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How to define light in laymans terms

  1. Apr 24, 2013 #1
    So ive got to give a speech for a class soon with the informative topic of what is light. Light is just what we call the visible portion of the spectrum. How can i go about defining this to a group of college kids in a way they can understand?

  2. jcsd
  3. Apr 24, 2013 #2

    jim hardy

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    So is light particles or waves?

    How long can you speak?

    I'd make a mystery story out of it - see this article.

    http://www.spectroscopyonline.com/spectroscopy/article/articleDetail.jsp?id=337288&pageID=1&sk=&date= [Broken]
    Last edited by a moderator: May 6, 2017
  4. Apr 24, 2013 #3


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    My first reaction to the question is - if a college kid doesn't understand light - then what the heck are they doing in college?! Are our educational standards set so low these days?

    In most folks everyday experience, light generally refers to 'visible' light, which corresponds to electromagnetic radiation (photons) of a narrow range of energy (E) or wavelength (λ), where E = hc/λ = hν, and ν is the frequency. Visible light originates in an atom in which an electron drops from an excited energy level to a lower (more stable) energy level. The emitted photon has wave and particle (discrete) properties.

    Is one interested in the origin or nature of light, or both?

    The Nature of Light - http://ocw.tufts.edu/data/30/365841.pdf (probably more than one would want to know)

    Photon - http://hyperphysics.phy-astr.gsu.edu/hbase/particles/expar.html
    Electromagnetic radation - http://hyperphysics.phy-astr.gsu.edu/hbase/ems1.html
    Visible light - http://hyperphysics.phy-astr.gsu.edu/hbase/ems3.html#c2
    Wave-particle duality of light - http://hyperphysics.phy-astr.gsu.edu/hbase/mod1.html
    Measurement of light - http://hyperphysics.phy-astr.gsu.edu/hbase/vision/radphocon.html#c1
  5. Apr 24, 2013 #4
    My speech is suppose to be between 5 and 6 minutes. In my opinion, light is both, its a wave of particles.(before you begin bashing my opinion take regard that it is just a thought) But yes college standards are such a joke these days, your jaw would hit the floor if you saw how basic the standards are and yet people still cant grasp anything.

    Technically, light(visible), is no different than the rest of the electromagnetic spectrum, its just energy on a different frequency. So in a sense its all the same. So does color exist?

    And i understand light travels at the speed of light in a vacuum until it hits matter, than it acts as a wave. So where does that leave us on the spectrum? we see a small portion, but everything else is happening all around us at the same time we just dont see it.

    this topic was definitely a hard choice, not even google can find me an answer to what is light lol Is it just something we have to experience for ourselves as language cant obviously find the words to use
  6. Apr 24, 2013 #5


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    Light is not a wave of particles.

    The light we see is a collection of individual photons. Photons have wave and particle properties, and essentially a photon is a particle.

    One needs to distinguish between the individual photon (and its behavior or interaction) and the collective behavior of a population of photons, much like one would distinguish between a molecule of water and a liter or gallon of water, or a gas atom and the atmosphere above one.

    Color is a rather arbitrary interpretation of light based on the interaction of light with our eyes. Once the photon interacts with our retina, it no longer exists, but there are other photons that precede or follow each photon that emanates from what we observe.
  7. Apr 24, 2013 #6
    I understand that, photons act differently when by itself vs in the light from the sun. But what im having a hard time grasping is when in nature do we ever experience photons by themselves. I mean obviously we can detect them with different sensors adjusted to certain levels of the ES, but essentially photons stay together and fly in packs.....

    When you flip the light switch off on a lightbulb, where does the light go?....
  8. Apr 25, 2013 #7
    oh boy, big splashy liquid helices of photons

    I would give mention to how we can direct light in a circular polarized orientation and also how nature does it.
  9. Apr 25, 2013 #8


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    Light from a source travels in many directions. Go our a few million km, and one would find light from a point source is diverging.

    When one turns off a light switch, the production of photons ceases, because the excitation of the atoms producing the light ceases. The light (photons) that encounters surfaces is either absorbed or reflected. Reflected light will ultimately be absorbed somewhere. Light that is absorbed simply contributes to the energy (usually in the form of low level heat) within the material that absorbs it.
  10. Apr 25, 2013 #9


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    Light is the opposite of dark.
  11. Apr 25, 2013 #10


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    This time of year, probably Cleveland.
  12. Apr 25, 2013 #11

    jim hardy

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    Back into the bulb, through the wires and into the generators which pull the water down the river.
    oops, conventional light goes the other way.
  13. Apr 25, 2013 #12


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    Surely the irony here is tragic. Emphasis added.

    Is this the topic of your discussion? You should be able to answer this question in far less than five minutes:

    Light from a light bulb is produced in different manners depending upon the type of light bulb in question. When you flip the light switch off, the production of this light ceases.​

    Your question is either simple or misleading. When you stop writing, where do the words go? When you stop yelling, where does the noise go?
  14. Apr 25, 2013 #13
    my question has gotten completely sidetracked. basicallly what im doing is a speech on what is light? simple as that but its also my final and i need to come up with something good, something that will make my fellow students pull their iphone out of their a** and look at life a little differently.

    But the best answer for the light bulb question i could come up with is the the light disperses into the room at the speed of light and energy is ceased to be produced.
  15. Apr 25, 2013 #14


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    What is wrong with simply explaining that light is an electromagnetic wave? How complicated do want to do this? Are you expecting to get questions on the photon picture of light?

  16. Apr 25, 2013 #15
    Yea, I wouldn't bother trying to explain light in the context of QED/QFT because you're really getting into some very murky water there when trying to understand what a photon is in terms of pictures and analogies which aren't mathematical equations, and honestly it's probably impossible to do so anyways.

    The classical picture is difficult enough for laypeople to get, and probably more accurate anyways when put in terms of simple pictures and analogies.
  17. Apr 25, 2013 #16


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    The two main theories of light are classical electrodynamics and quantum electrodynamics. In the former, light is described as a wave solution of Maxwell's equations for the electric and magnetic fields. Those two fields can be combined into one, the electromagnetic field. So light can also be described as a wave solution of Maxwell's equations for the electromagnetic field. If we simplify this statement a bit, we get what Zz said, it's an electromagnetic wave.

    In quantum electrodynamics, light consists of photons, but what are photons? This is really hard to explain. The theory doesn't really define "photons", it defines things like "1-photon states" (of the electromagnetic field). So it's going to be very hard for you to give them a meaningful definition of "photon".

    I think I would mostly talk about the classical description, and mention that there's also an even better theory in which light is described in terms of particles. I would also mention that these are not particles in the classical sense of "a pointlike object with a well-defined position", and that they would have to study quantum field theory to understand it.
  18. Apr 25, 2013 #17


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    Sorry to be nit-picky, but I need to correct some terminology here.

    The term "light" applies to infrared radiation all the way up through ultraviolet; visible light is small sliver somewhere near the middle.

    For some reason, frequencies below infrared are not considered "light" (such as radio waves and microwaves) nor are frequencies above ultraviolet (x-rays, gamma rays and cosmic rays). This is really just a terminology thing: they're all electromagnetic waves, and can all be thought of as photons too, but only the band from infrared to ultraviolet is called "light." (This is mostly a matter of language more than it is anything technical.)

    But there is some logic to this insanity. Light (from infrared through ultraviolet) is created and absorbed by electrons transitioning between excited states of atoms. When an electron is in an exited state it might fall to a lower energy state releasing a photon in the process. Similarly, a photon might bump up an electron to a more energetic state in an atom.

    Radio waves and microwaves are created and absorbed by moving electrons around in a conductor, but the electrons involved are not necessarily in any "excited state" They remain in the conduction band. Also microwaves can be created (and absorbed) by thermal interaction of polar molecules -- again though, nothing that involves excited electrons. Since it doesn't really involve "excited states" it's not called "light."

    X-rays involve completely ionized electrons accelerated to extremely high velocities and then smash into stuff, releasing a x-ray photons in the process. This is above and beyond the normal interaction of simply "excited" electrons, so its not called "light."

    Gamma rays involve nuclear interactions, not excited states of electrons. Thus it's not called light. Cosmic rays are more mysterious, but they are definitely not the simple, familiar excited electron state transitions.

    [Edit: When defining these bands of electromagnetic radiation, from radio waves up through cosmic rays, there is always some overlap when it comes to the photon interaction. There are not necessarily well defined boundaries.]
    Last edited: Apr 25, 2013
  19. Apr 25, 2013 #18
    Finally, someone actually understands me lol Collinsmark you are officially my hero that is exactly the response ive been looking for. So the reason we cant see xrays or microwaves is because its not in a range on the spectrum where photons are released from which our eyes than pick up?
  20. Apr 25, 2013 #19


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    Actually yes, that sums it up pretty well.

    Our eyes have rods and cones that are sensitive to certain frequencies of light. The mechanism involves electrons in atoms in certain molecules (in some sort of cellular sub-structure that I'm not at all familiar with) getting bumped up from one energy state to a different, higher energy state. Somehow or another (again I don't know anything about the details) produces a synaptic response. Although I don't know the details, I'm guessing that the excited state of an atom in a particular type of molecule induces a electro-chemical reaction (perhaps with some other molecules) which then is ultimately registered as the synapse, but that's just my guess.

    You might want to research rods and cones to figure out more about how they work (I can't be of much help there).

    But yes, x-rays won't cause the particular excited state transition to happen that would otherwise end up as a synaptic response: instead, an x-ray photon would just blow the electron clean off the molecule. Radio wave photons don't have enough energy to excite the electron, so they won't cause the necessary excited state transition either. (Edit: radio wave/microwave photon would cause the molecule to vibrate, but not cause an electron excitation.)

    On a subtle, yet important note, the same can be said of infrared photons and ultraviolet photons too. Although these types of photons can cause electron state changes in general, they are not the correct energy for the particular electron state changes that would cause the right state change that ultimately registers a synapse response. So we can't see ultraviolet or infrared light with our eyes. But for whatever reason, infrared and ultraviolet are still called "light," unlike x-rays or radio waves.
    Last edited: Apr 26, 2013
  21. Apr 25, 2013 #20
    Ok now it all makes sense, the point i was shooting for with regards to my speech, is along the line of "visible light" being the same thing as xrays or microwaves just energy level turned up or down. Kind of my way of trying to get people to think about life a little more.

    What gets me is i know snakes see more on the infared and bees see more ultraviolet, so with that being said this has to mean that were being subjected to most the spectrum all day every day but just cant see it...

    [Mentor's note: unscientific speculation removed]

    check this out though...http://www.physics.uc.edu/~sitko/ReflectanceSpectroscopy.pdf

    sorry if i keep jumping around topics, this speech has really had me thinking....
    Last edited by a moderator: Apr 26, 2013
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