arranged in various configurations, which determine their properties and behaviour.
- Materials can have natural origins, such as wood, stone, and metals, or synthetic origins, such as polymers and ceramics. Materials can also be classified based on whether they are organic or inorganic.
- The properties of a material depend on its structure at different length scales, from atomic to macroscopic scales.
- Materials can be classified based on physical and chemical properties such as mechanical, thermal, electrical, and magnetic properties.
- Materials are studied in materials science, a branch of engineering that focuses on structure, properties, and processing.
A material thing is made up of matter, which includes all substances that have mass and occupy space. Matter is composed of atoms and molecules, and its properties include mass, volume, and density.
- Material things include objects, living organisms, and even intangible things such as sound or light, which are considered material because they are made up of particles.
- An attribute of an object is called a property if it can be measured or observed through the senses (e.g. its colour, size, weight, odour, taste, and location).
- Objects can be identified or characterized through their properties, which manifest themselves in various ways.
- These manifestations often exhibit consistent patterns, indicating that there is a underlying cause or mechanism that governs the properties:
- For example when different metals are mixed to form alloys, such as bronze or steel, the resulting material often exhibits a consistent relationship between its composition (the types and proportions of metals) and its density. So increasing the percentage of a denser metal in an alloy tends to increase its overall density.
