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. F

    I Does the speed of light imply ....

    Does the speed of light imply that in its property as a wave it encompasses the entire known electromagnetic spectrum?
  2. E

    B Problem about postulate of the invariance of the speed of light

    Hello, I have a problem with the postulate of the invariance of the speed of light. When we move away from a light source it is redshift, it is the sign that the relative velocity between us and the light source has changed. If a stationary observer observes the phenomenon, he will measure that...
  3. P

    I Energy of Light in Accelerating Universe: Comparing Cases

    Assume two observers very far from each other, so far, that the accelerating expansion of the universe matters. (edit: But not outside of each others event horizons.) They will send light beams each other, and measure the energy of it. Also tie them together with a very long rope to fix their...
  4. S

    B Light Taking U-Turn: Transformation of Star to Black Hole

    sir , i always taught what would happen to light traveling away from the mega sized star at the time of transformation of this star into a black hole? will it trace back its path after it taking a u - turn?
  5. squeekymouse

    B How to understand the steradians equation for measuring a sphere of light?

    Hello and please know I am very greatful for your help! I am wanting to learn how to measure light. I have chosen a specific light for this to help me better understand. Lm- 7800 CD-620.7 So, I got that far, lol. I don't really know how to input the numbers for the Steradians equation, I have an...
  6. C

    Quantum Discover the Best Lightweight QFT Introductions for Self-Study

    I was a denizen of this forum some 15 years ago during undergrad. However I since joined the dark side working in software on ML, AI, and distributed data processing. Every now and then I pick up a physics textbook to get into the weeds of a topic I would have missed due to skipping out on grad...
  7. A

    I Light coupling in fiber optic cables

    Hi Everyone! I have a photonics problem that I was wondering if anyone may be able to shed some light on (no pun intended). I work for a company that is designing a fiber optic emitter/detector as a part of one of our products and we've noticed something that would appear to defy physics...
  8. MrSimp

    B Does Force Travel Faster Than Light?

    Imagine a plane 1 light minute across. Now imagine 1 person on either and of that plane. Between them, is a thin indestructible bar that is 1 light minute in length. In the center of this plane, a simple device has a wire that leads to a motion sensor on the left side of the bar. If the left...
  9. A

    I Can visible light cause fluorescence in colorful objects?

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  10. Cody Livengood

    B What if we cut out the Big Bang and inflation?

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  11. X

    I Lux & Lumens -- how do they work?

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  12. H

    I Spacetime interval and basic properties of light

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  13. A

    Light beam fired in a moving train (relativity)

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  14. L

    I Can we see light pollution from space?

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  15. A

    I Focusing UV light from a 275nm LED down to a small spot

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  16. A

    Pendulum made with a Ball and two light strings

    Sketch
  17. R

    Heat Transfer Through a Two Material System due to a Light Source

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  18. Element13

    Electronics Custom Multi Light Switch with Dimmer

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  19. A

    I Does Ohm's Law work for Light Bulbs?

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  20. F

    I Is the concept of a one-way speed of light meaningful and measurable?

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  21. K

    I Why do we still say “the speed of light” instead of “c”?

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  22. L

    I Do objects gain mass while approaching light speed?

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  23. A

    I Speed of Light: Spaceship & Gravity Law Impact

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  24. ohwilleke

    A What New Experiments, If Any, Would Help Determine Light Quark Masses?

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  25. H

    A Calculate the group velocity in EIT (famous paper: light speed 17m/s)

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  26. J3J33J333

    B Storing light and using it to propel an object

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  27. malawi_glenn

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  28. jerry222

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  29. mesa

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  30. R

    I Light Clock Relativity: Why Divide T_moving/T_stationary?

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  31. A

    I Question about polarization of light by scattering in the atmosphere

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  32. alan123hk

    B The relationship between mass, light and energy

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  33. flying_bear

    Medical doctor, ophthalmologist, industrial PhD, chief medical officer

    https://www.linkedin.com/in/hellandhansen/ CMO at an ophthalmological tech company; Bulbitech. Now initiated an industrial PhD at the University of Oslo, supported by a Norwegian Research Council grant, where I will work on one of our tests (retinal adaptation speed to different lighting...
  34. T

    Optical power of Smartphone (Galaxy S20) torch

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  35. L

    B How does the propagation of light work? How much time does it take?

    I'm quite aware that according to Einstein light is a particle and a wave at the same time. However, I try to imagine how exactly would light expand. A photon must likely have a certain timing for expansion. I mean, after a photon is originated it immedialy multiplies itself into many other...
  36. technotux

    Heavy mass vs light mass in circular motion

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  37. O

    B Parallel light reflection for a one-way speed of light measurement setup

    Hello, Is there a mirror that will reflect light in parallel trajectories ? If yes, is the reflected light in sync, and will all beams hit a flat surface simultaneously ? Thank you
  38. K

    I Rindler Wedge: Timelike or Spacelike? Intuition & Light Cone

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  39. A

    Inertial confinement fusion (First light, NIF, etc)

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  40. gehrenfeld

    B Exploring Infrared Light: Is It Visible to Our Eyes?

    I am 74yo and love Physics. If my basic knowledge of physics is correct, light is a photon. My question is: 1. Is the reason we cannot see infrared light because the spectrum travels at a wave link our eyes cannot see? 2. Does the light from the sun have all the different waves combined, and we...
  41. mef

    I How to Measure Speed of Light & Is It Constant?

    The question constantly arises how the speed of light is measured and what does it mean that the speed is constant, including at remote points for the observer, including at points beyond the local frame of reference, as you understand it in general relativity (GR). First of all, it should be...
  42. R

    B What happens when 2 beams of light interact?

    I realize turn into probably isn't the correct term. Also I know gravity can bend light but can gravity cause light to turn into other particles? I am layman so can someone explain this in layman's term? Also I am not even sure the quantum section is the correct section.
  43. C

    What Caused the Strange Flash of Light in My Bedroom at 3:57am?

    (edit.. not the red light, that's my emergency power outage light in every frame, I'm referring to 3:57am and exactly 15 seconds, blink and you will miss it) turn up the volume Strange white/yellow flash appeared in my room and the sound it made when the light hit my security camera sounded...
  44. SvenDahlhaus

    Is there really nothing faster than light?

    Homework Statement:: Einsteins Theorie Relevant Equations:: Theorie Hi. My name is Sven Dahlhaus. I'm from Germany and just got my high school diploma. Nevertheless, I am a big fan of theoretical physics. There is a question that bothers me a lot. According to Einstein's theory, nothing can...
  45. D

    Dispersion diagram of light source seen through prism

    I know that red light has a lower index of refraction than blue light, but that’s not what I’m seeing. The blue light is where the red light should be. I can’t afford to join CHEGG. Any chance that someone would help me out for free? I’d really appreciate it. [Link to chegg removed by the Mentors]
  46. H

    I Can we determine the one way speed of light by combined measurements?

    The idea is to have 2 clocks at position A and B. The clocks are synchronized by sending a light pulse from position S over 2 equal distances x. The receiver is at position R at a distance y rectangular to the direction AB and exactly in the middle between A and B and right below S. for proofing...
  47. F

    Finding Total Number of Optical Field Modes for Visible Light

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  48. G

    How to find the forces in a particular framework of light rods

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  49. PCNerdz

    B Speed of Push vs Speed of Light

    Been thinking about this since I was 12. Suppose you had a 1 lightyear long pole, and on the other end was a button 1 inch away that activates a light. The light is powerful enough for you to see it 1 lightyear away. You push the pole 1 inch forward. How long does it take for the light to reach...
  50. M

    I Light speed independent of source

    Please bear with me I am trying to get a grip with underlying principles. Starting to try and understand Einstein’s second postulate and distinguish that the speed of light is independent of the speed of the source – v - of objects, other than light with travel initiated independent of the...
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