Amplitude of a light wave

[deleted647690]

To increase oscillation freuqnecy of an electron (in a vacuum) stimulated by an EM wave, one must:
I. Decrease wavelength
II. Increase E field amplitude of the wave
III. Increase the speed of the wave


Answer was just I. It said that III is not possible because it is in a vacuum, thus speed is constant? What does this mean exactly? Even if speed is constant, if you increase the value of a constant, why would that not change the frequency since v = lambda* f?

Also, why is amplitude not affected by frequency? If increasing amplitude increases the energy of a wave, and E=hf, why would f not also increase?

---

Members don't see this ad.

 

[aldol16]

Answer was just I. It said that III is not possible because it is in a vacuum, thus speed is constant? What does this mean exactly? Even if speed is constant, if you increase the value of a constant, why would that not change the frequency since v = lambda* f?

A fundamental consequence of Einstein's predictions is that light speed is constant in a vacuum, regardless of frame of reference. Therefore, the answer to this questions turns on whether you understand actual physics and not whether you understand math. Yes, you are correct in that if the speed of the wave is increased, then frequency could increase. However, the idea is that light speed cannot increase. Therefore, III is impossible.

Also, why is amplitude not affected by frequency? If increasing amplitude increases the energy of a wave, and E=hf, why would f not also increase?

You err in assuming that amplitude increases the energy of a light wave. Amplitude and energy are related in classical physics but classical physics kind of breaks down when you're talking about light. Amplitude of a light wave is related to the intensity of the light. Light intensity is the number of photons emitted per second. It has nothing to do with the energy of those photons. This fact is central to the interpretation of the photoelectric effect, for which Einstein won the Nobel Prize.

 

[deleted647690]

A fundamental consequence of Einstein's predictions is that light speed is constant in a vacuum, regardless of frame of reference. Therefore, the answer to this questions turns on whether you understand actual physics and not whether you understand math. Yes, you are correct in that if the speed of the wave is increased, then frequency could increase. However, the idea is that light speed cannot increase. Therefore, III is impossible.



You err in assuming that amplitude increases the energy of a light wave. Amplitude and energy are related in classical physics but classical physics kind of breaks down when you're talking about light. Amplitude of a light wave is related to the intensity of the light. Light intensity is the number of photons emitted per second. It has nothing to do with the energy of those photons. This fact is central to the interpretation of the photoelectric effect, for which Einstein won the Nobel Prize.

Thanks! So if we were talking about a sound wave, amplitude would increase energy?

 

[sazerac]

Thanks! So if we were talking about a sound wave, amplitude would increase energy?

Yes. This question was testing if you know the differences between sound waves (which kinda make sense) and light waves (which have their own bizarre set of rules).

 

[deleted647690]

Thank you guys