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Theories of Light

  1. Aug 8, 2008 #1
    hey everyone, My names aaron and im a year 12 student studying physics. We have to learn about both the wave and particle theories of light, and i just have a few questions.

    I have read that light behaves as if it has a mass, and although we cant actually 'weigh' the light we beleive its mass to be one molecular layer of oleic acid. Im not too sure if i have the terminology correct or not but heres the guts of my question, If the light has a mass, Shouldnt it be affected by gravity?

    And if this is true then would the light itself speed up as it gets further away from the sun? I was wondering this because we learnt about gravitational feilds last semester and the further away from the gravitational feild an object is the less force towards the center of the gravitational feild, Therefore shouldnt the light have less force acting on it the further it gets from the sun? And does this mean that the speed of light gets faster the further from the sun it gets?

    I apologise if my terminology isn't correct, Im not too sure what acid i made reference too in paragraph 2 is not correct but thats the one that sprang to mind when i thought about it, your answers are appreciated:)

  2. jcsd
  3. Aug 8, 2008 #2
    Hi Azza,

    First of all, light definitely does not have mass. In the particle theory where light is described as made of photons, the photons all have zero mass.

    On the other hand, light is affected by gravity, even though it has no mass. Gravity cannot change the speed of a light wave, but it can change it's direction. This is called gravitational lensing, because a massive object like the sun can bend light similarly to a glass lens. This was predicted by Einstein's theory of gravity, and was first confirmed by measurement by Arthur Eddington in a 1919 solar eclipse. This surprising prediction, that gravity curves the path of light, gave Einstein a lot of fame when it was measured to be true.

    To be clear on your second question, gravity does not affect the speed of light but only its direction.
  4. Aug 8, 2008 #3
    ok thanks that helped clear up a few things, but i didnt say it had a mass lol i asked if it behaved as if it had a mass. any other answers are more than welcome
  5. Aug 8, 2008 #4
    Sorry, I got that from this part of your post:

  6. Aug 8, 2008 #5


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    Welcome to PhysicsForums, Azza!!!

    Light has energy (and therefore mass) proportional to its frequency according to the formula:


    ...Where h is Planck's constant and v is the frequency (sort of like the vibration of a guitar string, this is the number of cycles per second). It does not have a "rest" mass (which can be a confusing statement given the above).

    Light comes in a wide variety of frequencies (visible light and radio wave are all the same kind of particles, namely photons). I don't think the reference to Oleic Acid is worth following up on, there is no real analogy there.

    Light is affected by gravity as described by Einstein's General Relativity (don't confuse that with Special Relativity, which is a subset of GR). But it still always travels at c, the speed of light, whenever it is measured in any inertial frame of reference.
  7. Aug 8, 2008 #6
    In response to the last post by Crosson
    Dr Chinese, So light does indeed have a mass? you refer to frquency so i beleive your talking about the wave theory? Is there a uniform meausrement that we can use as the mass of light?
  8. Aug 8, 2008 #7
    ohh ok thanks alot, i think i was getting confused a bit because i thought it had momentum therefore it had mass lol,now for another question, is it possible for the light to have a rest mass and us not able to measure it? or is the light somehow dissapated as soon as its stops moving?
  9. Aug 8, 2008 #8
    Hello there,
    No no light definitely does not have mass! :)

    Look at it this way. Both light and matter are forms of energy and both carry momentum. But the biggest difference is that light always travels at the same speed - c - while matter can be accelerated or slowed down. Other than that minor detail - and the fact that they don't respond to the electroweak or strong forces - photons behave quantum mechanically like any particle would.
    Last edited: Aug 8, 2008
  10. Aug 10, 2008 #9


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    In case anything I said made anyone think otherwise:

    1. Light does not have a "rest" mass - as opposed to particles such as electrons, protons or neutrons which have a rest mass, and cannot be accelerated to c.

    2. Light does have energy associated with it, and because it moves it has momentum. This implies there is an inertial mass proportional to its frequency. A photon will "bend" around a massive object (like the sun) according to General Relativity. The amount of bending is affected by the amount of the inertial mass.

    3. According to Special Relativity, all observers will agree that photons move at c.

    So you will sometimes find that the above leads to some problems in the discussion of these terms, as it makes a difference which mass you are talking about, and which form of relativity you are talking about. Most often, photons (light) are described as massless to distinguish them from other particles that do have a rest mass AND also have an inertial mass. Particles with a rest mass cannot be accelerated to the speed of light c.
  11. Aug 10, 2008 #10


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    I thought the amount of gravitational bending is independent of frequency (or wavelength), that is, it has no dispersion.
  12. Aug 11, 2008 #11

    ok but for an object, for arguements sake a photon, will 'bend' around the sun doesnt this means thats its turning? and if so one side of the photon would have to be traveleing faster than the other side correct?
  13. Aug 11, 2008 #12
    "One side of the photon"? A photon has no sides. And even if it had, that reasoning would be correct in an euclidean geometry, but the geometry it's not euclidean...
  14. Aug 11, 2008 #13


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    That is a classical notion. However, spacetime is not considered to be "flat". The photon thinks it is traveling in a straight line.
  15. Aug 11, 2008 #14


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    Ah, you might be right (which would make me wrong). :)

    I looked up the formula for gravitational lensing, and wavelength (actually the photon's inertial mass) does not show up as a term in the calculation. I had it in my mind it did disperse ever so slightly (perhaps insignificantly compared to the object doing the bending). So my apologies and thanks for the correction.
  16. Aug 11, 2008 #15
    Hi Azza, you are a man of sound common sense.

    There are different kinds of "mass". Like everyone else said, light doesn't have rest mass. But it does have "relativistic mass" because it moves at the speed of light. This means that it has "inertial mass" and "gravitational mass" (Einstein said that inertial mass and gravitational mass are equivalent, although this issue is sometimes debated). Therefore, light is, indeed, affected by gravity.

    In addition to curving around massive objects, it experiences changes in frequency (and wavelength). As light gets further away from the sun, its speed stays the same BUT it experiences "gravitational redshift" (as viewed by someone in a gravitational field less than that of the sun). In other words, the wavelength of the light gets longer as it moves away from the sun, because it loses energy as it escapes the sun's gravitational field.
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