What is Light: Definition and 996 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. S

    B How did expansion take 380,000 years to let light travel freely?

    Here's as far as I've gotten in the research (please correct any errors I'm making). The stages of high density had included these periods: • light at high energy was in a cycle of forming into matter, forming back into light, and so on • until things cooled enough for nucleons to form •...
  2. cathrynliz

    I Where Do You Stop? (Do electrons or photons move faster than the speed of light?)

    The title is from a great book by Eric Kraft, who plays around with one's physical-being in elemental terms in an excellent novel. He is very funny. To get down to my question: Do electrons or photons on anything move faster than the speed of light?
  3. L

    I Are photons definitely massless?

    Recently saw this video. "Why No One Knows If Photons Really Are Massless: What if they Aren't?" Arvin Ash He says photons need not be massless, but they must be quite light nonetheless. He separates speed of light from speed of causality. Is it true that we can't know its mass below a...
  4. J

    How Does Flickered Light Affect Our Brain's Perceived Contrast and Brightness of Images?

    Bear with me this will be long, but everything here I feel is necessary to understand the context. I recently used a crt at varying refresh rates and noticed a few things that has made me question how the brain perceives colors on a display. First, is that my crt appears to have more color...
  5. A

    B Gravitrons vs Photons: how come light and gravity exist?

    if matter and anti-matter meet, they annihilate each other. Gravitons are anti-photons and photons are anti-gravitons. They MUST meet in immesurable quantities in our universe. Yet gravity exists, and light exists in our known universe. This denies the annihilation necessity. And annihilation...
  6. Pushoam

    B Scattering of light by dust particles

    Inside a dark room, when light comes through a hole, we see the dust particles as the particles scatter the light. But, when the whole window is open, we don't see the dust particles scattering the light. Why?
  7. CHAIM123

    B Measuring the speed of light on moving systems

    A spaceship moves at half the speed of light, what will be the results of measuring the speed of light for it: in relation to the light source it is approaching and in relation to the light source from which it is moving away?
  8. chemisthypnos

    I Best frequency of radio light to penetrate 1 meter of sea water?

    We are attempting to penetrate 1m of water featuring dissolved ions ( assume salinity of sea water) using radio waves. Aside for using wavelengths of very low frequency radio or below, what would be the best frequency for achieving this task?
  9. T

    I Why is there nothing faster than light?

    Hi guys! Is there a simple explanation to describe why light owns the faster speed at the universe ?
  10. Aromalsp

    I Constancy of Speed of Light: Postulate or Assumption?

    The theory of relativity is based on two key principles: the principle of relativity and the constancy of the speed of light. The constancy of the speed of light is one of the fundamental postulates of the theory of relativity. Which statement is wrong?
  11. S

    B To measure light's frequency, do we interact with each crest + trough?

    How are we interacting with light to measure its frequency? And how'd we learn the distance between its crests and troughs? What sort of interactions are giving us such info?
  12. S

    How to show light from laser pointer is plane polarised?

    The answer key is the light is directed through one polarising filter then filter is rotated and the light changes intensity. I don't understand how that proves that the light is plane polarised. I think if the light is unpolarised, the intensity will also change when it passes through...
  13. L

    B Does light propagate as a wave of little bullets?

    According to Einstein light would be a particle and a wave. So I infer that it propagates in vacuum in form of waves of little bullets (particles). This explanation is very insuficient. So tell me how do waves increase in size since it's made of little bullets (particles)... a wave gets...
  14. S

    I Optics: infinite light source illusion question - can you help?

    Hi. I’m trying to solve an optics problem and really struggling. The problem is best described as follows… Imagine you have a section of a wall that you want to look like a window on a spaceship. So you want to look at this “window” and see through it some “stars” (i.e. pinpoints of light) that...
  15. red65

    I Why do we use low-coherence light in Optical Coherence Tomography?

    Hello , I study the principles of optical coherence tomography, where we emit light and by the refraction that we detect we reconstruct and image, but I don't understand why we use low coherence light , if i want to measure the refracted light i would prefer to have coherent light so that the...
  16. Bluesy

    I Light Refraction, Biconvex Lens, Diagram Help

    I've read that the refraction of light at the boundary of a medium can be described as follows: -a line of connected people marching. one side of the line enters mud, and slows down. This causes the non-mud side to pivot towards the slower side, which then causes the line to change direction...
  17. S

    Exploring Propulsion with Light & Magnetic Fields

    while browsing for new propulsion methods I found the following interesting: https://en.wikipedia.org/wiki/Radiation_pressure "Radiation pressure (also known as light pressure) is the mechanical pressure exerted upon any surface due to the exchange of momentum between the object and the...
  18. Marcarious Thomas

    B Faster Than Light Travel: Exploring the Possibilities of Spacetime Curvature

    Based on the current understanding of general relativity, it is possible that curving spacetime in the back of a spacecraft would allow for faster-than-light travel. In general relativity, the curvature of spacetime is determined by the universe's distribution of matter and energy. If a...
  19. W

    B Interpreting light as Maxwell's EM wave

    Sometimes I cannot imagine light as the popular Maxwell's blue and red electric and magnetic wave https://simply.science/images/content/physics/Electromagnetism/em_waves/emv.jpg but I found the image below to be the more accurate representation of how light actually looks like as waves...
  20. O

    Misc. Let's change another light bulb

    You've helped me change a couple bulbs so far, hopefully this is the last one. There's a recessed light on a hinge (a gimbal light maybe?) There are three exterior white rings - the outermost is a trim, the middle is a flat piece, the inner is ridged. Both the middle and inner piece move on...
  21. sinus

    I Polarization of Light -- Please help me with some basic questions

    I'm studying optics in this semester and one of the topic is polarization of light. Please help me. For ease of understanding we only consider the electric field E, right? I'm confused, if light passes through a polarizer then there will only be one field whose direction of vibration is only...
  22. L

    B Does light ever go from the eye to the image?

    You can see from the picture that the teacher has circled the arrows which shows light coming from image to the eye, and drew it in the opposite direction saying the light goes from the eye to the image. The marking scheme of this paper only says the correct direction does not specify which is...
  23. sol47739

    I Is the light from Andromeda truly blue shifted?

    I asked a similar question on Quora not that long...
  24. KataruZ98

    I Light intensity required to shine a light through an opaque object

    Pretty much, I was picturing a 1x1x1 meters cube of a dark colored, mostly uniform material (limited porosity, no crevices) with a small chamber in its interior - exactly where the center of mass should be. In it, a body emitting light is activated with the goal to find the intensity necessary...
  25. F

    Effect of a polarizer/analyzer on partially polarized light

    This is problem 62 in Cutnell & Johnson's Physics (9th edition): Suppose that the light falling on the polarizer in the figure is partially polarized (average intensity \bar S_P) and partially upolarized (average intensity \bar S_U). The total incident intensity is \bar S_P+ \bar S_U and the...
  26. Luckyroad21

    Solving an Asymmetrical Inequalities Problem: Seeking Light

    I'm obsessed with this problem and need some light, I've tried solving it using inequalities but there seems to be an asymmetry that's hard to deal with.
  27. K

    I One-Way Speed of Light: Is it Possible?

    From what I've read, it is not possible to measure the one way speed of light. We must reflect it off a mirror and then divide its travel time by 2, giving us its round trip average speed. Time dilation makes synchronizing two separated clocks impossible. We just assume light goes at C in all...
  28. O

    I Has anyone measured the speed of light in one direction?

    I don't know if there are some here familiar with "Veritasium", a YouTube video channel dedicated to science and engineering. It was created and is hosted by Dr. Derek Muller. It has over 8 million subscribers and many of its programs have been watched millions of times. Today I was watching an...
  29. Haorong Wu

    I Can Light Rays Cross Near a BH? - Simulated w/ Matlab

    Hi. I use Matlab to simulate that two parallel light rays pass near a Kerr BH. The angular momentum of the BH points to the ##z## direction. The ##z## components of the start points of the two rays are ## 1\times 10^3 ~\rm{m}## and ##- 1\times 10^3 ~\rm{m}##, respectively. The result, as shown...
  30. A

    B Phase Change in Light: A & B Signal Reflections

    I would appreciate knowing if there is any error in the mechanics illustrated in this animation with respect to phase change in the signal reflections shown at A and B, or in the description accompanying it. I have embedded the description with the animation and included it in this post for...
  31. Mustafa Bayram

    B Is there a fractional frequency of light?

    If emw spectrum is continuous, possible wavelengths should be infinite and there should be fraction of frequencies like 25,2 hertz. Well is there a fractional frequency of light? In high school when we are teaching interference of light we say "only the same wavelength of lights interfere with...
  32. Policetac

    B What Powers Light? Exploring Photon Energy Across the Galaxy

    How can a photon of light literally travel across the galaxy? Where does it get the energy to do so and what then keeps it going? Why isn't yhe frequency and energy depleted? Does the coldness of space have anything to do with it?
  33. C

    Light passing though slab

    ##\gamma = \theta_1 - \theta_2## where do you get that from? Thank!
  34. S

    Delta-Scuti phase folded light curve equation

    data I'm trying to fit
  35. J

    B Speed of light and this Accelerating Rocket scenario

    Hello, I have a question linked to gravity and speed of light : According to the Special Theory of Relativity, the speed of light cannot be exceeded because it would need an infinitive energy to accelerate the relativistic mass of a moving objet (a space craft for example) : indeed if the...
  36. K

    B One light year distance

    Dear PF Forum, It's been a while since I logged in here. And I really do appreciate all the answers that I've been getting here. Now, I wonder. Is there any standardization for 1 light year distance? Is it 10 trillion kilometers, or 299,792,458 * 60 * 60 * 24 * 365.256 = ...
  37. Intel Xeon

    I Class debate: Everything we see is a result of reflected light

    Everything we see is a result of reflected light. True or False We had an all out war over this question. The question is not as easy as it seems. There are many valid arguments. It would be interesting to see any opinion or contribution to this argument.
  38. L

    B I still don't understand the propagation of light

    Hi, I've been struggling for a long time to understand the propagation of light. Here are my key problems: - since a star can be seen from any direction, it means that photons are emitted in all these directions, which are infinite, how is this possible? infinite photons? - in case there are...
  39. S

    I William Herschel's light experiment

    In Cosmos: A Spacetime Odyssey episode 5 Hiding in the Light, Neil Degrasse Tyson describes one of William Herschel's experiments with light in the year 1800. The episode shows Herschel's experiment as a cartoon. Herschel put two thermometers under a prism that was in the sunlight. Herschel put...
  40. I

    I Doppler Effect on Light: Constant Speed of Light Confusion

    If the speed of light in the vacuum of empty space is the same for all observers, how can there be a Doppler effect on light? Doesn't the shift of color/frequency indicate a change in speed? Sorry if I posted with the wrong prefix, I'm just a curious senior (as in old guy) :oldconfused:
  41. C

    B A thought experiment of the relativity of light

    A light source in uniform motion emits a flash of light. A spherically symmetric wavefront propagates from a central point, the source, or the “origin” of emission. The wave front remains at c relative to the origin as measured by all observers. How fast do these origins move? Am I missing...
  42. V

    I How to rule out that the speed of light was different in the past?

    The constancy of the speed of light is a fundamental principle in modern physics, and it is supported by a wide range of current experimental evidence. There is no evidence to suggest that the speed of light was different in the past, and the idea that it could have been different is at odds...
  43. J

    B Questions about the Speed of Light, Time, etc.

    1. How much is acceleration of light? 2. Why is speed of light constant if time is relative, to keep speed constant distance must be changed as well? 3.What is time by Einstien? If we state it is not absolute, it is illusion, dont exsit? 4. Can we say ratio between distance and time is...
  44. J

    I Light in an accelerated elevator

    Elevator (2m height) is accelerate upward, light is at the top pointed toward flor of elevator. Is time t=2m/c to light pass from top to bottom? Bottom moving toward light,so time would be smaller then t=2m/c?
  45. Bruh23

    The Physics of Light and Sound

    I brought time over to the other side and multiplied to get 686 m/s but that wasn't correct
  46. AlienGrey

    B Light Speed In Forward and Backward Directions

    In the illustration below, a laser beam is emitted, and the time recorded, from the rear of a frame which is 150,000 km long and is moving at velocity 0.5 c to the right. When the beam reaches a half silvered mirror at the front of the frame, the frame being shown as a rectangle and using...
  47. anorlunda

    I Feynman's QED; 36,000 revolutions per inch for red light.

    In Feynman's book, QED, The Strange Theory of Light and Matter, page 27 he says, "As long as the photon moves, the stopwatch hand turns (about 36,000 times per inch for red light); when the photon end up at the photomultiplier, we stop the watch." I've been puzzling about the significance of...
  48. marcosdb

    B Speed of Light Unaffected by Moving Emitter

    Is my understanding correct that if we have a moving vehicle moving to the right at speed v, as above, with a light source in center going in both directions, that (upon emitting the light at time T), a detector at D1 & D2 would both detect light reaching it at T2? (even though in the time it...
  49. mammoth13

    Unexplained bright flash of light inside house

    Last night I was awoken by what seemed like an extremely bright flash of light. I sat up and saw another bright flash of light just outside my bedroom that seemed to originate just down the hall. I was worried there was an electrical short so I ran out and nothing. I thought maybe I imagined it...
  50. S

    I Arguments leading to the speed of light as a dimensionless constant

    I once read (though I don’t remember where) that in the same way that you talk about a dimensionless ratio between Y and X in ordinary space, you can conceive of c as a dimensionless ratio between T and X in spacetime. Do you know where I can find a reliable treatment of that idea? As...