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. Haorong Wu

    How to model light from a star?

    I am aware that a laser could be modeled as a Gaussian beam, e.g., $$E=E_0\frac{w_0}{w_z}\exp (\frac {-r^2}{w^2_z}) \exp (-i(kz+k \frac {r^2}{2R(z)}-\psi(z))).$$ Now I want to study the propagation of light emitted from stars. But I am not sure how to model it, especially by some kind of...
  2. Justice Hunter

    B Thought Experiment : Try and get Alice to "0%" the Speed of light

    Someone asked a really interesting question on a comment thread somewhere's, and ever since, I could never really stop thinking about what the proper answer to it could be. It's a really basic question, but it unpacks a can of worms. The exact question I read was the following This question...
  3. O

    Connect: Coupling Light from 2 LEDs into a Single Fiber

    Hi, I am looking for a suitable method to couple light from 2 LEDs into a fiber and then join both fibers, so that I only have one fiber at the end so that both wavelengths can be emitted from a single fiber. The first coupling step from LED into fiber seems to be more easy, I already heard...
  4. O

    Deflection of light passing the Sun

    The book says the actual measured deflection from some experiments is about ##9\times 10^{-6}## radians, and this is double the value that a naive computation returns because of general relativity effects. So I guess my goal is to compute a number that looks like half that. If suggests to do it...
  5. W

    A About pair production using light

    Can light make two pairs? (assuming enough energy to make two pairs) If so, why? or If not, why not?
  6. O

    Misc. How do I change a really high light bulb?

    Your guys really helped me out with my last bulb problem so I thought I'd try again. There's a bulb over a staircase that is - a recessed floodlight - about 18 feet above the stairs directly beneath it (it's over the bottom of the stairs, and the staircase is exposed to the next floor) About...
  7. The Baron

    I One way speed of light measurement

    Hi so to those of you who don't know the problem here is a video that explains it very well. - I have an idea on how to measure it, can someone please tell me if, and why it is incorrect. Thank you! Okay so we will have a timer, and a lot of mirrors, set up in a circle, a light beam is fired...
  8. R

    Refraction of light question -- Flintglass submerged in oil

    Hello, hopefully the question made sense, it was hard to translate. i attached a photo about the question. I started with n1=1.4, sinΘ1=37◦ and n2=1.62 1.4(sin(37◦))=1.62sinΘ2 1.4(sin(37◦))/1.62=sinΘ2 arcsin(0.52)=31.34◦ Is it calculated correctly?
  9. J

    Exploring a New Theory of Gravity: Filling in the Gaps of Newtonian Physics

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  10. R

    I Probability of finding a particle outside its light cone

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  11. J

    B Exploring Light & Dark: A Universe of Questions

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  12. M

    I Less known electric/magnetic interactions with light

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  13. S

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  14. Y

    Difference in destructive power between a heavy stick and a light stick?

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  15. Janos Meri

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  16. L

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  17. S

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  18. GhostLoveScore

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  19. The Baron

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  20. H

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    I know what happens initially, calculated as follows. 1/∞ ＋ 1/q = 1/50, q=50 1/-25 + 1/q = 1/-25, q= ∞ However, how do we know about the after when the two lenses get closer to each other?
  21. G

    Mirror box with 1 candela of luminous intensity light source

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  22. B

    B Understanding Light Quantization

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  23. J

    B Does Gravitational Time Dilation Affect the Speed of Light in Outer Space?

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  24. MoistOxi

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  25. The Baron

    I A problem with how the speed of light connects to special relativity

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  26. A

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  27. A

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  28. M

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  29. T

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  30. J

    I Constructive Interference of Light

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  31. S

    I Uncertainty and light through a prism

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  32. greg_rack

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  33. J

    Trigonometry Help: Calculating the Angle of Light Entry & Exit

    So what I did first was made the face of the triangle flat and calculated the angle the light entered it. This means the light enters the triangle from the base corner angle (so (180-38.8)/2) of 70.6 degrees. 1sin(70.6)=1.47sin(angle) angle=39.915 Now I need to find the angle it exits. But...
  34. Mr_Allod

    Rotation and Polarisation of Light using Jones Matrices

    Hello there I am having trouble with part b) of this exercise. I can apply the rotation matrix easily enough and get: $$ R(-\theta) \vec J= \begin{bmatrix} A\cos\theta + B\sin{\theta}e^{i\delta} \\ -A\sin\theta + B\cos{\theta}e^{i\delta} \end{bmatrix} $$ I decided to convert the exponential...
  35. N

    Question on how much intensity of light has been scattered

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  36. complete_noob

    B What Happens When Light Approaches a Black Hole?

    Hello I am not a physics student and i don't know anything about science, but i was curious if someone could tell me about what happens when light approaches a black hole i have heard that nothing goes faster than light but i have also heard that black holes can suck in light, combined with the...
  37. K

    Calculating the degree of polarization of reflecting light

    I am looking for in an equation that's spits out the degree of polarization of reflected light, with incidence angle and the refractive indexes as inputs. an article online article had this graph decribing the degree of polarization as a value between 0 and 1 plotted against the angle of...
  38. A

    I Isotropy of the speed of light

    It has been put to me that a simple spectroscope could in theory demonstrate the isotropy of the speed of light . By using a frequency standard (laser comb or Th Lamp for example) with the spectroscope in various orientations the lack of shift of the spectral lines would prove its isotropic via...
  39. G

    B Light Speed Invariance: Experiments, Difficulties & Clarification

    Let me clarify my question, is there any experiment directly proved the invariance of light speed to observers? Let's not get to the argument of equivalence between source and observer. SR was based on the postulate that the light speed is constant and independent of both the motions of source...
  40. Strange07

    Calculations for the movement of a light beam reflected by a mirror

    The only knowledge I have about such reflection questions is theta=2theta. But I missed class when this was thought. Please I need help and guide
  41. C

    Why a 20 Ohm lightbulb won't light up in a closed circuit?

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  42. K

    B Speed of Light: Questions Answered

    Yess! speed of light it's driving me crazy. I know it might sound too noob and silly but please enlighten me, First of all how does we concluded that speed of light is constant for every observer in all possible inertial frames of reference. Next thing how come a photon doesn't experience...
  43. idktbh1

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  44. M

    I Doppler effect - can't imagine how the frequency of light can change

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  45. F

    I The Speed of Light: Comparing Photon and EM Wave Velocities

    Velocity of photon allways is c(photon is massless particle).While velocity of EM wave in medium < c.So does velocity of photon need not allways equal velocity of EM wave?
  46. H

    Monochromatic light, Pattern on the screen

    A point source of monochromatic light is placed in front of a soccer ball and a screen is placed behind the ball. The light intensity pattern on the screen is best described as: Answer: a dark disk with a bright spot at its center and bright rings outside Why is there a bright spot at its...
  47. T

    Question on Cosmology Solution for Tired Light Hypothesis

    Good evening, I have a question on a cosmology problem I have solved from Barbara Ryden’s Introduction to Cosmology 2nd Edition. I believe I have answered the question correctly, resulting in the following linear redshift relation when using separation by variables and some algebra manipulation...
  48. Pyter

    I Speed of light not an invariant in GR

    Hi all, I need help understanding the light ray bending in the original GR 1916 paper, Die Grundlagen.... First of all, Einstein states the ##c## is not an invariant in GR. In fact, from (70) and (73), it stems that $$\gamma = \sqrt{ -\frac {g_{44}}{g_{22}} }, $$ where ##\gamma## is ##|c| <= 1##...
  49. G

    Two 95 watt light bulbs walk into a bar ....

    Two 95 W (120V) light bulbs are wired in series, then the combination is connected to a 120 V supply. How much power is dissipated by each bulb? Answer 24W I followed the explanations as such: r1 = 120^2/(95) = 151.8 R = 303 r2 = r1 W = 190 I = P/V I = 190/120 = 1.58 v1 = 190/1.58 = 60 p1 =...
  50. Cosmic Lollipop

    I Speed of Light in Different Mediums: Exploring Maxwell Equations

    Based on Maxwell equations, we can determine the speed of light as: $$c=\frac{1}{\sqrt{\epsilon_{0}\mu_{0}}}$$ Where ##\epsilon_{0}## is the electric permittivity of vacuum and ##\mu_{0}## is the magnetic permittivity of vacuum. This notation makes me believe that in other mediums those...
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