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. DaveC426913

    Can Light Experience Doppler-Equivalent Slingshotting Near Massive Bodies?

    Someone asked an intriguing question: Does light passing close to a massive body experience the Doppler-equivalent of slingshotting? i.e. blue-shifted or red-shifted, depending on its path? (I guess the light would be acting as the third body in a three-body system?)
  2. D

    The microscopic expl. of how light is slowed in materials

    In wikipedia I found: If what I underlined is correct, it means that from one incident photon we will get at the other end countless similar photons, as the "shaken" charges radiate "their own electromagnetic wave" (photons, right?). This sounds like light amplification, but the explanation...
  3. ORF

    Do carnivores have a higher proportion of light isotopes than plants?

    Hello, because of the "kinetic isotopic effect", https://en.m.wikipedia.org/wiki/Kinetic_isotope_effect can the ratio of the common isotopes of N, C, P, H, O, S determine the position of an individual in the food chain? (eg, have carnivore animals higher proportion of light isotopes than...
  4. Hieu

    Optical Making a vein viewer with LED lights

    I'm trying to optimize my vein viewer with LED lights but I'm not sure which factors affect the penetration of the light through the skin. (I'm following this guide https://www.instructables.com/id/How-to-make-an-affordable-Vein-Finder-for-use-d/) I can say those factors are the angle of the...
  5. pixel

    I Bending of light - Newton vs. Einstein

    Using Newton's equation for gravity and assuming a corpuscular theory of light, one can calculate the angle that light would bend in a gravitational field. General relativity predicts a bend that is twice as large. In the Newtonian limit of GR (which includes weak gravity), does the GR...
  6. gleem

    Light sabers from the movies to the piste

    Well maybe. The International Fencing Federation (FIE) ishttps://www.msn.com/en-us/sports/more-sports/fencing-body-interested-in-frances-embrace-of-lightsaber/ar-BBTOQ6y?li=BBnba9I progresses. Apparently this sport is popular in France and has caught the attention of the FIE, How long will...
  7. askbbj

    What happens to a light photon when it hits glass?

    Homework Statement Using quantum mechanics, what happens when a photon of light hits glass? Homework Equations Momentum= Mass x velocity The Attempt at a Solution I am not sure of my answer but suffice to say the following: Assuming the collision between the photon particle and those in the...
  8. M

    Perceiving white light from Lightbulb Illumination

    What is exactly hitting the retina for us to perceive white light in a room radiated by light from a lightbulb? The light from the lightbulb caused by heat radiates wavelengths randomly through the space around it. But how could it be that the superposition of these random frequencies and...
  9. H

    Electric arc: How to calculate the required voltage for a light bow?

    <Moderator's note: Moved from a homework forum.> Calculate the required voltage to produce a electric arc between 2 iron nails (distance: 3cm). I´ve read in the internet that you need 1 kV per mm. But how can I calculate this value, that I need 1kV per 1mm?
  10. DarkMattrHole

    I Question about the limits of space contraction near light speed

    Hi all. I have a question about something Nima Arkani-Hamed said in his lecture on space-time about space contraction near light speed. I included a link to the lecture at the point where he refers to contraction of two space ships with a 'cable' between them, they are accelerating towards the...
  11. H

    Why do higher frequency light waves bend more in a prism?

    For example, why does blue light refract more than red light in a prism?
  12. A

    Can you make a battery out of light slowing materials?

    I've read about light slowing materials, and I'm wondering if you could slow down light enough, could it be used to say store light from the sun during the day so solar panels could work at night? Or would it not work because the process of slowing light down gradually absorbs some of the...
  13. D

    Stargazing Seeing star light -- Why does it take so long?

    O.K. I'm new here so if this has been answered I don't know how or where to find the answer. I'm not a scientist just a mechanic so if you could explain this simply maybe i can understand it. Maybe ELI5. Light I see in the sky has been traveling for a long time and "is here now". As far as I...
  14. S

    Is the EM field the medium for light waves

    EM waves can propagate through empty space, but there is also the EM field. Is this field really empty space or is there something that exists that the light wave disturbs, like some kind of fluid? In areas of space where values of the field is 0, is this like empty space or more like calm...
  15. A

    I Speed of Light in Materials: Explained

    It is commonly said that the speed of light when traveling inside materials is lower than that of light in vacuum, but I don't understand how this can be true. It is the same light traveling, so how can it act differently? Does light appear to be slower in materials because it is not following a...
  16. laurieke

    Wavelength of light in a dielectric material

    Homework Statement In a physics lab, light with a wavelength λ0 travels in vacuum from a laser to a photocell in a time t1. When a slab of glass with a thickness d is placed in the light beam, with the beam incident along the normal to the parallel faces of the slab, it takes the light a time...
  17. JD_PM

    Size resolution limitation on Dynamic Light Scattering

    I have been looking online and it is stated to be 1nm, but these are posts around 6 years old: https://www.azonano.com/article.aspx?ArticleID=3662 https://www.researchgate.net/post/Whats_the_measurement_limit_of_dynamic_light_scattering I am wondering if the limiting resolution has improved...
  18. skyshrimp

    B What am I Missing Here? Play from 3:00

    Play from 3:00 It states that we will age slower if we were traveling at the speed of light compared to being on Earth. Would we not age the same but just be further away hypothetically? Next it states that if we travel away from a clock at the speed of light at 12:00, we would always see...
  19. hagopbul

    A Can multiple layers of thin films increase infrared light absorption?

    hello : i have a small question , can we use multi layer thin films to increase absorption of infrared spectrum , on certain surface , ? taking in mind that we use the same kind of thin film for each layer . is that possible ? best hagop
  20. TachyonLord

    Fizeau's Experiment (Speed of light)

    Homework Statement In the Fizeau's Experiment to determine the speed of light, let the gear have N teeth, the frequency of the rotating gear being f, the distance traveled by the light beam/ray L (distance b/w the gear and the mirror) and let there be n eclipses(blocking of the light beam)...
  21. fresh_42

    Illustration about the "relativity" of light

    I just found this on the internet. It reminded me of a joke I once made: https://www.physicsforums.com/threads/science-jokes-p2.847743/page-8#post-5603278 It's a nice illustration about the "relativity" of light - kudos https://www.linkedin.com/in/james-o-donoghue-astronomer/
  22. Wrichik Basu

    Classic double-slit experiment in a new light

    An intense beam of high-energy X-ray photons (violet) hits two adjacent iridium atoms (green) in the crystal. This excites electrons in the atoms for a short time. The atoms emit X-ray photons which overlap behind the two iridium atoms (red) and can be analyzed as interference images. Credit...
  23. J

    Why does PPG (photoplethysmography) Use Infrared and Green Both Light

    Hi Everyone, I'm doing research in Implanted Devices in Human Body (Sensors, Hearing Aid, Pacemaker) for my college course. I've started with FitBit that uses the PPG Principle. But I'm struggling to understand why the FitBit uses both Green and IR LED whereas the sensor at the receiving end is...
  24. Boltzman Oscillation

    Question: What happens to absorbed light?

    Suppose I have two apples, one is white and the other is black. Both apples are exactly the same except for their colors. The black apple is darker than the white apple because it absorbs more energy than the white apple, correct? Would that mean that the black apple would have a higher energy...
  25. G

    I Why does a mercury Franck-Hertz tube produce visible light?

    Hi. I've recently conducted a Franck-Hertz experiment with mercury. I was able to see bluish glowing regions just as here: However, theory predicts 254 nm, which is far below visible. Are there other energy levels at play here? Wouldn't that mess up the 4.9-V-spaces of the drops in the current...
  26. PainterGuy

    The speed of light in glass and water

    Hi, The speed of light is given c=1/√(ε0μ0) where ε0 is permittivity of vacuum and μ0 is vacuum permeability. The permittivity and permeability of a material is given as ε=εrε0 and μ=μr)μ0 respectively where ε is absolute permittivity of the material and εr is relative permittivity of the...
  27. B

    Why is light emitted from an object when it is heated?

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  28. B

    I Info traveling faster than light, and quantum entanglement

    OK, going to ask a question that I sort of know is going to be shot down but at the moment I can't make sense of this. If I send a machine/robot with a particle that is quantum entangled with another particle that is left on earth. When one particle is blue the other is red. The machine also...
  29. A

    Normal incidence of light on a denser medium

    When light, traveling through a rarer medium, gets reflected off at an interface with a medium of higher refractive index, it suffers a phase shift of half-wavelength. Now if it was normal incidence, the phase-shifted light would retrace its path. Does that mean the wave would cancel out?
  30. Dipra Irham

    Why Does a Prism Periscope Focus Light Instead of Scattering It?

    a prism is supposed to scatter light rays and separate it into rainbow,then why in a prism periscope the prism send light rays to a particular direction instead of scattering?
  31. The brachistochrone problem

    The brachistochrone problem

    Brachistochrone problem’s original proof by Snell’s law and a story of Newton.
    • YoungPhysicist
    • Media item
    • brachistochrone problem light
    • Comments: 0
    • Category: Calculus
  32. R

    I Polarization of light and probability

    I Dont understand how light polarized along a certain axis in a prepared state can then pass through a second polarizer that is at an angle to the first. The experiment I’m trying to understand starts with unpolarized light which then passes through a first filter which vertically polarizes the...
  33. S

    Infinite light polarizers in an array

    I was wondering if I placed an infinite number of light polarizers in an array, each rotating an infinitesimal amount from the next, would I be able to get 100% of the photons shun thru them on the other side?
  34. G

    Electrical Flourescent light ballast wiring

    Ok very much amateur here, appreciate any advice. So, the guy at home depot suggested the ballast was the reason my flourescent kitchen light goes out occasionally. The light worked well for the first 3 years of ownership. Now, it doesn't buzz or flicker, it just occasionally doesn't work...
  35. iVenky

    What's the mode of light in single-mode fiber?

    Hi, We talk about single-mode and multi-mode fiber and I assume they talk about the way of sending just one modes (TEM or TMmn or TEmn). Which mode is generally used for single-mode fiber? Is this set by the geometry of the single-mode fiber (especially the opening)? Also, this means that the...
  36. P

    B Speed of Light -- Why doesn't it add to the speed of the emission source?

    My question is about the speed of light. Our current understanding of light says that light is constant for all observers, and uses time dilation to explain this. Have we proven this? The speed of light emitted from a stationary object is equal to C. The speed of light emitted from a moving...
  37. Frederick T

    B Travel at 0.5c & Observe Redshifted Light Traveling at c

    It's come to my attention that if we were to travel in a spaceship at 0.5c directly away from our sun, I would observe that the light from the sun as being red shifted. What I would also observe was that this red shifted electromagnetic wave would in fact travel away from me in the spaceship...
  38. M

    On Rayleigh Scattering and UV Light Absorption

    I've read that UV light cannot "penetrate" the atmosphere as easily during winter. But what does this exactly mean? My hypothesis is that due to rayleigh scattering, since light with smaller wave lengths scatter more, the UV light doesn't end up reaching the surface of the Earth (i.e. gets...
  39. T

    Calculation of Light Intensity

    Hi all, I would like to calculate light intensity of a bulb from 0.5 meter. Which formula shoul i use? I have a PV. Dimensions are 12x12 cm. Bulb is 100 Watt. Thanks
  40. Johnnyallen

    Polarized Light -- Idea for headlight safety

    I've been a semi-pro photographer for a long, long time. I know the value of having a polarizing filter in my camera bag. I also wear polarized sunglasses for driving during the day. You can't beat 'em. I know some people who are complaining about these LED head lights from oncoming cars...
  41. J

    Understanding Light Reflection: Exploring Photons and Reflective Surfaces

    Simple question. Are photons reflected as is from a surface like a mirror, or is the reflecting surface atoms capturing the photons and re-emitting them?
  42. M

    B Layman question about faster than light communication

    Okay, first of all so I'm in no way educated in the concepts and especially the notation of quantum physics; my knowledge is confined to a very simple superficial understanding. However, if someone could educate me about why faster than light communication is impossible in the scenario I'm...
  43. Srihari05

    I Exploring Feynman's Theory on Reflection of Light from Glass Surfaces

    I recently started reading Feynmans book QED. There are a couple of questions I have regarding his theory on the percentage of light that is reflected of two surfaces of glass. My question is as follows, A piece of glass in fact has four surfaces. The front of the glass the back side of...
  44. L

    Does Gravity Deflect Light according to Newtonian Mechanics?

    I have read in several popular physics texts that general relativity predicts that gravity deflects light, but that Newtonian mechanics, in contrast, predicts that the trajectory of light is not affected by gravity. However, I am very skeptical and confused about this result. We of course have...
  45. Ennio

    I Expanding universe or tired light in a static universe?

    Has anyone read the book of this author? What is your opinion about this topic? https://www.google.de/search?q=tired+light+lyndon+ashmore&source=lnms&tbm=isch&sa=X&ved=0ahUKEwjbo_exj5vfAhXK2qQKHdHsD6AQ_AUIDygC&biw=1280&bih=623
  46. Spinnor

    Freezing fog creates rare light pillars in Wisconsin on Sunday

    From, https://flipboard.com/topic/crystals/freezing-fog-creates-rare-light-pillars-in-wisconsin-on-sunday/f-ca02faf45c%2Faccuweather.com For physics see, https://www.nytimes.com/2017/01/06/science/light-pillars.html
  47. Swedeson

    B Discuss Dark Matter & Light: Photons Affecting Space-Time?

    I would like to discuss the topic of light (photons) being emitted from stars potentially being the source of dark energy and dark matter. Photons have mass and velocity; therefore they will affect space time, and their impact could have an effect on objects causing acceleration. Would love to...
  48. I

    Optical Path with a light source inside a gradient medium

    Hi, I am wondering if anyone could point be to any references for gradient optics. The current literature seems a little haphazard. The model I am looking for would need to consider the following. Single medium whose optical density (index of refraction) is some gradient index of refraction...
  49. wlc88

    B The Red Light Shift and Space Debris

    Hi, As I understand it the theory that the universe is expanding is in part based off the red light shift. My question is could atoms in space, dust etc cause the red light to travel faster. I realize that space isn’t a medium, but could the aforementioned atoms and dust exist in such...
  50. L

    I Can two inertial frames differ by the speed of light?

    I know that special relativity is based on the postulate that an object that travels at the speed of light with respect to one inertial observer must travel with the speed of light with respect to all inertial observers. This postulate implies that no two inertial frames can differ by a relative...
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