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. platosuniverse

    B Do photons interact with spacetime?

    The way I understand this is that Relativity says space-time is like a field that's affected by the way mass moves through it. Photons are massless so is this why the speed of light is the same in all reference frames?
  2. CristianHernandez

    How to concentrate white light in a very small point?

    Good morning everyone. I have a question for the community. I am doing an optical project in which I need, as the title says, to concentrate white light in a very small point, with a diameter of the order of microns, so that I can pass light through a pinhole. I've been trying it with some lens...
  3. Sorcerer

    I Does infinite one-way speed of light violate p conservation?

    Suppose for the sake of argument someone said the outward speed of light is infinite and the return speed is c/2, creating a two-way speed of c. Wouldn't this violate the conservation of momentum? p = E/c. That means on the way out, the momentum of light would be zero, but on the way back it...
  4. D

    Engineering Combination circuit with Series & Parallel light bulbs

    Homework Statement One 18 watt lamp and two 60-watt light bulb are plugged into a 120V circuit. For either DC or AC, the two bulbs are connected each other in parallel and in series with the lamp in the same circuit. Calculate; i. the current flow through each light ii. the total...
  5. M

    B Light and Reflections in Space

    I am reading Feynman's book on QED and something struck me about light. I know that we can only calculate the probability of where a photon goes. After that I came across how a partial reflection affects light. My question is, is there a place in the universe where there is a great thickness of...
  6. B

    IR camera 1550 nm IR light source

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  7. okandrea

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    Homework Statement Light of wavelenghs 4.80x10^2 nm and 632nm passes through two slits 0.52 mm apart. How far apart are the second-order fringes on a screen 1.6m away? λ₁ = 4.80x10^2 nm = 4.80x10^-7m λ₂ = 6.32x10^-7m d = 0.52mm = 5.2x10^-4m n = 2 L = 1.6 Homework Equations (Maxima/Bright)...
  8. R

    Light Reflection: Understand How Light Interacts with Materials

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  9. K

    CAM photosynthesis saturation / trying to parse a paper

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  10. Sanborn Chase

    B What is the Speed of Light in Dark Matter?

    Does anyone know the speed of light in dark matter?
  11. K

    I Consequences of light moving in a medium

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  12. E

    Why the Frequency of Light Does Not Change Through a Medium

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  13. Johnnyallen

    Why is the speed of light what it is?

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  14. Sorcerer

    I Can Light Create Sound in Air? Exploring Momentum Transfer

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  15. Matthew Bristow

    Can the speed of light be constant and absolute?

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  16. Greg Bernhardt

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  17. Wrichik Basu

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  18. yrjosmiel

    B Effects of a moving mirror on a ray of light

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  19. G

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  20. S

    A General relativity: time dilation and speed of light

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  21. sweet springs

    B Transformation of velocity exceeding light speed

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  22. S

    How do we know that light that comes from the stars is in past?

    So for some reason, from time to time, i always come back to this question and i can't remember that part of the physics while i was studying and most of the explanation are pretty generic. Basically how do we know that light actually travels and not just oscillate and transfer energy when...
  23. Malek

    B Speed of Light & Virtual Particles: Is There a Connection?

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  24. E

    I Does gravity impact the speed of light?

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  25. T

    B What, in essence, distinguishes light from matter?

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  26. starstruck_

    I Relativity and speed of light basics

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  27. P

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  28. flyguille

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  29. Seven of Nine

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  30. R

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  31. ephphatha16

    Find λ of light using a laser beam and a diffraction grating

    1. What is the advantage of using class 2 laser light? I would really appreciate if reference source is also given!Homework Equations Not needed.[/B]The Attempt at a Solution Well, I thought that using monochromatic light would made it easier to measure the distance between fringes. And also...
  32. S

    What will stop me from going at the speed of light?

    Using Newton's 2nd law F=ma, If you provide a constant force of 1mil Newtons then an object will accelerate at 100m/s. Using V = U + AT I can say that (speed of light) 299,792,458 = 0 + 100T thus T = 2997924.58 seconds or I can achieve speed of light in 35 days or so. Why is this not...
  33. sherzadakhan

    Visible light communication modulation schemes?

    visible light contains a range of frequencies i.e not of single frequency, then how visible light is used as a carrier in Visible light communication. Further more is it possible to modulate light (in tera HZ frequency range). what modulation scheme is possible and how it is implemented in VLC...
  34. tma73

    I Speed of light around a (massive) object in space ...(i.e.sun

    Just wondering how a photon reacts when it is affected by gravity of an object in space like a star. Does the gravity actually bend the light/photons? Say you have a series of photons in a perfect line, all travelling, well, at the speed of light toward a massive object(x). The photons in the...
  35. Passionate Eng

    I Measuring Speed of Light: Reference Guide

    I don't know what's the appropriate title for my question but here it is: What's the reference we consider to measure the speed with respect to when we say that as the speed of a moving object approaches the speed of light the time becomes slower in the frame of this moving object.
  36. Sandeep T S

    B Experiments to Measure Speed of Light

    Any one please tell,all experiment attempt to find speed of light
  37. O

    Colored Light or Colored Object?

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  38. T

    Refractive index of light from permittivity

    Homework Statement Given that the relative magnetic permeability and relative electric permeability of water are respectively, ##\mu_{rel}=1## and ##\epsilon=1.769## calculate the following 6 parameters. 1. Index of refraction of water ## n ## 2. speed of electromagnetic waves in water ## c...
  39. M

    Can a photon travel slow enough to be seen by the naked eye

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  40. R

    B Light cone and Cherenkov radiation

    Hi All I read somewhere that at close to C the light emitting from a regular light globe ie diffuse light in all directions, will form a cone. what is the thinking behind this and does anyone have a link where I can read about it ?
  41. P

    Unpolarised light: Orientation of the E-field

    Homework Statement How does this picture represent a unpolarised light?Homework EquationsThe Attempt at a Solution [/B] I thought light waves were perpendicular to the direction of propogation so if it has diagonal components then isn't that not perpendicular to the wave direction If it is...
  42. Clara Chung

    White light interference pattern and its resolution criteria

    Homework Statement Homework EquationsThe Attempt at a Solution I don't know how to do part d. For di, i know that separation of arm d should be small in order to have spatial coherence. For dii, what is the meaningful criterion, can I say the primary max of blue light lies on the first zero...
  43. P

    How does this spectroscope work?

    I build the following spectroscope: However, I'm not sure how it works. I thought that the light entered the slit, diffracted from the CD (because the tracks in them are comparable to the wavelength of visible light) and then we observe the different components in the...
  44. D

    I Electrons Emitting Photons And Ambient Light

    Ambient for lack of a better term... I'm reading "Beyond The Cosmic Landscape". Perhaps out of date, but a very understandable explanation of QED. Am I right to deduce that the air which surrounds us is jam-packed with electrons emitting photons? Thanks...
  45. Georges

    Light Reflection: A 3D Designer's Guide

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  46. MermaidWonders

    MHB Solving for the Speed to Reach a Star 240 Light Years Away

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  47. S

    I Young's double slit: why fringes gets dimmer and inconsistent?

    Hi all! In Young's double slit experiment, there are two things I cannot explain. Any help is appreciated! The first one is why the bright fringes get dimmer as you get further from the central/brightest spot. My theory, after looking in the two books I have, is that each single slit decreases...
  48. P

    How many batteries does it take to light this bulb up?

    Hey guys So lately, I've been interested in circuits. I am good at maths, but my physics isn't as good (hence why i signed up to ask questions). So I have a question. Imagine you have a simple circuit, like below: Simple circuit, 1 bulb and X amount of batteries. So my question is, how many...
  49. S

    Bending light with temperature gradients

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  50. S

    B Semiconductor's sensistivity to light

    Hello guys , I will make this brief.Does anyone know why Semi Conductors are more sensitive to light than other materials? For example Semi Conductors are used in Photovoltaic cells.
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