Light
Light is a form of electromagnetic radiation. It is the natural agent that stimulates sight and makes things easy to see. Light is the opposite of darkness.
Light has a wavelength. Some wavelengths can be seen by the human eye. Others cannot.[1] Many animals can see light. They need it to find food, water, and other things.
Light makes up a part of both the electromagnetic spectrum and radiation given off by stars, like the sun. The light that comes from the sun reaches the Earth and makes it bright. This is called daytime.
The study of light is known as optics. When light hits an opaque object, it makes a shadow. When light hits a transparent object, it passes through it almost completely without making a significant shadow.
Light is a form of electromagnetic radiation that shows properties of both waves and particles. It is a form of energy. Light also keeps the Earth warm. Light exists in tiny energy packets called photons. Each wave has a wavelength or frequency. The human eye sees each wavelength as a different color. Rainbows show the entire spectrum of visible light. The separate colors, moving in from the outer edges, are red, orange, yellow, green, blue, indigo and violet. Other colors can be seen only with special cameras or instruments: Wavelengths up the frequency of red are called infrared, and higher than of violet are called ultraviolet.
The main properties of light are intensity, polarization, phase and orbital angular momentum.
In physics, the term light sometimes means electromagnetic radiation of any wavelength, whether it can be seen or not.[2][3]
Physical properties of light
In a vacuum, the speed of light is 299,792,458 meters per second,[4] or about 186,282 miles per second. This means it takes about 8 minutes for light to reach Earth from the Sun.[5][6]
Light moves in a straight line. The straight line path is often drawn as a ray that moves in one direction. Ray diagrams are used to illustrate light traveling from one place to another. A beam of light can be thought of as a set of light rays.
Objects that block rays of light can create shadows. Opaque objects block all light rays and create a darker shadow. Translucent objects block some rays and scatter others. They create a lighter shadow. Transparent things will allow all light rays to pass straight through them. They have no or very little shadow. Light can easily move through transparent things. When light is not in a vacuum, it travels slower than the speed of light. The slowest speed of light ever recorded was 39 miles per hour.[7]Opaque objects make shadows.
Our eyes react to light. When we see something, we see the light it reflects, or the light it gives off. For example, a lamp gives off light. Everything else in the room the lamp is in reflects the lamp's light. As the viewer, one cannot directly determine where the ray of light came from prior to reflecting off an object. Humans can only see rays of light traveling directly into the eyes.
Every "color" of light has a different wavelength. The shorter the wavelength, the more energy the light has.Red has the longest and purple has the shortest wavelength. The speed of light does not depend on its energy. Going through translucent objects can slow light down by a very small amount.
White light is made up of all the many colors of light added together. When white light shines through a prism, it splits up into different colors, becoming a spectrum. The spectrum contains all of the wavelengths of light that we can see. Red light has the longest wavelength, and violet (purple) light has the shortest.
Light with a wavelength shorter than violet is called ultraviolet light. X-rays and gamma rays are forms of light with even shorter wavelengths than ultraviolet. Light with a wavelength longer than red is called infrared light. Radio waves are a form of electromagnetic radiation with a wavelength longer than infrared light. The microwaves that are used to heat food in a microwave oven are also a form of electromagnetic radiation. Our eyes cannot see those kinds of energy, but there are some cameras that can see them. The various forms of light, both visible and invisible are the electromagnetic spectrum.
When light is refracted in raindrops, a rainbow is made. The raindrop acts like a prism and refracts the light until we can see the colors of the spectrum.
Color
Light and color are forms of analog information. However, electronic cameras and computer displays work with digital information. Electronic cameras or document scanners make a digital version of a color image by separating out the full color image into separate red, green, and blue images. Later, a digital display uses pixels of just those three colors. Computer screens use only these three colors in different brightness levels. The brain puts them together to see all of the other colors in the image.
People think objects "have" a color. The color of objects is because the molecules that make up the object absorb certain light waves, leaving the other light waves to bounce off. The human eye sees the wavelengths of all of the light that was not absorbed. This gives the brain the impression that there is a color.
Light is very hot if it is centered in one direction (usually on focus). That is why if you point a magnifying glass at an ant, it burns the ant. Centered light can also burn metal due to it being so overwhelmingly concentrated.
Light Media
A triangular prism dispersing a beam of white light. The longer wavelengths (red) and the shorter wavelengths (green-blue) are separated.
The electromagnetic spectrum, with the visible portion highlighted. The bottom graph (Visible spectrum) is wavelength in units of nanometres (nm).
Beam of sun light inside the cavity of Rocca ill'Abissu at Fondachelli-Fantina, Sicily
Due to refraction, the straw dipped in water appears bent and the ruler scale compressed when viewed from a shallow angle.
Portrait of Pierre Gassendi (1592-1655)
- ChristiaanHuygenspainting
Thomas Young's sketch of a double-slit experiment showing diffraction. Young's experiments supported the theory that light consists of waves.
A sinusoidal electromagnetic wave propagating along the positive z-axis, showing the electric field (blue) and magnetic field (red) vectors.
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References
- ↑ International Commission on Illumination 1987. International Lighting Vocabulary Archived 2010-02-27 at the Wayback Machine. Number 17.4. CIE, 4th edition. ISBN 978-3-900734-07-7.
By the International Lighting Vocabulary, the definition of light is: “Any radiation capable of causing a visual sensation directly.” - ↑ Gregory Hallock Smith (2006), Camera lenses: from box camera to digital, SPIE Press, p. 4, ISBN 9780819460936
- ↑ Narinder Kumar (2008), Comprehensive Physics XII, Laxmi Publications, p. 1416, ISBN 9788170085928
- ↑ Cox, Brian; Cohen, Andrew (2011). Wonders of the Universe. HarperCollins. p. 43. ISBN 9780007395828.
- ↑ "Seeing in the Dark · Astronomy Topics · Light as a Cosmic Time Machine". pbs.org. Retrieved 13 August 2010.
- ↑ "Cosmic Distance Scales - The Solar System". heasarc.gsfc.nasa.gov. Retrieved 13 August 2010.
- ↑ Hau L.V. et al 1999. Light reduction to 17 metres per second in an ultracold atomic gas. Nature 397, p594/8. [1]