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. astrodummy

    I Solving the Flickering Light Paradox

    This post builds on a previously closed thread here: https://www.physicsforums.com/threads/lightbulb-paradox.141191/#post-1145674 I will not describe the problem here without the copyright owner's permission. I, like the OP in the original thread, am keen to see if anyone else who has this text...
  2. Atstill77

    B Why do you need a light clock?

    what is wrong with the thought experiment below?! If I consider a light beam going straight towards an observer in motion it makes it look the moving clock ticks faster, but if you use a light clock it makes it look like the clock turns slower. What am I missing here? “Two observers convene in...
  3. M

    Can a Resistor Simulate a Light Bulb Load for LED Bulb Installation?

    Hello All, I have a snowmobile that uses a shunt type regulator to control voltage. They require a constant load (headlights) to work proper. The 2 stock bulbs are 55 watts on low with the system running 12v. Racers install another set of hand warmers, wired to high, and tape them to the...
  4. E

    Waveform produced by a collapsing magnetic field

    A thought experiment: A electron is moving in a straight line at velocity v. It instantly stops dead. It doesn't move another femtometer. Obviously its magnetic field collapses and produces light. What is the waveform of the light produced? Is it something like this...
  5. F

    B Why do we know all photons have same speed as the speed of light?

    Why do we know the velocity of all photons are the same as the velocity of light?Can we deduce this or we must have experiment test?What is the experiment test?
  6. person123

    Optical Compact Device For Measuring Speed of Light

    Hi. This is an idea which I just happened to think of, and I was curious if it would be at all feasible. Here's a quick sketch I drew: The two curved mirrors should have a laser attached on one end and a video camera attached on the other. The laser would be tilted very slightly above...
  7. B

    B Did the Big Bang expand faster than light?

    I read that our telescopes pointing in any direction show light coming from the early days of the Big Bang (like 13.7 billion years ago). So did the expansion of the Big Bang to "fill" the universe happen faster than the speed of light?
  8. R

    Can heavy objects be used to modulate light waves with small amplitude?

    Question: Can heavy objects be used to operate light waves with smaller amplitude, or waves with high amplitude be used to operate objects with small mass? This is a diagram I drew that illustrates my point. Heavy objects are meant to be celestial bodies, and the light source generates a beam...
  9. Nantes

    What causes this curious light phenomenon? (video inside)

    Here is the video: What is the physical explanation to why I see the light make patterns strikingly similar to magnetic lines on that spinning electric fan, depending on how I move the point of view? (the phenomenon happens equally to the naked eye and to the camera).
  10. morrobay

    B Light Clock: Why Aren't Delta T and 2D/c Equal?

    With this light clock delta t' in S' would be 2 Ls proper time. Since time measurements were done in same location delta x'=0 . Then from stationary S frame delta t = gamma delta t' . With gamma equal to 1.25 then that is 2.5 Ls. However the path the light takes as calculated from S frame in...
  11. G

    A Can Data Travel Faster Than Light Computation?

    Hi bear with me I have a conundrum I want to ask you. If data traveled many times the speed of light could the results of decrypted cypher message be computed quicker than any system we currently have? For instance if we sent a burst of data at many times the speed of light across the solar...
  12. M

    I Photon behaviour across EH threshold

    Physics is not my area of expertise. That being said, philosophy of science is, but I'm not here to discuss philosophy. I recently found myself trying to imagine how light behaves once it crosses the event horizon of a black hole. Presumably, between the event horizon and the singularity...
  13. T

    I Speed of Sound & Light: Is it True?

    Are the following statements true? --------------------------------------------------------------- If a speaker and a listener are stationary with respect to the air, a listener will measure the speed of sound at about 343 m/s. Since the speed of a sound wave in air is dependent only on the...
  14. jaketodd

    B E & B Field Oscillation with Time Dilation: Explained

    If time for something approaching the speed of light will slow down and approach zero, then how is it that light itself can exhibit changes - the oscillation of the E and B fields? Is it because those fields are perpendicular to the direction of light travel? If so, then something other than...
  15. N

    How Do I Perform a Galilei and Lorentz Transform?

    c^2 * t^2 = x^2 + y^2 + z^2 = r^2
  16. richengle

    How does the speed of light change when it enters glass?

    its has been said that light slows down in glass, but some talk about phase velocity and group velocity... then there is the velocity of the photons, which some say is always c. What are those speeds [Vgroup, Vphase, Vphoton, Vlight] in glass, and what do they mean?
  17. H

    I Does photon self energy contribute to the speed of light?

    Hi Pf I read that in the light propagator there are loops of electrons. What would be the consequences if we could switch them off (or neglect them)? would it modify the speed of the photons?
  18. M

    B Do Photons Decay or Have a Half-Life?

    I don't think this is what is observed, so where is my understanding wrong?
  19. richengle

    Can Light Travel Faster Than the Speed of Light in Glass?

    i made a simulation of waves in glass and in air. my simulation has led me to believe that if light begins in glass, it can travel to the air, and then move much faster, and then go back into the glass at a point further down. Could this happen in real life? here is the video of my...
  20. Byron Forbes

    Circuits and the Speed of Light (2)

    https://www.allaboutcircuits.com/textbook/alternating-current/chpt-14/circuits-and-the-speed-of-light/ Sorry, my other post must have been too brief. In another thread shut down by Dale, this very site (allaboutcircuits) was used by him to reference wave guides. This particular page is 100%...
  21. Byron Forbes

    Circuits and the Speed of Light

    https://www.allaboutcircuits.com/textbook/alternating-current/chpt-14/circuits-and-the-speed-of-light/Hmmmmm. :))
  22. danielhaish

    B Why can't energy or information go faster than light?

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  23. K

    I Is it feasible to measure one way speed of light this way?

    Theoretical experiment for measuring one way speed of light From 1905 to this day we have not experimentally measured the one way speed of light between a source to the detector only the roundtrip from the source to the detector and back again. We just assume that the speed of light is the...
  24. A

    Problem Involving Refraction Of Light

    N1sintheta
  25. T

    Why doesn't a light bulb draw 100 amps?

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  26. adafus

    B Are time and light at the same speed?

    I'm probably am wrong in this but i guess you guys could light me in this one
  27. amjad-sh

    Diffraction of light and conservation of energy.

    Suppose that we shined a source of light on a wall with infinitismal small opening. As the opening is infinitismly small, only one ray of light will pass through the opening ( suppose it has an intensity ##I_0##) and this ray of light will diffract into an infinite number of light rays with the...
  28. A

    I Verifying Speed of Light Varies by Direction

    Note, it possible to determine if the speed of light varies depending on direction, with the use of a single clock. Simply send a beam and reflect it back, but on the return trip introduce a medium to slow the beam. Three cases, T1, T2, T3 are total travel times. T1: assume light is the...
  29. Daveopg

    B Light Behaviour at High Speed: Explained

    First, I am way out of my field of understanding here so please keep it simple. I watched some videos on E=MC2 which led to how light reacts differently than matter at high speeds causing time to slow down when moving fast. My question is, if I was to shine a flashlight perpendicular (90...
  30. T

    I Can Light Curve Spacetime?

    Hello there.The question is as stated:does light curve spacetime?We know that bodies with mass do curve spacetime but does a massless particle or wave like light curve spacetime?Thank you.
  31. S

    I The Speed of Light: Can it Travel in Opposite Directions?

    Is it really possible for light to travel at different speed in opposite directions? This video seems to say that this is assumed as a fact (with of course Einstein being the one to make the assumption), and that since it is impossible to measure the speed of light in one direction, we really...
  32. xpell

    B How small is the smallest hole that would still allow you to see through it?

    This is not for a homework or anything, I'm just a curious person who was wondering... that: how small is the smallest hole in an opaque material that would still allow to see the "world beyond it", using your eye or any kind of existing or prospective technology? (English is not my mother...
  33. R

    B Why is the speed of light constant?

    Visualize the following scenario. A star such as the sun, and a planet such as Earth are located in a solar system. A photon travels from sun (x0) to Earth (x1), and a satellite is in the midway observing the photon. This is relates to Einstein's special relativity. I ask this since it take...
  34. R

    Two light sources are a distance D apart

    I let E1 be the event where source 1 emits the photon and E2 for the second source with the respective coordinates in O as $(x_1, t_1$) and $(x_2,t_2)$ such that $t_2=t_1 \because$ simultaneous and $x_2-x_1 =D$. Using Lorentz transformation I obtained that in O', $$x'_2-x'_1 = \gamma...
  35. RandyD123

    B Measuring the One Way Speed of Light

    Does this video even make sense? And if so, is it right or wrong?
  36. Abdalrahman Azab

    Question about two-stage light gas guns

    Hello ... I had a question about the capabilities of the two-stage light gas guns which are used to study the effect of small meteors on spacecraft shields ... is it theoretically possible to achieve velocity above 7 km / s for a projectile of about 16 grams weight and 15 mm diameter using a...
  37. Y

    Battery Operated Ceiling Light and Light Switch?

    Does anyone know of any products that would allow me to install a battery operated light switch and ceiling light? This would save me a lot over tearing down all the sheetrock, replacing sheetrock, painting, etc.. I'm thinking a switch with a faceplate. A simple button cell would go behind the...
  38. S

    Does a material change the direction of polarization of light?

    The polarization referes to the direction of the electric field of a light wave which is to be one direction. The unpolarized light interacts with a material in various ways such as reflection, scattering, refraction. and each of which can transform an unpolarized light into a vertically or...
  39. F

    Angle of reflection of a beam light on a moving mirror

    Before to open this topic, I found this there. It's quite similar, if not the same, but I'm a little confused, so I'm here. The situation is represented in this image. From optical geometry, ##\theta_{incident} = \theta_{reflected}## The four-momentum in ##S'## is the following one...
  40. danielhaish

    I Would light keep on a straight path after it bends?

    I am asking this question because I dint find a source that explain why the light go in straits line Let say that a light go near very big mass that bend it ,and then the light escape it . would the light change his angle after it band like that the red part is the range where the mass effect...
  41. A

    B Kugelblitz at the speed of light misconception

    I have a question regarding Kugelblitz black holes. I know that they are purely theoretical, and I am perfectly fine with the matter-energy equivalence so I have no problem in assuming that concentrating a sufficient amount of energy in a certain radius might generate an event horizon. However...
  42. Nick tringali

    B Near the speed of light time slows down but not at everyday speeds?

    Hello, just some general questions I have been thinking about 1. So as you approach the speed of light time slows down, but why not at everyday speeds like walking around? I know people are going to say time slowing down only matters at extremely high speeds. However, shouldn't time dilation...
  43. A

    Light refracting and reflecting in a drop of water to form a rainbow

    I drew the red and green tangent lines and I found that the angles in blue are equal to theta 1. Also , as the BCD triangle is equilateral, theta 2 = 30. With this I can calculate the side of this equilateral triangle as a function of the radius R of the circumference. After that, I can't go on...
  44. J

    What is the Wavelength of Corona Discharge in High Voltage Testing?

    Hi, I am doing high voltage testing (20-35 kV or more) where I am trying to capture corona discharge. My DUT is in a dark test room with no light. I am using a Canon EOS T7 (2000D) DSLR to capture the corona. What I would like to know is the wavelength of the corona so I can do further...
  45. T

    B How does a wave collapse not violate the speed of light?

    If you want to detect a particle in the 2 slit experiment on a detector. And we state that the electron is traveling as a wave so there is a wave front...that must mean that the wave front hits the detector at the same time in more than one place where there is constructive interference. But...
  46. 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...
  47. J

    Optical Converting non-visible light to visible spectrum for photography

    I am trying to capture corona discharge on metal conductors. I am using a Canon DSLR and it works pretty good. I think there may be light from outside of the visible spectrum being emitted during these discharge events so I would like to see if it's possible to capture this light and actually...
  48. iVenky

    Can light cause eddy currents?

    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...
  49. I

    B Fundamentals of Lightspeed: Questions & Answers

    Hey there, I'm aware this is a bit of a stupid question, and I think that I understand the principle fundamentally, however, my intuition is still having a little trouble catching up, and I'm trying to figure out if it is because of an important detail that I have missed/misinterpreted. I think...
  50. G

    I Speed of light for a Rindler observer

    I've read that speed of light for a Rindler observer is not constant. I wasn't sure why and I tried to do this as an exercise for myself, I think I came with the correct answer, but I'm not sure, is the following argument correct? Let's describe the (flat) space-time using Rindler coordinates...
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