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.

View More On Wikipedia.org
Recent contents Top users
  1. Pushoam

    We see colour of an object due to reflection of light.

    Homework Statement How to demonstrate that we see colour of an object due to reflection of light? Homework EquationsThe Attempt at a Solution We see an object because photons from that object come to our eyes. The photons from the object comes to us due to radiation or reflection. If the...
  2. K

    B Does Gravity Have the Power to Slow Down Light?

    Hey guys, I'm itching to find out what you know or theories about gravity and light... My first question that I pose to you is this.. If the cosmic speed limit is set by light.. then why can gravity affect it? Consider this. Light is traveling away from a black hole.. gravity is so...
  3. Lunct

    Can Light Accelerate When Changing Direction?

    I know that light is always at constant speed but does it ever accelerate when changing direction, if you put light through lots of different densities it curves into the Brachistochrone curve, would that be an instance of light acceleration.
  4. B

    B Exploring Proxima Centauri: The Effects of Light Speed and Time Travel

    I am a physics "layman", so please forgive me if these questions seem basic. Let's say we can travel at light speed, we send a crew to Proxima Centauri, & it's exactly 4 light years away. They explore for exactly one day, then return. 1. Since it takes 4 years for light to reach it & 4 years...
  5. A

    Unknown light-based Phenomenon

    Can you help me figure out how this light-based phenomenon works? Please explain with an intuitive answer using the simplest principles possible. Note how the location and width of the dark horizontal bands is constant with respect to the edge of the video frame. The LED light looks normal...
  6. Z

    Photoelectric Effect, Light Intensity and Stopping Voltage

    In a physics investigation we were required to use a Photo-Electric Effect Instrument (Shown in the attached pdf file), which was able to measure the current produced via the photoelectric effect. One of the objectives of the investigation was to alter the aperture size, and hence intensity of...
  7. C

    B Question regarding Light by Light Scattering

    I recently read this article from Science Alert regarding Light by Light Scattering (http://www.sciencealert.com/light-continues-to-behave-really-weirdly-in-the-large-hadron-collider). They seemed to be astonished to witness light actually bouncing off each other, as they state that photons pass...
  8. S

    Polarization of light and diffuse/specular reflection

    How would polarized light behave after being reflected by a diffuse surface, such as a white wall? Would it still be completely polarized or would it be more randomly polarized? Would whatever effect takes place vary significantly with different diffuse materials? What about a mirror? Surely...
  9. T

    I Really parabolic light ray in an accelerated elevator?

    In an elevator "vertically" accelerated at g in outer space, the equivalence principle says a "horizontal" light ray in the elevator looks like a parabola. I completely understand that the light ray is curved but don't understand why the deflected light ray is an exactly parabola. Almost all...
  10. Wrichik Basu

    What is the cause of this sound from the tube light?

    I have a tube light at my home which produces this sound: The sound can be heard for a long time every time the light is switched on, but it stops after about half an hour of switching on the light. What is the cause of this sound?
  11. stargateengineer

    What topics are best for study of how gravity light works?

    I've seen a little bit about the gravity light. I think it works in a simple way, a weight at a certain height has gravitational potential energy depending on how far the weight will fall, this quantity represents the total energy we have at our disposal to power a light. Now we normally think...
  12. C

    I Michelson Morley experiment: relativistic explanation

    I don't uderstand how, using special relativity theory (time dilatation and length contraction), one can explain why in the Michelson interferometer there is no delay between the two rays in the reference frame where the interferometer is moving. Consider the picture ##2.## Setting...
  13. Fervent Freyja

    Are mosquitoes attracted by UV light?

    Are mosquitoes attracted to UV light? This PF thread isn't very clear on whether it does or not- some of it suggests that it does... So, I looked it up and found this article on Wikipedia: The American Mosquito Control association also http://www.mosquito.org/page/faq: Is the information...
  14. P

    I Bending of light - caused by gravity or relativity?

    The mainstream interpretation of GRT equations is, that additional double amount of angle of bending of light (Newton vs GRT) is caused by gravity (which is interpreted as curvature of spacetime). But when looking on the equations, it seems that this additional amount of bending is caused by...
  15. D

    Would HE light applied to mercury vapor produce electricity?

    I'm likely going to have a ton of questions in the following weeks about a wide range of particle physics etc. If (electricity X mercury vapor) = light, then multiplication being transitive, (light X mercury vapor) = electricity. Can someone inform? I've found only mercury vapor as a metallic...
  16. K

    Diffraction work differently on water waves vs light waves?

    When the water waves go through one wide slit (slit wider than wave length), there's no fringes, the water waves spread all over the sides, like in this pic: http://electron6.phys.utk.edu/light/images1-3/misc3b.jpg or in this pic...
  17. Telemachus

    Photodiode for light detection

    Hi. I am willing to make an experiment with light. The idea is to have a light source (specifically a led light) and a photodiode to detect the light and make some spectroscopy. The photodiode has to detect the light in the same frequency that is emitted by the led. I'm not an expert in...
  18. B

    B GR Curvature at Light Cone Surface: Smooth, Bent, Blocked?

    For example, the curvature due to a mass; does that curvature continue passing from within to outside the mass's light cone? If so, is the mass subject to the external curvature? If not, does the curvature have a discontinuity at the light cone surface?
  19. D

    Do light and sound waves roll up and break like ocean waves?

    When sea waves approach the shore they roll up and break due to different velocities of water layers formed due to the gradual change in water depth. The highest wave peaks move faster than all other layers and thus falls down. All other layers fall the same way but in a delay. this ends up with...
  20. I

    Light reflecting through a geometry

    I am discussing physics with a friend and we need someone to confirm a thing that we're not agreeing on. We are discussing incident light that is passing through different geometries, and I want to know how the light behaves when it reflects inside a half sphere (of glass for example). Maybe...
  21. mishima

    Stargazing 'losing light' 1.25" DSLR coupling vs 2"?

    I use a 2" coupling to mount my DSLR on the scope. However, I have an 90 degree mirror angle in the 1.25" size, and would like to use it sometimes for convenience getting my eye on the viewfinder/lcd screen. I was wondering what drawbacks, if any, there were in stepping down the diameter of...
  22. M

    Effect of electrical permittivity on the speed of light

    Why does an increased electrical permittivity reduce the phase velocity of light in a medium? Furthermore, what interactions do we see on an atomic level? I am aware of the equation that defines the speed of light in terms of the electrical permittivity and magnetic permeability, but I do not...
  23. K

    A Correlation properties of quasi-thermal light

    Hello! I'm measuring the second order correlation function g(2) of quasi-thermal light, generated by focusing a 633 nm laser onto a rotating sandpaper-like surface at 45 deg. Part of the dispersed light is collected into a fiber and split with a fiber beam splitter. A simple HBT setup. g(2) is...
  24. H

    I Understanding the transmission of an etalon

    Hi there, I have a question concerning the etalon (One would think that I find an answer for this in every standard physics textbook or this forum, but I actually did not): An etalon can be built from a solid block of glas (for example) with dielectric high reflectivity coatings (let's say R=99...
  25. M

    Effect of electrical permittivity on the speed of light?

    Why does an increased electrical permittivity reduce the phase velocity of light in a medium? Furthermore, what interactions do we see on an atomic level? I am aware of the equation that defines the speed of light in terms of the electrical permittivity and magnetic permeability, but I do not...
  26. S

    I Traveling at a speed faster than light

    Imagine yourself to be bat. You can't see anything. You don't have eyes. All you can do is echolocate, using ultrasound. Now imagine something is moving away from you faster than the speed of sound. Can you locate it ? Can you perceive its existence ? If the answer is yes, how ? If no , then...
  27. K

    A Gravitational Bending of Light: Equation for Photon Trajectory in Strong Fields

    I'm looking for an expression for the deflection of light in a static gravitational field. Referring to 'deflection of star light past the sun' in Sean Carroll's "Spacetime and Geometry" - equation 7.80 for the "transverse gradient": \nabla\perp\phi = \frac{GM}{(b^2 + x^2)^{3/2}}\vec b...
  28. D

    I Einstein's light clock thought experiment

    I'm an engineer who has an amateur interest in physics. I have been reading about Einsteins light clock experiment. I understand the principal that when a light clock on a train etc is moving relative to a standing still observer then the light must travel a longer distance per tick. given that...
  29. L

    Single-slit Diffraction Problem

    Homework Statement A spy camera is said to be able to read the numbers on a car’s license plate. If the numbers on the plate are 5.0 cm apart, and the spy satellite is at an altitude of 160 km, what must be the diameter of the camera’s aperture? (Assume light with a wavelength of 550 nm.)...
  30. S

    Solar Thermal Optics - Concentrated Light

    Hi, Can someone start me out with creating predictable temperatures and inputs for calculations. Here is a miniature example of the type of parabolic circulated water heating devices I have been building. My first questions would be what is the potential heating energy of the sun per cm2? And...
  31. M

    Light and oxygen acting upon the eye and the lung during the process of Evolution

    Did the evolution of the eye include interaction/reaction with a specific frequency of light? Did the evolution of the lung include interaction/reaction with the air density, humidity, and temperature of its environment?
  32. M

    How does one dimensional light warp multiple dimensions?

    If light is one dimensional, yet has gravity, and gravity is the warpage of spacetime, and spacetime has four dimensions, then how does a one dimensional wave/particle warp multiple dimensions?
  33. EspressoDan

    Light - Black-bodies & Spontaneous / Stimulated Emission.

    Hello, I have a short question: In relation to the Sun - is Black-body radiation ultimately the result of a combination of spontaneous (majority) and stimulated (minority) emission of photons? In relation to the IR emissions from everyday objects due to their temperature - is Black-body...
  34. K

    I Does the wave function shorten when approaching light speed?

    Relative to the observer, objects shorten when approaching the speed of light exponentially. Does this rule also apply to the wave function? Does this rule also apply to massless particles like Photons? Or am I just simply forgetting something?
  35. I

    Light bulbs -- is there a continuous wave spreading out?

    When a light bulb is emitinting light, is there a continuous wave spreading out, or is there a large number of particles (photons) emitted in random directions, which in the conglomerate, mimics a single continuous wave spreading out.
  36. NaukowiecGirl

    Uses for laser light in measuring hair width

    I have a question that I feel I cannot find enough on. We just did a practical in Physics class where we used a laser light shined on a single hair strand in order to create a diffraction pattern and use formulae to determine the hair's width. Because of Babinet's principle, the hair managed to...
  37. C

    Interference of light, emission spectroscopy prac. report

    Ok, so I've got a practical report due tomorrow and I just wanted to clarify a few things. A bit of background knowledge first. The practical involved taking angular measurements for the emission lines of the Mercury spectrum using the apparatus, a spectrometer and a Mercury spectral tube...
  38. H

    Light in a medium: do phonons store light energy?

    When light travels through a transparent medium and is absorbed then re-emitted, do the phonons just move to a higher energy state after absorption? Is it the same as absorption and re-emission of light by electrons, aside from having a continuous range of available energies?
  39. W

    B Being on a different IRF, wouldn't light appear to slow?

    I realize the basics of general and special relativity, and I picked up a copy of "Relativity" by Albert Einstein, though there's one question I just couldn't find the answer to despite my hours of surfing online. I understand the basics of inertial reference frames, length contraction, and time...
  40. T

    B How can light travel around an object in space?

    I came across an article about one of the many observations that helped to confirm GR. This observation took place during the 1919 total solar eclipse. Stars that were located directly behind the sun could be seen during the total solar eclipse because the gravitational force of the sun bent the...
  41. C

    Electric circuits -- Finding the V-I characteristics of a light bulb

    Homework Statement Homework EquationsThe Attempt at a Solution I know Ammeter and bulb goes in series and Voltmeters in parallel. But am I supposed to connect the voltmeter to the little arrow on the potential divider, or just across the bulb?
  42. genphis

    I Galaxies moving away at the speed of light?

    I have as advised done some catching up on basic cosmology, looking at various discussion and lectures on the nature of the universe. Something that has always played on my mind, is that i remember reading and watching a documentary about the potential to achieve the velocity of light. It was...
  43. J

    Speed of Light Thought Experiment

    A thought experiment about the speed of light. Say I build a 600,000 km long tube around the circumference of the Earth at the Equator. The tube's inner diameter is constant at 54.4505 mm. A snooker ball is perfectly manufactured to its lowest tolerance by a special new machine. Each one is...
  44. S

    B If energy increases with decreasing wavelength, then...

    ..then a wavelength of zero nanometers has infinite energy?
  45. W

    B Help me understand light traveling through vast distances in space

    Hi everyone.Ive been looking for a beginners section on this site but no joy.I am uneducated but physics and Astro physics in particular really interest me.One of the things I don't understand is the vast distance in space.We talk about things being thousands of light years away.Yet light is...
  46. A

    A Speed of Gravitational Waves confirmed by Experiment?

    It seems that with the 2016 LIGO and VIRGO confirmation of the gravitational chirp that we have also experimental confirmation that the speed is indeed the speed of light. True? https://en.wikipedia.org/wiki/First_observation_of_gravitational_waves#Direct_observation
  47. Quantum Velocity

    B Exploring the Dual Nature of Light: Is it a Wave or Particle?

    I heart from my friend that light is both wave and particle but i don't know that is true or not. So can you guy tell me what light actually is? And how light travel from the source and then to our eye?
  48. alex4lp

    A Solve Bending of Light Formula Problem

    Good morning everybody. I have a problem with this wikipedia passage https://en.wikipedia.org/wiki/Schwarzschild_geodesics#Bending_of_light_by_gravity 'cause it says "Expanding in powers of rs/r, the leading order term in this formula gives the approximate angular deflection δφ for a massless...
  49. KT KIM

    How can light propagate through Hollow optical fiber.

    Now I am studying a bit of optical fibers. And of course generally optical fiber works because of total reflection property of the light, which happens when the light from higher medium hits the lower medium material. However I recently found out that there also is a thing 'Hollow optical...
  50. M

    What is a good material for staging furniture

    It must be light strong and inexpensive. This would be used to make staging furniture. So it is moved often.
Back
Top