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Name: Raymond
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Question:
I understand Snell's law explains that light bends when going from one medium to another because of the different speeds of light in different mediums. Many claim that different colors of light are bent by a prism to different angles because each frequency has a different speed of light in the second medium (say glass). If there is different speeds of light for red and blue light in glass, then through thick glass, the angle between the red and blue light should continue to increase after it enters the glass. In essence it should curve. If however it is a surface effect the angle between the frequencies would remain constant through out the glass. You do not need to invoke different speeds of frequencies to get the frequency separation at the interface. Has anyone actually measured within glass the speed of blue light and the speed of red light?



Replies:
If there is different speeds of light for red and blue light in glass, then through thick glass, the angle between the red and blue light should continue to increase after it enters the glass. In essence it should curve.

---No, this is incorrect. The change occurs at the interface, not in the bulk. The velocities of both return to equal (albeit slower) once the wave has fully entered the medium.

If however it is a surface effect the angle between the frequencies would remain constant through out the glass. You do not need to invoke different speeds of frequencies to get the frequency separation at the interface.

---The velocity does change, for all frequencies. The change in velocity at the interface (as the wave crosses the interface) is required to explain the change in direction.

Has anyone actually measured within glass the speed of blue light and the speed of red light?

---Yes. I chose these links at random; there are lots and lots of others. Mostly they are older publications -- current research is more about negative refractive index materials than well-established materials like glass. You might notice that these scientists were from Bell Labs (think 'fiber optics').

http://prola.aps.org/abstract/PRB/v7/i8/p3767_1

http://prola.aps.org/abstract/PRB/v3/i4/p1338_1

Hope this helps,

Burr Zimmerman


Raymond,

There are two explanations. One relates to individual photons. One relates to the entire wave. Both together can be quite useful.

When light enters material, individual bits of light called photons are absorbed by atoms. If the energy is correct, the energy will move an electron to a higher orbit for a long time. The energy usually ends up as increased temperature in the material. If the energy of the photon is not right, then the atom cannot hold the energy. It is quickly released as a new photon. Some energies can be held longer than others. As the light passes through the material, these small delays are what slow the light. Between atoms, the speed is standard. It is like driving at 60mph while moving, but having to stop every few blocks. Frequency is so important because the energy of individual photons is proportional to frequency.

Many photons combine to make a wave. Consider an open area. One half is concrete and the other half is dirt. The gas pedal is held at a constant position, and the wheels are kept straight. The car will move slower on the dirt. Small pauses due to the wheels slipping in the dirt are what slow the car. When driving on concrete, the car moves in a straight line. When driving in the dirt, the car moves in a straight line.

When crossing the border between concrete and dirt, direction can change. If crossing at an angle, the wheel to first reach the dirt slows first. One side slows before the other. This makes the car turn a small amount away from the border, toward the perpendicular to the border. After all wheels are in the dirt, the car moves straight again.

Dr. Ken Mellendorf
Physics Instructor
Illinois Central College



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