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. RedRum69

    Courses Can a background in computer science lead to a career in astronomy or physics?

    Good Day, all! Currently having a semi-half-mid-life crisis xD I am currently enrolled at the Open University (UK) nearly finished with first year (120credits), though this is distance learning and I would prefer to go to Birkbeck University of London which will be evening classes. Now...
  2. ZMacZ

    Bending light by use of a strong field...

    Like the title says, bending light by means of a strong field.. And before you say "it's electromagnetic field by itself and therefore can't be changed by such.." A strong gravitc field (also an electromagnetic field) can in fact change photons to never escape it (in effect bending the light...
  3. R

    Composition of Light: Questions & Answers

    Hi everyone, I have a few questions about the composition of light: First, what is it? Is white light the result of all color wavelengths present in an area? Second, if so, then why is there no interference in waves of light (or is there)? Third, if photons all travel at the same speed, then...
  4. navneet9431

    Is light eye colour dominant or recessive trait?

    Homework Statement A study found that children with light-coloured eyes are likely to have parents with light-coloured eyes. On this basis, can we say anything about whether the light eye colour trait is dominant or recessive? Why or why not? Homework Equations Not Any The Attempt at a Solution...
  5. dumpling

    I Exploring the Relationship Between Light Emission and Wavefunction: An Analysis

    Hi! I would like to wrap my head around a relatively simple issue. Lets say that you have an excited atom, which rests in your refernce frame. When it emits light, the atom will have a backreaction, and it will "gain momentum" with the opposite direction as the photon. Of course, without...
  6. Afina

    B Scattering of light in a strange fashion

    :smile::smile::smile::smile:Have you ever observe in night a source ( spherical) emitting light in strange fashion. If not then observe today. I saw that it was more towards the corner making plus sign or cross sign which can be seen through our eyes . Is there is something which is attracting...
  7. B

    Mass, Light Speed, and the Mystery of Photons Explained

    A particle without mass is supposed to travel at speed of light. But isn't it that a photon has no mass but has relativistic energy.. therefore why does it travel at speed of light.. or perhaps if a photon had no relativistic energy.. it would travel a bit faster than the current lightspeed?
  8. W

    B Light spectrum of planet Earth as seen from space

    Has the spectrum of light that is reflected off planet Earth ever been measured from outer space? (In the same sense that we measure spectra of the light emitted/reflected from other celestial bodies in astronomy). If so, would it be possible that there could be dips in the spectrum resulting...
  9. C

    Condition for circular-wire polarizer to transmit no light

    I want to make a wire grid surface that transmits no light. For that I wanted to use quarter-circular arc wires. My question is what dimensions would I need to apply to the wires, would an arc with radius = wavelength of incident light be sufficient (since then it is the size of a wavelength in...
  10. S

    B Why do photons stimulate emission in atoms?

    I can't get my head around stimulated emission.Why does a photon traveling towards an atom stimulate emission? The best way I could answer is that a photon has an electro magnetic field and this causes an excited atom to oscillate.This induces an excited electrons to fall down an orbital. But...
  11. C

    Where do antireflective coatings reflect light?

    Lets consider an index matching antireflective coating on a conducting substrate (i.e. salisbury screen). The waves reflected destructively interfere in the direction of specular reflection. However, the energy has to be conserved, and since it is not transmitted, it most me reflected in a...
  12. zdcyclops

    B If the moon were a light bulb how many watts would it be.

    If the moon were a light bulb how many watts would it be.
  13. S

    B Light Clock Traveling in Z Direction

    Hi, I'm studying GR these days. But then I got to thinking about something about SR and got kinda stuck. I know that if a light clock is moving in the x direction(from left to right), the light beam has to traverse more space and then you will see this person's clock running slower in the...
  14. C

    Does a Single Metal Stripe Reflect Polarized Light at the Angle of Incidence?

    I know that only light with polarization in the plane of the "stripe" gets reflected. What i need to know is if a single metal stripe could reflect such polarised light at ONLY the angle equal to the angle of incidence or do only a whole array of stripes produce specular reflections? Sources...
  15. Rigel84

    B Gravitational Lensing: Gravity's Effect on Light

    This is my first (well second if we exclude introduction forums) topic here. I just want to make clear that I am not professional nor involved in field in any way. I am just regular Joe who wants to know more about nature. I could say that I understand fact that mass distorts space but there is...
  16. C

    Is it possible to see objects at 600m distance in infrared

    Hi I am wondering if it is possible to see the full distance of a 600m beam from a 500lm torch with an infrared filter and infrared goggles?
  17. ISamson

    Faster than light: Instantaneous lever

    Hello. I have been thinking on faster than light things. I thought about: chemical engines, lasers, wormholes... All that kind of cool, advanced physics. But, I stumbled on faster than light mechanical actions. I thought about a lever: first class, if you wish, if you apply a force to one side...
  18. C

    I Time Delay on Multiple Photons when travelling from one plac

    I'm trying to understand the time delay induced on each photon when several individual photons travel in an open space from a sender to a receiver for example in the application of Quantum Key Distribution. So what I understand so far is light(photon) travel around 299,792 km (186,282 miles) per...
  19. J

    B Speed of Light & Time Dilation: Travel Distance in 1 Sec?

    10 seconds somewhere can be one on Earth due to time dilation, right? So In the case of distance wouldn't light have traveled more than 186,282 MPS since it had 10 seconds to travel relative to the 1 second on earth. So if light traveled at 186,282MPS for 10 seconds it would have traveled...
  20. M

    Light Bulb in a Box: Velocity Change After Release of Photon?

    Homework Statement [/B] A box is traveling with a certain velocity in space. Inside the box is a light bulb, which is capable of releasing one photon of light. The light bulb harnesses the energy from the box; it does not require an outside power source but instead uses the energy on the...
  21. scottdave

    Death by light saber - YouTube video

    I came across this YouTube video describing some of the "science" behind light sabers, and what it would be like to be hit with one. I thought it was worth sharing.
  22. Chris84567

    I Measuring Speed of Light Relative to Earth

    The question: If the max speed I can go in the universe is the speed of light measured from? Background: If there are three rocks in a vacuum along a line (the x axis), one is stationary (rock 1), one is moving 5 m/s in the x direction (rock 2) and one is move -5 m/s in the x...
  23. aatari

    Light wave Interference - Angler Fish Problem

    Hi Guys, I have attempted to solve the following question and was hoping someone can have a look and give some feedback. Any help will be greatly appreciated. Thanks! 1. Homework Statement Angler fish emit light through bioluminescence. An angler fish is swimming in a fish tank that has a...
  24. B

    Combustion Emit Light: Reasons & Benefits

    Why does combustion emit light? For heat, for excited electrons or for other reasons?
  25. Vivek98phyboy

    Rectilinear propagation of light

    How is it possible for different parts of a single rectangular wavefront to produce different intensity at particular point(P) despite the fact that wavefront is something which has all the particles at same phase?
  26. S

    Emergent light produced by two lenses

    Homework Statement Two thin lenses are placed 5 cm apart along the same axis and illuminated with a beam of light parallel to that axis, The first lens in the path of the beam is a converging lens of focal length 10 cm whereas the second is a diverging lens of focal length 5 cm. If the second...
  27. Richie Smash

    B Faster than Light and wormholes

    If the concept of a wormhole is like folding a piece of space-time like a piece of paper, for near instantaneous travel faster than light, what is keeping us back right now? I am aware that it's really complicated and far beyond my scope of understanding right now, however I am fascinated by...
  28. J

    Reasoning for photon having no mass

    Other than relativity is there any theory, proof, experiment, etc. that tells us a photon has no mass? i.e. Is the concept of zero mass solely derived from relativity
  29. E

    Interference of Multiple Waves: How to Determine Phase for Maximum Intensity?

    Homework Statement Four identical wave sources (s1,s2, s3,s4 ) are located at the corners of a square. We know the phase at three sources: 0 at s1, π/4 at s2, π/2 at s3. Whet is the phase we have to give to s4 in order to have a maximun of intensity at the center of the square? Homework...
  30. N

    What is SSL Lighting and How Does it Impact Our World?

    Hi everybody, I'm a curious worker and passionate student, I haven't got the possibility to study at the university but my passion for SSL lighting and light phenomena in general started some years ago, has helped me to start a long journey, trying to understand a little more day by day of what...
  31. paulo84

    Electrons move faster than the speed of light?

    In school, I was taught that the position of an electron relative to the nucleus is only an approximation. It is very likely that the electron will be close to the nucleus and where it is 'expected' to be, but it may in fact be anywhere in the universe. Is this correct? And does this allow...
  32. Q

    Light bulbs best for seedling growing?

    I have heard that LED light is not as good for growing plants as incandescent or fluorescent, can someone explain this? And what is the best color or wavelength to keep seedlings from getting all tall and falling over...? or is it the intensity?
  33. R

    Macroscopic Maxwell's equations and speed of light in media

    So I followed the derivation of the Macroscopic Maxwell's equations by averaging the fields / equations and doing a taylor series to separate the induced charges and currents from the free ones. But why does light now "suddenly" travel slower in dielectric media? I mean, sure, it comes out from...
  34. Q

    How light rays from a fireplace wood fire on Earth compare to the Sun's fire?

    While lying in front of a 500 degree woodfire, I wonder if besides light and heat that we take as ordinary does it also have ultraviolet rays of light like the sun?? What are the types from light from an ordinary on Earth fire?? Would they be beneficial to plants as main light source?, would...
  35. Paul Colby

    I Measuring Light Speed Anisotropy

    Sorry, me again. In another thread #47 I gave a experimental setup to detect an anisotropy in light speed of the form ##c(\hat{n}) = c_o + \alpha (\hat{n}\cdot\hat{v})##. It was pointed out quite correctly that the error incurred in the long arm to the sensor would cancel the anisotropy making...
  36. bremsstrahlung

    I Does light exist between events?

    Does light exist between events? [mod: unapproved source] The above perspective will resolve a lot of paradoxes in QM. Is the above view viable or is it untenable? Are there any evidence to show that photons exist in flight? Are there any evidence of photon trajectories?
  37. Mohammad Fajar

    B Why Light Experienced a Doppler Shift?

    It's already known in physical communities that the velocity of light always constant and not depend on motion of observer or source. But it is also teached to us that there are aslo Doppler shift experienced by light wave like that when measuring CMB radiation. But for me it is makes a...
  38. J

    Why is the photon's mass zero in classical equations for light?

    Mentor's note: this thread has been forked from https://www.physicsforums.com/threads/why-the-photon-has-no-mass.899792/#post-5912751 But momentum = mass x velocity so that part of the equation is E2 = c.(mv) If the mass is zero then that formulation also yields zero energy (which we know is...
  39. S

    Understanding Malus Law and Polaroid Intensity: Conceptual Doubts

    Homework Statement It is just a conceptual doubt Homework Equations According to malus law intensity of light through polaroid is I=(Io)cos^2(theta) where theta is the angle between axis of polaroid and incident light. The Attempt at a Solution But when Theta=0 then I=Io but in my book it is...
  40. T

    What is light exactly? Electromagnetic wave or photons?

    When why sey that light is electromagnetic wave i understand this. But what i do not understand is where photons come into picture ? can somebody explain me the relationship between those two . My knowledge in this respect is not great so I would ask that the answers be adjusted .
  41. GAURAV DADWAL

    B Light Propagation: Doubts Answered Here

    Hello There:woot:, I have basic doubt about light propagation .So I'm hoping to get some help here. The problem is ...Let us assume two frames, 'MOVING FRAME'[one moving with a certain velocity(constant)] and 'REST FRAME'[one wrt to which the first frame is moving]. If now moving frame emits a...
  42. S

    Measurement of the Speed of light

    Hi, What is people measuring when they measure the speed of light (c) actually? the photons or the waves? or some kind of manifestation of light? I ask because I was reading about photons and that they "might" have a very small mass and therefore are not traveling at c. If photons have a...
  43. Abhimessi10

    What is the Number of Fringes in a YDSE with Given Parameters?

    Homework Statement Find number of fringes separated by 1 cm on a screen at a distance of 2mm from slits which are 0.2mm apart from each other.The wavelength of light used is 500nm. Homework Equations The Attempt at a Solution I really don't know any way to solve it.I have tried putting unit...
  44. Domenicus

    How to project a high power white line

    Hi I have a personal art project and I am stuck, because of very limited knowledge. I'd like to project an 1-2 meter white line from 1-3 meter distance. The projected line should be as thin as possible (3-10mm) and as powerful as possible. I created a DIY projector, but my problem is that, the...
  45. H

    Eye's response to electromagnetic (light) waves?

    Does the eye's retina, rods and cones detect the E or B or both components of a lightwave? I've not been able to find much Googling. Thanks Hank
  46. stefan3423

    2 Polarizers attenuating a light beam

    Homework Statement An angle is given between 2 polarizer's (45 degrees), through them light passes (unpolarized than after passing through the first one it polarizes), some of the light its shown on the display. For how much does the angle needs to be increased for the intensity of light to be...
  47. S

    Device with which I can measure the frequency of light

    Does anybody know where i can buy a device with which i can measure the frequency of light from various sources such as bulb light?
  48. T

    Light Signal Travels Between Earth and a Spaceship

    Hi All, An observer, F, stands on Earth. A spaceship, F', is also on Earth. Their clocks are set at 0. The spaceship then leaves Earth at .5c. After 10 seconds, F sends a light signal to F'. As soon as F' receives the signal, F' sends a light signal back. When does F receive this signal from...
  49. SW Dad

    UV light + electric current = boom?

    Hi, so I was hoping to get an EE's explanation for what just happened. So my son was apparently over by the light switch pointing a UV pen light at the switch. Suddenly there was very loud, gunshot-like pop, sparks flew, and I had to go reset the breaker. Maybe correlation doesn't imply...
  50. C

    I What causes disturbance on my phone camera?

    Hi I have a cheap smart phone with a low quality camera when I go to take a picture there is what I would describe as like old television static appearing in front of the image as it appears on the phone screen (although what is beyond it can still be seen). I am wondering if this is just dust...
Back
Top