What is Light: Definition and 1000 Discussions

Light or visible light is electromagnetic radiation within the portion of the electromagnetic spectrum that is perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), between the infrared (with longer wavelengths) and the ultraviolet (with shorter wavelengths). This wavelength means a frequency range of roughly 430–750 terahertz (THz).

The primary properties of visible light are intensity, propagation-direction, frequency or wavelength spectrum and polarization. Its speed in a vacuum, 299 792 458 metres a second (m/s), is one of the fundamental constants of nature, as with all types of electromagnetic radiation (EMR), light is found in experimental conditions to always move at this speed in a vacuum.In physics, the term 'light' sometimes refers to electromagnetic radiation of any wavelength, whether visible or not. In this sense, gamma rays, X-rays, microwaves and radio waves are also light. Like all types of electromagnetic radiation, visible light propagates as waves. However, the energy imparted by the waves is absorbed at single locations the way particles are absorbed. The absorbed energy of the electromagnetic waves is called a photon and represents the quanta of light. When a wave of light is transformed and absorbed as a photon, the energy of the wave instantly collapses to a single location and this location is where the photon "arrives". This is what is called the wave function collapse. This dual wave-like and particle-like nature of light is known as the wave–particle duality. The study of light, known as optics, is an important research area in modern physics.
The main source of light on Earth is the Sun. Historically, another important source of light for humans has been fire, from ancient campfires to modern kerosene lamps. With the development of electric lights and power systems, electric lighting has effectively replaced firelight.

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  1. danielhaish

    B Light's Speed & Moving Observer: What Happens?

    for example the car headlight would look faster then c speed. (it speed would be the speed of light +the speed of the car) to observer in front the car. because the light come from the car so the speed of the car would connect with light speed (like when you walk on a train) . so does the light...
  2. J

    Optical Converting non-visible light to visible spectrum for photography

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  3. iVenky

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    This may sound dumb.. We know metals reflect microwave. One way to look at this is considering an eddy current formed inside the metal that cause the reflected EM Flux. When glass reflects light, does something similar happen? Can we have eddy currents (or something analogous) with light since...
  4. I

    B Fundamentals of Lightspeed: Questions & Answers

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  5. G

    I Speed of light for a Rindler observer

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  6. danHa

    Why can't light pass through to the dark side of a window?

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  7. G

    B Speed of Light & Sound: Revolutionary Phenomenon?

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  8. S

    B Light Clock & Length Contraction in GR

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  9. David Lewis

    B Einstein's Train: Light Travel Time Explained

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  10. U

    B The speed of light: Was Einstein's second postulate not so revolutionary?

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  11. Drizy

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  12. S

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  13. T

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  14. danielhaish

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  15. aspodkfpo

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  16. L

    Light interacting with Matter -- Does the wavelength make a difference?

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  17. docnet

    B Speed of Light & Expanding Universe: Does Hubble's Law Violate Rule?

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  18. G

    Question about whether UV light can penetrate a cardboard box

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  19. E

    I Photon Momentum: The Impact of Light on Movement in Space

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  20. S

    Electrical How to make the illumination direction of an overhead LED light adjustable?

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  21. A

    How far from the mirror should the light source be positioned?

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  22. terminator 87

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  23. H

    Please Explain My Polarized Light Experiments

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  24. Mayan Fung

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  25. A

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  26. J

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  27. danielhaish

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  28. L

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  29. Ranku

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  30. X

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  31. greg_rack

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  32. M

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  33. Stephen Bulking

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    Radio wave travels at the speed of light 3x10^8 (m/s) Converting the distance to meter: 1.3 x 3.1x 10^16 = 4.03x10^16m The time it takes in our Earth frame of reference is: 4.03x10^16m/3x10^8 (m/s) = 4.26 years The answer is B But wouldn't the time in light's frame of reference be 0 and it's...
  34. F

    Why does light take a longer path in a rarer medium?

    When traveling into an optically denser medium, the speed of light reduces and as per the principle of least time, light bends towards the normal and takes the shortest path. But why isn't this followed when light passes into a rarer medium? With its speed increased in the rarer medium, if it...
  35. ZapperZ

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  36. S

    B Black Holes & Light: Questions from an Average Joe

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  37. darthgoon

    B Dumb Q, let's discuss :: How would quantum communications work at the speed of light?

    I feel like the emerging idea of quantum communications; is that you can exchange data via the synchronous states of entangled particles, across any distance, in real-time. But, there's also this dilation in the physical world, at the speed of light, according to relativity. So- how would the...
  38. A

    Light as an electromagnetic wave

    light is electromagnetic wave ,so does it also have magnetic and electric field,like all others waves(micro,gama,xray,radio waves etc..)? i never heard that some one talk about light in sense of magnetic and electric field.. if it has ,why than compass don't response to light?
  39. S

    Understanding Reflection of Light on a Concave Mirror

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  40. H

    I Does quantum theory describe light as a wave?

    Hello! I recently had a discussion with a person who's well-read on quantum physics and I was suprised by his claim that "light is in no sense regarded as a wave" in quantum mechanics. His support for this claim was that there are no wave crest or wave trough, there is nothing moving. What...
  41. bob012345

    I Meaning of the Orbital Angular Momentum of Super Chiral Light

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  42. I

    B Can an Object Exceed the Speed of Light?

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  43. L

    Focus or parallelize the light from an LED

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  44. hairey94

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  45. ItsKurious

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    In the above diagram, I have illustrated what is written in the summery. So, if the thickness and refractive index of the material are chosen such that the part of the wave that travels through the slab acquires a path difference of λ/2 and at the right end if I keen another convex lens so as to...
  46. U

    I Big bounce theory in light of the closed Universe finding

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  47. Z

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  48. bhobba

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  49. Elbraido

    B Compressing Light: Reflection in Small Gaps

    Hi thereLet’s consider a photon of wavelength λ is being reflected across of gap d meters. The photon is a reflected back and forth between the same points on two horizontal sheets that reflect 100% of the light. What happens when the distance is smaller than wavelength? Thanks
  50. Lotic7

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