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How does light from the sun cause atoms to vibrate?

  1. Aug 14, 2011 #1
    Is the reason behind the vibrating the movement of electrons from a stable to a excited state?

    My quest for knowledge began with this question..."How exactly does light transform into heat--for instance, when sunlight warms up a brick wall? I understand that electrons in the atoms in the wall absorb the light, but how does that absorbed sunlight turn into thermal energy?"

    I later found out that the atoms are perpetually vibrating due to the resonance of the solar energy. Soon enough, the vibration causes atoms to collide and heat. But, how does light cause the vibration of the atoms?
    Last edited: Aug 14, 2011
  2. jcsd
  3. Aug 14, 2011 #2
    First of all, it is the molecules of the brick that absorb the photons of sunshine, not the electrons. Photons are massless parcels of electromagnetic energy. When absorbed by a molecule, the photon ceases to exist and the kinetic energy of the molecule increases. When the mean kinetic energy of a large number of molecules is increased, we sense this as an increase in temperature.

    Kinetic energy can be of three kinds: translation (movement of the molecule from place to place), rotation along any or all of the molecule's axes of rotation, and vibration/libration. For solids, such as brick, most of the kinetic energy is vibration/libration. The brick, in its turn, will emit photons at thermal infrared wavelengths. We sense this radiation as sensible heat.

    In this manner, electromagnetic energy (light) becomes sensible heat.
  4. Aug 14, 2011 #3

    In my textbook, it states that the electrons absorb the light and cause it to become 'excited'. Electrons in a excited state are rather unstable so in a few nanoseconds would return to ground state. Then, it says that heat is released. Is it the emission of photons that causes the brick to be hot once touched? I really appreciate you answering by the way.
    Last edited: Aug 14, 2011
  5. Aug 14, 2011 #4


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    Mm. Yes and no.

    There are three forms of heat transfer: radiation, conduction and convection.

    When you touch the bricks, that's conduction. The molecules of the brick are directly transferring kinetic energy to the molecules of your hand.

    If you feel heat from the bricks without touching them, that's partly convection (bricks heat air, air brings heat to your hand) and partly radiation (bricks radiate infra-red photons, which strike your hand).
  6. Aug 14, 2011 #5
    I have a question related to this paragraph. What causes the atoms to vibrate?"Light from the sun excites electrons in the atoms which constitute the brick wall. How does that electronic energy get converted to heat, you ask. The key is 'radiationless transitions.' Here's how it works: the atoms of the brick are perpetually vibrating. Some of those atoms vibrate sufficiently vigorously that their vibrational energy is roughly equal to the electronic energy (photons) absorbed from the sun--in essence, they are in resonance with the solar energy. Those atoms then make a quantum transition from 'electronically excited' to 'vibrationally excited,' meaning that the energy causes the whole atom to move. We feel that motion as "heat." The atoms which make the jump to vibrational excitation soon collide into neighboring atoms, dissipating their vibrational energy throughout the entire brick, making the brick hot throughout.
  7. Aug 14, 2011 #6


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    You say you have a question, then you proceed to make claims.

    You're using terms and mechanisms that make no sense. We have a policy here about speculation and personal theories.
  8. Aug 14, 2011 #7
    I'm sorry, I have no physics background. I was told to come here from a biology forum because it was more physics related. While looking for an answer to my original question, this result (the paragraph) came up from what seemed to be a reliable source. I do understand the three forms of heat transfer, but I do not understand how light can generate heat. What makes a car that's been sitting in a parking lot day hot? Light causes the molecules that constitute the car to vibrate which creates heat. That's a fact. What I want to know is what makes the molecules vibrate to begin with.
  9. Aug 14, 2011 #8
    In any solid substance there will always be a distribution of molecular energies of vibration, from the base level to extremely vigorous. In a molecule, these vibratory energies include electrons moving rhythmically in and out along their bonding axes (vibration) and the flexing back and forth of these bonding axes (libration). In addition, the atoms of the molecule do the same two things relative to one another. The total of these energies is the total kinetic energy of vibration/libration.

    The mean value of this kinetic energy distribution will be measured as temperature. When light is absorbed and its electromagnetic energy transformed into kinetic energy, the mean value of the energy distribution increases. We sense this as an increase in temperature.
    The absorption of EMR does not start the molecules vibrating, it simply increases the mean energy level of this vibration.
  10. Aug 14, 2011 #9

    thanks :smile: You cleared a lot of things up for me. May I ask though, HOW does it increase the mean energy level? Is there a scientific explanation behind it, or is it just accepted that it does?
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