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Name: Jonas S.
Status: student
Age: 16
Location: N/A
Country: N/A
Date: 9/30/2003


Question:
How is light energy converted into thermal energy to do work?


Replies:
Yours is a "large" question. There are many mechanisms. However, the general principle is this. A photon (packet) of light energy has an energy, E (Joules), proportional to its frequency, f (sec^-1). The constant of proportionality is Planck's constant, h (Joule-sec). So: E = h*f. The value of h = 6.63*10^-34 so you can see that any single photon carries very little energy. When photons encounter other matter several things can happen. It can be reflected (angle of incidence same as angle of reflectance), scattered (angle of incidence different from angle of reflectance), or absorbed. Usually all three possibilities are occurring simultaneously. In the case of scattering and absorption some or all of the energy of the photon is transferred to the matter it strikes. That energy is re-emitted by photons of lower energy (frequency). This process of absorption / re-emission continues to re-occur until the photons being re-absorbed / re-emitted take on a frequency distribution called the "black body" distribution. Here the re-absorption / re-emission has reached an equilibrium energy that is characterized by its temperature, T (kelvins). This is heat. Work can be "extracted" from a hotter body by transferring some of its energy to a cooler body. The amount of work (w) that can be obtained by this process is: w = q*[(Th-Tc/Th)] where the amount of heat transferred is (q), and Th and Tc are the hotter and cooler temperature, respectively, and Th>Tc>0. From this inequality you can see that only a fraction of the available heat can be converted into work. This is one statement of what is called the "Second Law of Thermodynamics". There are other equivalent ones.

Vince Calder


Jonas,

Visible light is a form of electromagnetic wave. It is just like the radio signal you pick up on your stereo, only with a higher frequency. The only thing special about visible light is the tiny antennae, called "rods" and "cones", on the eye's retina are tuned to the frequencies of visible light. Just like radio waves, light waves make electric charges vibrate. A certain frequency, or color, makes the electrons in certain materials vibrate. This absorbed light energy may be immediately released: reflection. This energy may also be transferred throughout the material: absorption. When absorbed, all the atoms eventually vibrate. This is thermal energy.

Dr. Ken Mellendorf
Physics Professor
Illinois Central College


Light contains energy. You may think of it as the kinetic energy of the photons which, according to quantum theory, are the quanta of the electromagnetic field. You may also think of it as the energy contained in the electric and magnetic fields which make up the light wave. This duality is a basic part of quantum theory. Which picture is the most convenient (both are correct) depends on the experiment used to detect the energy.

If you use the photoelectric effect where the light photons bang into electrons, giving them kinetic energy and knocking some out of the material, the photon picture is most appropriate. If you let the light hit your skin, hoping to get a tan, or focus sunlight on a pot of water to heat it up, the wave picture is more convenient. Then the electric field exerts an electric force on the electrons, causing them to accelerate and gain kinetic energy. The electric field is oscillating, reversing sign about 10^15 times per second (that is 1 followed by 15 zeros). So the electron is pushed back and forth, colliding with other electrons and atoms in the material, heating them up.

Incidentally, light also exerts a force on the material it strikes. In the photon picture, the force is due to the photons colliding with electrons and pushing them back. The electrons, of course, collide with atoms in the material, thereby transmitting the force to the material. In the wave picture, the electric field pushes the electrons sideways (the electric field and magnetic fields are always perpendicular to the direction of motion of the light and to each other) and the magnetic field then exerts a force on the moving electron. Because of the relation between the directions of the light, the electric, and the magnetic fields, the force is always in the direction the light was travelling.

Perhaps not as simple an answer as you would like, but I do hope it makes sense to you. If not, ask again!

Best, Dick Plano...



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