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B Silly questions about the behaviour of photons

  1. Apr 20, 2016 #1
    Please bare with me as I haven't studied much at all about light in high school physics.

    When we see an object as red, does that mean the way that photons bounce off of it cause only light at the wavelength of 'red' to be reflected? If so, wouldn't that mean that the light reflecting off of a red object would be warmer than the light reflecting from a blue object(since red light with its higher wavelength warm things up more) And so, doesn't that mean that it would have less intensity since it's carrying the same amount of energy but as 'heat' instead?

    Also, we describe the electromagnetic spectrum (light) as having different wavelengths, but also describe it as photons which are 'energy packets', how does this work at all?

    Are photons the purest form of energy? Since they can never be still and only bounce off of electrons, they transform into heat and therefore disappear?
    Also, since heat is kinetic energy of particles, how do we see heat being radiated from objects? How does the kinetic energy of particles create electromagnetic waves (and therefore protons?)

    Is time described as another dimension or simply the flow that energy undergoes inside of 'space'?
  2. jcsd
  3. Apr 20, 2016 #2


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    Basically, yes.

    You can't really talk about the temperature of light if it does not follow blackbody radiation. So red light is not "warmer" than blue light. In addition, blue light is actually more energetic than red light, as energy goes as the inverse of the wavelength. This has nothing to do with intensity, which would be the amplitude of the electromagnetic wave in the classical picture, or number of photons in the quantum picture.

    You'll find plenty of threads on PF treating this subject. Unless you are looking at things like absorption of light by an individual atom, keeping it to the classical picture of an EM wave is much better.

    I don't understand what "pure" energy means. What would electromagnetic radiation by purer energy than gravitational potential energy?

    First, I guess you mean photons, not protons. Heat is not only kinetic energy. If you take for example a molecule of nitrogen, it stores heat as translational, vibrational, rotational, and electronic energy. But for symmetry reasons, it actually doesn't emit any radiation at normal temperatures (i.e., where electronic excitation is negligible). You, on the other hand, emit infrared radiation, mainly due to the vibration of the molecules that you are made of. As these vibrations involve the motion of electrically charged particles (protons and electrons), they can lead to emission of EM radiation.
  4. Apr 20, 2016 #3


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    [The answer that follows is OK for high school physics and a 'B' thread. The real stuff comes from quantum electrodynamics, which you won't encounter until after four years of fairly demanding college-level physics and math.]

    It's easiest to think of light as an electromagnetic wave, always. Photons only come into the picture when the electromagnetic radiation is interacting with matter; we observe that it always transfers its energy and momentum in discrete amounts at single points, and when this happens we say that "a photon hit there". The probability of this happening at any given point is proportional to the intensity of the radiation at that point; you get more photons in areas where the electromagnetic radiation is brighter.

    One implication is that you should not think of photons as little particles of light, nor that a beam of light is a stream of photons moving by you the way a river is a stream of water molecules moving by. Except when the light is interacting with matter, there are no photons, just electromagnetic radiation.

    Thus, the entire model of photons "bouncing off of" objects is basically misleading; the electromagnetic radiation interacts with objects, including being absorbed and reemitted. Photons only appear when we grind through the calculations describing exactly what happens as the radiation interacts with the surface of the object.

    You might want to give Feynmann's book "QED: The strange theory of light and matter" a try.
    Last edited: Apr 20, 2016
  5. Apr 20, 2016 #4


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    You'll get better and more complete answers to that question over in the relativity subforum, but the quick answer is "another dimension". I can specify the position of a point on a two-dimensional sheet of paper with two numbers (the x and y coordinates) and the position of a point in three-dimensional space with three numbers (x, y, and z coordinates; or latitude, longitude, and height above or below sea level; or ....), and it turns out that the laws of physics can be cleanly and simply described by treating time as fourth coordinate in a four-dimensional spacetime. For example, "the tip of my nose at noon" and the "the tip of my nose at one second past noon" is usually thought of as the same point in space at different times - but we could think of that as two points in spacetime, one second apart.

    This stuff is the basis for Einstein's theory of relativity. The mathematical price of admission is much lower than for quantum electrodynamics (which I mentioned in my previous post) so it's realistic to expect to be able to study and understand it as a high school student. Taylor and Wheeler's "Spacetime Physics" would be a good start.
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