Light Definition and 1000 Threads

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. V

    B Reflection of light from the mirror

    What is exactly happening when the light hit a surface of a mirror? I know it is not same as a bounce of ball from the wall, because of constant speed of light for example. So I suppose the light is absorbed by electron of reflecting material. This probably define the materials who are...
  2. M

    A system comprising blocks, a light frictionless pulley, and

    Homework Statement A system comprising blocks, a light frictionless pulley, and connecting ropes is shown. The 9-kg block is on a smooth horizontal table (μ = 0). The surfaces of the 12-kg block are rough, with μ = 0.30. In Fig. 5.7, the mass M is set so that it descends at constant velocity...
  3. B

    Explore Polarized Light: Tilted Polarizer Effects

    https://www.dropbox.com/s/t5lv4nlunn35ok8/phy1.PNG?dl=0 https://www.dropbox.com/s/ktc9pj7qmqhejrv/phy2.PNG?dl=0 https://www.dropbox.com/s/qbjz1p1gokvsgj2/Capture3.PNG?dl=0 I googled "polarizer film diagram" and "malu's law" and "polarizer physics" to obtain the pictures in this thread. I...
  4. Allen_Wolf

    Single Ray of Light: Visible from All Angles?

    IMAGINE an infinitely large space where there exists no particles or energy fields at all. If we introduce a single photon or a single ray of light, will it be visible from all angles?
  5. R

    B 2 clocks -- Using orthogonal light path detectors in a space ship

    Hi I can't see the error in this can someone please explain where I went wrong? A man is in a spaceship traveling at a constant velocity He makes 2 identical tubes of length L with a mirror at one end, tube a and tube b He has a single light bulb. Next to the bulb is a detector. He carefully...
  6. L

    What types of light can safely pass through the human body?

    What types of light are able to pass through the human body without causing damage to tissue or individual cells? Are there any truly safe alternatives to x-rays that could allow doctors to see internal body parts but not cause damage from extended exposure?
  7. A

    A few questions about light and how things work

    Hello first time posting anything like this to people that have a better grasp on these ideas. Please pardon the lack of proper language structure. So to my understanding that light travels on photons. The light from a source comes out in waves and particles. As it travels redshift will happen...
  8. marmstrong941

    B Is anyone looking for this: reaching speed of light

    is anyone looking for what might be the effect of something going into or coming out of the speed of light?
  9. A

    I Does slowing light with a BEC change any of its properties?

    So I understand that scientists have been able to slow light to extremely low speeds using Bose-Einstein Condensates and even without them (https://physics.aps.org/story/v3/st37) and if I understand this correctly they slow light the same way water or air does; atoms absorb the photons and...
  10. C

    B Speed of light and modern physics

    What will happen if something traveling faster than light is found?What effect will it have on modern physics?
  11. E

    I Speed of gravity vs. speed of light

    GW170817 showed that gamma rays came 2 seconds after gravitational waves. (after 130M years) Are these two seconds uncertainty at determination of difference of these speeds? Or is it possible to explain, why 2 seconds of difference, or is it possible to explain, at least, a part of this...
  12. Physicsislifetome

    B Is Light Traveling Through Time?

    If we need light speed to travel in time and light already have it , so is light traveling in time
  13. H

    B Does a gravitational wave change the speed of light?

    Suppose at the instant a gravitational wave passes through an interferometer, one of the interferometer's arm get stretched by 1%. Would the wavelength of the photon traveling in the arm also get stretched by 1%? If so, then would the frequency of the photon remain the same and hence increasing...
  14. Erik Ayer

    I Light amplification with open-ended laser tube

    Is it possible to get a laser tube with the ends unmirrored, designed such that a photon being sent through will have a good chance of stimulating another photon to be emitted? If this is possible, then a beam of light going into the tube would be roughly doubled in intensity. When stimulated...
  15. M

    B How is light influenced really close to the event horizon?

    I was just wondering as it seems pretty counter intuitive that there is a really defined horizon where light can't escape from a black hole. It would make more sense to me if light gradually curved into one. Or does it do this? Please enlighten me ;)!
  16. E

    Applications of Interference of Light Waves

    Hi! I just had some questions. Is there any jobs or careers that directly relate with the interference of light waves. I know that an optical engineer would be one but they would create lenses for telescopes and stuff. I don't understand how that would relate to the interference of light, are...
  17. G

    B Concept of the stability of the speed of light

    Help with the question of the concept of the stability of the speed of light. I gather that current atomic clocks are based upon the speed of the electron around their nucleus, which is a function of the speed of light. Measurements of the speed of light that depend upon atomic clocks therefore...
  18. J

    I Speed of light on a moving platform

    1. We have a moving platform traveling 700 miles per hour to the Right-Direction. 2. In the middle of the platform, we have a gun which fires a laser both at the front and back of the platform at the same time. 3. We have at both the front and back of the platform, a mechanical device. These...
  19. K

    Does light refraction play a role in bubble iridescence?

    We know interference of waves play a role in causing "colors" on soap bubbles. But does light refraction play a role in bubble iridescence? When the light wave travels into the soap bubble surface, won't it refract since the wave is into a different matter (what matter the soap bubble surface...
  20. K_Physics

    Relative Intensity of Various Light Sources - Lab Experiment

    Homework Statement I've linked my data table down below. My problem is the relative intensity column. From the table, the units of relative intensity are w/m/s. I'm assuming this is power/velocity, where velocity is the speed of light. However, nowhere in the lab manual did it mention the...
  21. 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...
  22. 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...
  23. 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.
  24. 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...
  25. 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...
  26. 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...
  27. 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...
  28. 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...
  29. 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...
  30. 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?
  31. 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...
  32. 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...
  33. 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...
  34. 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...
  35. 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...
  36. 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...
  37. Telemachus

    How Can I Use a Photodiode to Detect LED Light for Spectroscopy?

    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...
  38. 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?
  39. 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...
  40. 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...
  41. 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...
  42. 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...
  43. 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...
  44. 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...
  45. 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...
  46. 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...
  47. 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...
  48. 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...
  49. 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.)...
  50. 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...
Back
Top