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Light question

  1. Aug 13, 2004 #1
    Hi all -

    This is the first time I have posted here, and at this point I have a very limited understanding of physics, and science in general, but I have found much of what I've read here (the parts which I could follow) to be very interesting.

    I have a very simple question, but it's something I don't really understand, so I'd appreciate some feedback if anyone can spare a moment...

    I understand that there are physical laws which state that matter as well as energy can be neither created nor destroyed, but that they simply change form.

    I know light has unusal properties, something about being both a wave and a particle, and that nothing can travel faster than it as it is a physical impossibility. So if I were driving a car at close to the speed of light, the headlights in the front would still be lighting the way as the light emitted from them would be ahead of me - right?

    My general question is what happens to light as it dissipates, or "fades"?
    That is, picture someone standing in a dark wooded area with no light source other than a lantern hanging on a tree branch. The light is moving away from the lantern at the speed of light, thereby lighting the surrounding area, but as you move away from it, the world gets darker and darker (ie, the light fades).

    I know we can see stars as they were years ago from billions of miles away and I assume this is because of their incredible luminosity, the light has enough energy to get here, but about 500 feet (let's say) away from the lantern the light has completely vanished.

    OK - So here's my actual question:

    I believe that light has no mass, correct me if I'm wrong, so I assume it's pure energy, and nothing but. What is light "made of" which gives it the properties of being visible, and if it loses these properties (or energy) after a certain distance (or time?) like the lantern light, where does that energy go and what is replacing it ?

    Thanks for your patience, I'm so new at this type of discussion.

  2. jcsd
  3. Aug 13, 2004 #2
    Light has a limited amount of energy which falls off as the inverse square law. At one foot lets say the light energy level is one. At 100 feet the energy level is 1/10000, at 1000 feet the energy level is 1/1000000, much smaller. It might be too dim to see clearly at 1000 feet. Hopes the helps. Wes Hughes
  4. Aug 13, 2004 #3
    Nice to see another Torontonian...
    Yup. The speed of light is constant, so it doesn't matter it your car is travelling at 99% of c, the light from your headlights still races away at the same speed as if you were sitting still.
    An excellent belief
    As far as what light is made of, it depends on who you talk to. In quantum theory, the discrete packets of energy forming light are called photons with are described by a probability wave, massless particles that act as messengers for the electromagnetic force. String theory says these particles are the result of a tiny oscillating "string" under tremendous tension, and its properties are the result of the way it's vibrating. These properties include the way they interact with objects, allowing us to see.

    Light doesn't lose it's properties of being visible as it travels greater distance, or lose energy. It spreads out. Or more specifically, a point source spreads out, which is the kind of light you are most familiar with. The reason that the farther you are from a light source, the dimmer the light appears, is because light spreads out on a spherical wave front. Since the motion of light can be described with waves, picture dropping a stone into water. The ripples surrounding the stone start out small and increase in size as they move away from the disturbance. This is also how light travels. Because the total energy of the wave must stay the same (conservation of mass/energy), then as the ripples spread out their height or amplitude is decreased, which is why at a certain point they seem to disappear- they're just to small. Likewise, in order for the total energy of a light wave to stay the same as it spreads out, it's intensity must drop. Since intensity is what determines brightness, the light appears dimmer when you move away from it. The relationship that determines the intensity, or brightness, of a light source based on how far you are from it is called the inverse square law, as Wes has already said. The law says that the intensity of a point source (this applies to things other than light, it applies to sound, the stone falling into the water, even the Newtonian equation for gravity) falls off as the inverse square of the distance from the source. In other words, if you double your distance from the light source, the intensity will drop by a factor of four. Again, this isn't because energy is disappearing- it's just that the area of the sphere of light surrounding the light source is four times greater at twice the distance, hence the energy is spread over an area four times larger and so the intensity must drop by a factor of four in order that the conservation of mass-energy isn't violated. In the example of the stone falling into the water, the circumference of the circle of waves increases by a factor of four when you double your distance from the source, hence the height of the waves is quartered. The inverse square law is often written as
    where I is the relative intensity and r is the radius of the sphere/circle surrounding the point source, or equivalently your distance from it.

    It's important to mention that not all light follows the inverse square law, only point sources. Lasers don't spread on a spherical wave front, at least not a perfect laser. That's the why they're so useful- they travel in almost perfectly straight lines.

    Hope that helps.
  5. Aug 13, 2004 #4
    That totally helps. I guess my main problem was that I was picturing light fading away, rather than spreading out. Makes more sense now, when you spread a teaspoon of butter over a piece of texas toast it looks thinner and eventually seems to dissappear if the toast is big enough, but it's still the same amount of butter after all...

    Thanks very much for your feedback !
  6. Aug 13, 2004 #5


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    "I feel thin, worn out, like butter scraped out over too much bread.."

    Bilbo B., old gent
    Last edited: Aug 13, 2004
  7. Aug 14, 2004 #6
    Oh, slight correction. Not that it changes anything, but in the stone in water example, the inverse square law doesn't apply since there's no r^2 in the equation for circumference. Twice the radius equals twice the circumference. It's easy to see why the inverse square law appears to spheres though- the equation for the surface area of a sphere is of course SA=4(pi)r^2...so you can mathematically see why doubling r quadruples SA. Anyways, just thought I'd correct myself before someone else did.
  8. Aug 14, 2004 #7


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    Ah, a JRRT fan, I see.

    Sidewalk, there's just one more small detail that you might want to keep in mind. Lastonestanding has given a very nice explanation for why the light fades away, but there's one little thingy that he left out, perhaps to maintain simplicity.

    There's a difference between light travelling through vacuum (light from stars) and light travelling through some medium (light from lantern travels through air). Light from a point source will perfectly obey the inverse square law in vacuum. However, in air, or some other material medium, there will be an additional loss of intensity because of light being partially absorbed by the particles (atoms, molecules, etc) of the medium. That's why the lantern is harder to see on a foggy night.
  9. Aug 15, 2004 #8
    Of course, my mistake. Thanks for pointing that out!
  10. Aug 15, 2004 #9
    That's difficult to answer without a definition of "matter." But if one were to use the definition given by Einstein in his 1905 general relativity paper then yes, that is correct.
    To be precise - light may be considered to consits of particles, i.e, photons. As with all particles, they behave like waves under some circumstances and particles under other circumstances, but they never behave like both a particle and a wave simultaneously.
    I dunno. I saw Star Trek last night and they seemed to be able to do it. :biggrin: Seriously though, there is some speculation that spacetime may be manipulated in such a way as to circumvent the limiting speed of light and get from point A to point B faster than a photon would if it is simply traveling through space. There's some talk of wormholes and warp drives within the general relativitiy community.
    Stand in the middle of a parking lot with a ton of marbles and throw them in random directions. The closer a target is to you the more likely it is that it will get hit by a marble. Think of the marbles as photons and if yopu understand this then your question is answered.
    There is a thread in the "Special & General Relativity" forum called Those who use relativistic mass and why. I think that thread we discussed this question. To be 100% precise, light has zero proper mass (aka "rest mass") and a non-zero and finite relativistic mass aka inertial mass. For precise definitions of these terms please see

    The inertial mass of a photon is m = p/c where p is the magnitude of the photons momentum and c is the speed of light. So the correct answer to your question will depend on whether you're refering to inertial mass or proper mass.
    The term "pure energy" has little meaning. I recommend that you don't think of something with zero proper mass as being "pure energy".
    Light consists of photons. You can also think of light as an electromagnetic wave. Energy never goes anywhere. Only radiation does. Energy is more of a bookkeeping system than it is a thing.

  11. Aug 15, 2004 #10


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    When I was in high school, my physics teacher had what he called a 'butter gun". It was a squirt gun (presumably loaded with melted butter) with a frame work of four wires exending in a cone shape from the barrel.

    His point was that you could fit 4 slices of toast in it at twice the distance at which you could fit one slice (since area is proportional to distance squared) and so the same amount of butter was spread over 4 times the area: the thickness was inversely proportional to the square of the distance.
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