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Visible light: origin and characteristics
It is present all around us, allowing us to see, to perceivecolors, and to understand our environment. Yet behind this apparent simplicity lie intriguing questions that date back to the beginnings of science. Where does light come from, and what are the fundamental differences between natural light emanating from the sun and the artificial light we create?
(Sunlight)
Light is a form of electromagnetic energy that travels in the form of particles called photons. It is visible to the human eye in the electromagnetic spectrum between wavelengths of approximately 380 to 750 nanometers, which corresponds to colors ranging from violet to red. However, light also exists outside of this wavelength range and encompasses other types of electromagnetic radiation such as x-rays, microwaves, and radio waves.
(Electromagnetic spectrum: Source: lightzoomlumiere.fr)
Radiation can be emitted by a so-called natural source, for example: the sun, stars, fire, fireflies, etc. or it can be emitted by a so-called artificial source, such as a light bulb or LEDs.
(Light bulbs)
(Starry sky of the constellation)
Light carries energy, called light energy. When light energy comes into contact with a surface, it is converted into thermal energy. This is the case of photovoltaic solar panels which transform solar energy, available free of charge, into electricity.
(Solar panel operation)
Light travels at a speed of 300,000 km/sec. This speed in a vacuum is a fundamental constant of the universe denoted “c” and called celerity.
Light travels in a straight line. She cannot bypass an object, rather she will be blocked by it. However, it can be deflected if it passes through a different medium, for example: from air to water, this phenomenon is called refraction.
White light is made up of all the colors of the electromagnetic spectrum. When white light separates into constituent colors we observe the dispersion of light. Light can be broken down into a set of colors using a prism. This results in a spectrum. Note that when light hits an object, it is reflected by it. This is the phenomenon of reflection.
(A prism breaks white light into lines of different colors)
Light is picked up by our visual receptors and to form sharp images on our retina, allowing for clear vision. It will also play a role on the perception of colors, on contrast and brightness, shadows and reliefs.... Light is the main vector of information for our visual system.
Primary and secondary sources of light refer to the different ways in which light is generated and emitted.
Primary sources of light are those which have generated their own light by characterized processes. These sources produce light without requiring incident light. Incident light is light that comes directly from a light source and is reflected off a subject, object or surface. Examples of primary sources include the stars including our own sun. They emit light through nuclear reactions within their core. Some living organisms like fireflies have generated their own light through chemical reactions in their bodies.
(Natural light: Sunset)
Secondary sources of light do not produce their own light themselves, but due to their interaction with a primary source of light or by reflection, diffusion or transmission. We will therefore have the objects illuminated by a primary source; the object reflects incident light in different directions, allowing us to see the object. Some materials absorb light in a range of wavelengths and re-emit it at longer wavelengths, creating a fluorescent glow. Examples of this are fluorescent tubes and some fluorescent inks.
Artificial light: fluorescent neon lights
(Natural light: Mount Fuji, Mountain peak)
Natural light refers to light from natural sources, such as the sun, moon, and sky. Natural light plays a crucial role in our daily environment and has significant effects on biology, human behavior and architecture.
The sun is the main source of natural light. It has a wide range of wavelengths, and is also positioned in the field of visible light. It provides energy for life on our planet. Then we have the moon, although the moon does not produce its own light, it reflects light from the sun and illuminates the night sky. And lastly, the sky itself can act as a diffuse light source, especially during the day when illuminated by sunlight. Bright skies during the day help illuminate outdoor and indoor environments.
Natural light has several important characteristics. First of all, it presents a continuous spectrum, covering all the colors of the rainbow. This property is essential for color perception. Then, the color and intensity of natural light changes throughout the day due to the angle of the sun relative to Earth. Natural light also plays a crucial role in regulating circadian rhythms in living things, including humans. It influences our sleep, wakefulness and hormone production cycles. And finally Natural light provides quality lighting and helps reduce eye fatigue and visual stress compared to artificial lighting.
(Artificial light: Play of light (LED) in an evening)
Artificial light refers to light produced by man-made sources for the purpose of illuminating environments or objects. It is produced using electrical or chemical devices.
The main sources of artificial light include light bulbs, which work by heating a metal filament until light appears. There are also fluorescent tubes which produce light by exciting gases inside the tube, which creates ultraviolet light. This ultraviolet light is then converted to visible light by fluorescent coatings inside the tube. And finally the LEDs which emit light when the electric current passes through a semiconductor junction. LEDs are very energy efficient. They last over time and allow a wide range of colors and intensities.
Artificial light also has several important characteristics: it allows precise control over the intensity, color and direction of light. This allows more flexible adaptation to specific lighting needs. Compared to natural sunlight, artificial light can have a limited spectrum, meaning it may lack certain wavelengths found in natural light. Artificial light source technology has evolved to include dimming, color and color temperature options to meet different needs and scenarios.
(Spectrum and color of light)
To conclude, the essential properties of light include its electromagnetic nature, its constant speed in a vacuum, its ability to propagate in a straight line, its color dispersion in the visible spectrum, and its ability to be reflected, refracted, absorbed when it interacts with matter.
It exists in the form of natural (such as the sun) and artificial (such as human lighting) sources, and its effective management involves considering its different properties to optimize its use in various contexts.
This naturally leads us to think about light intensity, its influence on our daily exposure to light and the implications for our health. Likewise, when the sensor of a camera receives too much light, it alters the shot. A photographer must then control his exposure.
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