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. 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
  2. Lotic7

    Geiger Counter and ultraviolet light sources

    I purchased a RadiationD-v1.1(Cajoe) geiger counter off ebay, and attached an arduino nano and a 16x2 display ( I got the instructions off this site https://www.instructables.com/id/Arduino-DIY-Geiger-Counter/). I noticed that the counter was sensitive to UV when I had it out in the sunlight, so...
  3. sbrothy

    B Lensing Gravitational Waves Like Light?

    I was reading (or at least skimming) this paper: https://arxiv.org/abs/2005.10702 in which they seem to be discussing gravitational wave lensing. Is this an analogue of light lensing or is it another subject entirely? I mean, as I understand it, light is bend using gravity (as for...
  4. Lynch101

    "How the light gets in" online festival

    Has anyone here heard of the the HTLGI festival? Is anyone attending?
  5. C

    Size of a light beam before and after Refraction

    The size of light beam is same or different before and after refraction from a medium. If same then why we can not prove from mathematical expression. If not same why?
  6. PainterGuy

    Brewster's angle and the intensity of reflected light

    Hi, Would it be correct to say that at Brewster's angle, all the incident light which has its electric field parallel to the plane of incidence gets refracted, and the rest of light whose electric field is perpendicular to the plane of incidence gets reflected? For example, if the light whose...
  7. J

    I The Speed of Light -- Why can an object not travel faster than the speed of light?

    I apologize if this question is in the wrong forum section - but I could not find a proper place for what is a basic question or perhaps a nonsensical one. Is the reason why an object cannot travel faster than the speed of light because the object itself is ultimately made of waves? Is the...
  8. Jaffer2020

    Refraction of Light: Myth or Reality?

    In my latest 10th grade physics lesson, we were learning about the refraction of light. I decided to share what I knew about why light slows down in a vacuum, which is, in short, because the electric field of the electromagnetic wave exerts a force on the charged electrons of a medium, which in...
  9. Shubh Goel

    B Time & Speed of Light: Not a Universal Constant

    Time as we perceive is not a universal constant. It depends on the speed of the bodies and the distance between them.
  10. J

    Will closing the slit affect the clarity of observing light interference?

    My guesses are that if slit is almost closed, then the width is comparable to the wavelength and diffraction will occur. does this means that he will observe the slit S clearly or less clearly? [Picture cleaned up a bit by a Mentor]
  11. 0

    UV-C light sterilization test: Yeast seemed unaffected?

    Hi, I have a 25w 254nm bulb ozone free UVC bulb (draws about 19watts from my kil-a-wat). This was purchased from amazon, produced in China. This light definitely omits a funny smell, it pulls 19w, and the glow in the dark stickers are illuminated after my test (so I know it is omitting some...
  12. V

    Classical Textbook for light, heat, waves

    There's an undergraduate physics course at my uni that covers these topics and the course description is: Mathematical descriptions for classical physics: oscillations, mechanical waves, electromagnetic waves, physical optics and thermodynamics. Are there any good studying materials/textbooks I...
  13. IamVector

    Exploring the Nature of Light and Relativity in Space Travel

    so I was recently studying some ray optics and then suddenly a weird question came across my mind I just don't know if it's the correct thread to ask but let's continue anyway So the question is : suppose there is a spaceship traveling at light speed and someone fire a laser inside it what...
  14. R

    Doppler effect and how it affects the light spectrum

    Looking for a beginners explanation to the following question: How is Doppler effect separated from the original spectrum of light emanating from a moving body (in astronomy or other physics branches)? If the question does not make sense, here is the reasoning to ask it: If a certain color is...
  15. J

    B How can the annihilation operator for photons be realized experimentally?

    Light consists of transversely varying electric and magnetic fields. A point charge in the path of a vertically polarized light beam experiences a vertically varying force from which energy can be extracted. In an unpolarized beam, the electric fields of individual photons act randomly in all...
  16. Sokolov

    Irradiance of light trapped between two parallel mirrors

    The setup of the problem is shown in the image below. I know that I must add all the contributions of each reflected ray and that its amplitude will be reduced by a factor ##r## each time it is reflected. So after the n-th reflection, its amplitude will be ##E_0r^n##, with ##E_0## the amplitude...
  17. E

    Path of light in material of continuously varying refractive index

    I've been playing around with this for quite some time now this morning but can't get the last bit out. I defined the time functional to be $$T[y] = \int_{x_1}^{x_2} \frac{\sqrt{1+(y')^{2}}}{c(y)} dx$$ which follows from consideration of the time taken to cover an infinitesimal section of arc. I...
  18. S

    Engineering Light absorption in a semiconductor

    The doped a-Si: H layers in a HIT solar cell do not contribute to the photocurrent. The light they absorb (according to their absorption curve below) is lost. For a doped a-Si: H layer at the front side of the cell that is 25nm thick, what percentage of light at 400nm will be lost due to...
  19. S

    Is it possible to build a laser from a light bulb?

    In my drawing you can see: A generic bulb covered by a mirrored hollow ball. The hollow ball has a small hole (1 mm diameter). So the complete light will go through this small hole. The lens will bundle the light in a very distant (100 km) focal point. I guess it might be hard to create such a...
  20. halleff

    Signal Speed Below the Speed of Light for Circuit Abstraction

    I've read a couple of other topics on the Physics Forums about the lumped circuit abstraction requiring that signal timescales (i.e. the period of the highest frequency signal component) be much longer than the propagation delay of the signals though the circuit and that the wavelength should be...
  21. Greg Bernhardt

    Anyone have a light bulb blow up?

    Was shaving just now and one of my mirror wall light bulbs blew up. Luckily it's encased in the light fixture with an opening at the top so it was contained a bit, but some glass blew up through the top and rained on me. The glass was very hot, so I'm guess it wasn't LED.
  22. C

    Is light from an asteroid visible to the naked eye?

    d)NONE OF THESE
  23. T

    B Accurate measurement of the speed of light

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  24. J

    Light Reflection and Particle Size

    I'm studying particle counters and one of the detection methods uses reflected light to determine the sizes of the particles. What principle does it base on? Thanks!
  25. S

    B Is there a thought experiment to show that the speed of light is constant?

    I know the amazing thought experiment by Albert Einstein with the two light clocks. (The observer at the train station has a light clock and the person in the train.) It's amazing because you can even deduce the formula to calculate how fast the clock in the train goes. But this experiment...
  26. jisbon

    Polarization and Light Intensity

    Looked upon this and can't seem to find anything. The formula I=Icos^2theta seems to only work for polarization filters, which is not this case. On the other hand, the brewster's angle only c.aclates the angle when the REFLECTED light is polarized, not the REFRACTED light. Hence I am confused...
  27. S

    Why is the edge of the light from my flashlight beam so blurry?

    On the photo you can see: Red 1: the edge is very sharp (as expected) Red 2: the edge is very blurry (but why?) Same for green. You can also see (Red 2) that the blurry edge is yellow. What is this effect called?
  28. F

    B Understanding the Age of Observed Light: A Scientific Perspective

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  29. ramim

    B Is it possible to see the history of space if it expands faster than light?

    I was wondering if it would be possible to see the history of space if one was to stand at the edge of the universe as it expanded faster than the speed of light. If the universe was to be expanding faster than the speed of light right now, and I was able to somehow go there and keep up with it...
  30. J

    MHB Light Beam Speed on Shoreline from P - 16km

    A lighthouse is located on a small island 16 km off-shore from the nearest point P on a straight shoreline. Its light makes 5 revolutions per minute. How fast is the light beam moving along the shoreline when it is shining on a point 3 km along the shoreline from P?
  31. R

    I Exact historical explanation of deducing speed of light constancy

    As much as i search Google, in an effort to find out how exactly the constancy of speed of light was historically deduced before 1905, from Maxwell equations or by any other means, i am not able to find such an explanation. In all of the search results that i could find, it is just stated that...
  32. K

    Exploring the Limitations of Diffraction: Fresnel vs. Fraunhofer

    I have only seen Fresnel and Franhoufer diffraction being discussed in textbooks. Is it because they are the only two tratable types of diffraction? I mean, in the sense that one can really work out the equations to get results.
  33. seb7

    I Measuring One-Way Anisotropy of Light Speed

    I've always thought that light could possibly be transmitted faster in one direction than the other depending on the velocity of its container - despite what the current understanding of physics says! The problem is that its very hard to prove, and to this end I've read through all the...
  34. K

    B How do orthogonal waves interfere?

    How do two highly directional, orthogonal light beams (or any other kind of waves) with the same frequency interfere with each other?
  35. cianfa72

    I Speed of Light in Vacuum in Non-Inertial Frame

    Hi, I read various threads in PF about the concept of invariant speed and the speed of light in vacuum that in our universe happens to be the same as the 'invariant speed'. My doubt is about the speed of the light in vacuum as measured from a non-inertial frame (basically in the context of SR...
  36. P

    (Relativity) A flash of light is emitted at x1 and absorbed at x1 + l

    My answers are quite different and here are my attempts a) What is the spatial separation ##\ell'## between the point of emission and absorption? (In the frame going ##v = \beta c##) From what I know is that moving causes lengths to be contracted according to the formula ##\ell_{travel} =...
  37. thaiqi

    I What is the meaning of constant speed of light?

    Which one does it mean: "phase velocity" or "group velocity" or "speed of the wave front"? In the postulate of constant speed of light .
  38. G

    I Strongest Evidence for Trapping Light in Schwarzschild Radius

    The concept of that when a photon's trajectory intersects with the Schwarzschild Radius/event horizon, said photon will never exit the Schwarzschild Radius/event horizon. Or any other object besides a photon for that matter. So far what has been the strongest evidence for this prediction?
  39. TaurusSteve

    B Is E=mc² the Formula for the Speed of Light?

    What is the equation/formula for the Speed Of Light? E=mc²? I thought it would be simple to find in a search.
  40. K

    I Will anti-parallel light beams between two stars stay parallel?

    In our galaxy, two stars, A and B, move about 20 km/s in slightly different directions to each other. The stars are about 15 light years apart. Initially, the light beams are exactly anti-parallel. Will light beams emitted from star A traveling towards light beams emitted from star B stay...
  41. M

    I Rare Kaon decay may shed light on new physics....maybe

    The phys.org summary had me intrigued and the resulting new paper makes for heavy reading at my level of knowledge, but it was the underlying reasoning of the methodology that I really question: One of the authors notes "It's so rare, that they should not have seen any." From that, the...
  42. TaurusSteve

    Stargazing Convert Light Years to Miles: 5.879e+12 Explanation and Conversion Guide

    5.879e+12. So that is what comes up if you convert one light year to miles. My question is, what does the 'e' mean? I see it quite a bit.
  43. K

    Polarization of light -- Determine the thickness of the crystal

    I don't even know where to start with this problem. What kind of slit makes linearly polarized light circularly polarized? The correct answer is d = lambda/(4(n1 - n2)) = 856nm. But how do I get there? Thanks in beforehand!
  44. LawH

    B Energy, Mass, Speed of Light: Can We Reach It?

    Hello everyone! Let's say that you were to attempt to go as fast as possible on a spaceship with the mass of an average car in an absolute perfect vacuum. What I am wondering is, that if you were to reach a certain speed, and stop applying energy to this imagined spaceship, would the spaceship...
  45. L

    Faster than light thought experiment

    Hi :) I just had a (most likely totaly unoriginal) thought and thought I should try to ask someone who's thinkings on physics are somewhat more refined than mine. So after thinking twice about private messaging Mr Tyson and Mr Cox :) I decided to google 'physics forum'. So here I am with my...
  46. skyshrimp

    Is the Stationary Light Near the Moon Venus?

    I'm in Surrey, London, UK. I've noticed this stationary light in the sky near the moon when there are no stars visible. What is it?
  47. R

    B Meaning of "Speed of Light is Constant": Explained

    Isn't the meaning of speed, a variable of distance divided by a variable of time? Therefor isn't the meaning of a constant speed of light, a constant distance divided by a constant time? If there is any truth in this saying and there probably isn't, then what is the meaning of light constant...
  48. R

    B Constant speed between 2 objects close to the speed of light

    If i am moving away from an object at a certain constant speed close to the speed of light, is that object also moving away from me at the same constant speed?
  49. R

    B Moving Faster Than Light: Are There Symmetrical Rules?

    Would it be correct to say, that we are moving away from stars at the edge of the universe, at the same rate that these stars are moving away from us? I am relating to stars that are moving in relation to us, at a speed that is faster than the speed of light. Is the symmetry that maintains that...
  50. A

    I Why is the speed of light absolute?

    To describe the movement of the planets, Newton assumed that there was such a thing as gravity. But he didn't know what gravity was. To derive the Lorentz transformation, Einstein assumed that the speed of light was absolute (not relative), but is it also known why the speed of light is absolute?
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