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. T

    I Light clock running faster than light?

    Twin Paradox. I am on the outgoing rocket and look back at my stay at home twins light clock (two mirrors with a ray of light bouncing in between) I get to the nearest star and on the outbound journey my twins clock apears to run slow. When I turn around my plane of simultaneity shift and I...
  2. jzz

    A Is the quantum mechanics explanation for the propagation of light adequate enough to be understood?

    Ask any informed man on the street for the quantum mechanics explanation of light and his answer would probably be something like this: “Light as it travels from point A to point B is not something real, it exists as an abstract mathematical wave function that exists everywhere and nowhere...
  3. elou

    B Light beams and their reflection

    I must add, that, when used in a lit space, or in daylight, spraying before the mirror does not make any light rays visible. Is it simply a matter of light scattering? Also, I am not sure whether this question belongs here or in classical physics.
  4. Danielk010

    How does one calculate the number of photons striking a surface per second?

    I am more so stuck on where to start with this problem. I know dividing the photons per second by the area gets me the photon per area, but I am not sure how the distance is related to this part of the problem. If anyone can help, thank you.
  5. BiGyElLoWhAt

    I Newtonian path of light in a gravitational field

    I think this is the appropriate subforum. I'm curious as to what approaches have been taken. I know this prediction isn't correct. I can think of at least a couple ways that I could go about this. They may or may not give the same prediction. One approach would be to simply use kinematics, and...
  6. A

    B Why does light diffraction create circular patterns?

    We see under trees circular light that goes from sun through the leaves gaps. Why does it create a circular shape and not other shapes? why does this diffraction phenomenon create it in circular patterns?
  7. H

    I Seeing history when crossing event horizon

    Looking at Kruskal diagram, it appears that light from all previous events of something crossing event horizon at that same point, reaches the falling observer when it crosses the event horizon. Is my interpretation correct?
  8. rekha1804

    B Medium of light travel in space

    What is the difference between space and vacuum? Since light waves require a medium to travel, then what is the medium in space?
  9. mister i

    B Where does the light energy lost due to expansion go?

    Sorry for my questions as an amateur interested in physics: If light changes its frequency during a long time of travel in space (vacuum?) for all observers (redshift) and therefore its energy decreases (E=hf), what "entity" absorbs this energy? I suppose the answer will be that the wavelength...
  10. N

    I Speed of information in a medium

    I am aware of the explanation for light slowing down. From what I understand the EM field causes electrons in the medium to oscillate interfering with the wave and slowing its progress through the medium. The question I am asking is about a light source (or any source) that is turned on. If I...
  11. A

    I Lens Calculations to Focus Light

    I am looking for help getting started with focusing light into my monochromator. This is all just hobby level stuff with limited budget for high end lab gear. I have a 35mm slide projector I am going to re-task as a light source. It has a 200 watt tungsten bulb that is passed through a...
  12. T

    I How do I know it is light?

    I approach a traffic light. It is red. I know it is a light. What is happening in my brain to inform me that I am looking at a red light, and not a brightly colored red circle on the canvas of my perceptions of the physical world? If my eyes are moist and I squint, I see radiating red lines...
  13. H

    White light passing through prism

    I don't really have any idea. I know that shorter wavelengths refract more on entry to prism; e.g. ultra-violet refracts more than infra-red, but I don't know why they don't join up again when speeding up on exit from the prism.
  14. S

    B Redshift of a light pulse between 2 accelerating rockets

    I'm reading book from here. Suppose two rockets are accelerating with the same acceleration ##a## and are separated by some distance ##z##. At time ##t_0## the trailing rocket emits a light pulse. The book tells that pulse reaches leading box after time ##z/c## as seen in background frame. But...
  15. P

    B Is the Speed of Light Consistent Across Rotating Reference Frames?

    Does it violate the postulate of special relativity in any sense?
  16. Vanadium 50

    Light Heavy and Semiheavy Water Equilibrium

    If I start with a mix of half H2O and half D2O, when it equilibrates it will be half HDO, a quarter H2O and a quarter D2O. My question is "how long does this take?". Ballpark is fine - microsecodnds? Days? Centuries?
  17. 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 •...
  18. 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?
  19. 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...
  20. 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...
  21. 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...
  22. 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?
  23. 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?
  24. 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?
  25. 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 ?
  26. 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?
  27. 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?
  28. 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...
  29. 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...
  30. 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...
  31. 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...
  32. 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...
  33. 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...
  34. 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...
  35. 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...
  36. 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...
  37. 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...
  38. 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...
  39. sol47739

    I Is the light from Andromeda truly blue shifted?

    I asked a similar question on Quora not that long...
  40. 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...
  41. 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...
  42. 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.
  43. 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...
  44. 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...
  45. Haorong Wu

    MATLAB 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...
  46. 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...
  47. 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...
  48. 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?
  49. C

    Trigonometric Relationship for Light Passing Through a Slab

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