Classical or quantum electromagnetic theory?

In summary, the conversation was discussing a theoretical question about the energy of a beam of infrared radiation compared to that of visible radiation of the same intensity. The classical electromagnetic theory states that if the intensities are equal, then the energy densities are also equal. Therefore, the statement is FALSE. However, the quantum theory of radiation states that the energy of a photon is proportional to the frequency, and since infrared radiation has a lower frequency than visible light, it has less energy. Therefore, the proposition is TRUE. The conversation ends with a question about which theory is correct. However, it is important to note that the number of photons in each beam is unknown, which could affect the accuracy of the conclusions.
  • #1
orlan2r
31
0
In the last review of admission to a university in Peru (UNI) proposed a theoretical question true / false.

The sentence was: A beam of infrared radiation has less energy than a visible radiation of the same intensity.

This sentence is True or False?

According to classical electromagnetic theory the intensity of an electromagnetic wave is equal to the average energy density by the speed of light. Therefore, if both have the same intensity radiation will also have the same average energy, and therefore this statement is FALSE.

But there is another point of view logically valid.

According to quantum theory of radiation, the energy of a photon is proportional to the frequency of electromagnetic radiation, and the frequency of the infrared radiation is less than the average frequency of visible light, we conclude that the radiation energy infrared is less than that of visible light, and therefore the proposition is TRUE.

According to this view, the intensities of radiation are equal because there is greater amount of infrared radiation photons.

Who is right?

I would like to know your opinion about it.

Thanks in advance
 
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  • #2
If the intensity is the same, and the beam sizes the same, the energy carried by each is the same...

Besides, even going by classical theory, how can you assume that energy densities of the two different beams are the same?

"According to this view, the intensities of radiation are equal because there is greater amount of infrared radiation photons."
Yup I guess that has to be the case.

I think they were really only trying to see if you knew the definitions of these various terms well o.0
 
  • #3
orlan2r said:
According to quantum theory of radiation, the energy of a photon is proportional to the frequency of electromagnetic radiation, and the frequency of the infrared radiation is less than the average frequency of visible light, we conclude that the radiation energy infrared is less than that of visible light, and therefore the proposition is TRUE.

True only if the beams have an equal number of photons. Is that the case here? What do they say is equal about the two beams?
 
  • #4
Redbelly98 said:
True only if the beams have an equal number of photons. Is that the case here? What do they say is equal about the two beams?

From my point of view, if the intensity I of radiation is the same and the energy E of each photon infrared is less than photon visible light, we deduce that exist more photons infrared than photons visible light.

http://cepreuni.org/wp-content/uploads/2009/08/intensidad_onda1.gif

For you the original sentence is true?

Thanks for your response
 
  • #5
queenofbabes said:
If the intensity is the same, and the beam sizes the same, the energy carried by each is the same...

According to this paragraph the orginal sentence is FALSE.

queenofbabes said:
Besides, even going by classical theory, how can you assume that energy densities of the two different beams are the same?

According to classical electromagnetic theory, if the intensity of radiation are equals then the energy densities are equals.

queenofbabes said:
"According to this view, the intensities of radiation are equal because there is greater amount of infrared radiation photons."
Yup I guess that has to be the case.

According to this paragraph the orginal sentence is TRUE.

This is, true or false?

Greetings and thanks for your response
 
  • #6
Indeed the energy densities are the same precisely because the intensities are the same! The original sentence is false. The third paragraph you quoted does not imply in any sense that the statement is true, does it?
 
  • #7
queenofbabes said:
Indeed the energy densities are the same precisely because the intensities are the same! The original sentence is false. The third paragraph you quoted does not imply in any sense that the statement is true, does it?

I am of the opinion that the sentence is true.

My point of view the energy of infrared radiation is less than the average energy of a mixture of visible radiation (white light). The justification for this is the Planck's formula.

According to this, if the number of photons of both radiations are equals then intensity of radiation infrared will be less than the intensidad of visible radiation.

Then, if the intensities of these radiations are equal, there must be more photons of infrared light than visible light (see formula: http://cepreuni.org/wp-content/uploads/2009/08/intensidad_onda1.gif).

What is your opinion.
 
  • #8
orlan2r said:
According to this, if the number of photons of both radiations are equals then intensity of radiation infrared will be less than the intensidad of visible radiation.
But as Redbelly pointed out in the very first reply to this thread, are the number of photons in the two beams equal?
 
  • #9
Hootenanny said:
But as Redbelly pointed out in the very first reply to this thread, are the number of photons in the two beams equal?

Another hint: the 3 possible answers to this are either yes, no, or we don't know.
 
  • #10
Hootenanny said:
But as Redbelly pointed out in the very first reply to this thread, are the number of photons in the two beams equal?

This question has led diverse opinions in my country (Peru) and I appeal to you to respond.

Is it possible that two of the radiaciación accepted theories of the classical electromagnetic theory and quantum theory of radiation reaching diametrically opposite conclusions? I think this is not possible.

According to classical electromagnetic theory this statement is FALSE.

According to quantum theory of radiation this statement is TRUE.

Who's right?

http://cepreuni.org/?p=143
 
  • #11
orlan2r said:
This question has led diverse opinions in my country (Peru) and I appeal to you to respond.

Is it possible that two of the radiaciación accepted theories of the classical electromagnetic theory and quantum theory of radiation reaching diametrically opposite conclusions? I think this is not possible.

According to classical electromagnetic theory this statement is FALSE.

According to quantum theory of radiation this statement is TRUE.

Who's right?

http://cepreuni.org/?p=143

If you translated the problem correctly, the answer is most certainly FALSE: by definition, same intensity means same energy. The infrared beam has less energy per photon, but it has more photons. Quantum and classical theory do not disagree on the definition of the word "intensity".
 
  • #12
orlan2r said:
This question has led diverse opinions in my country (Peru) and I appeal to you to respond.

Is it possible that two of the radiaciación accepted theories of the classical electromagnetic theory and quantum theory of radiation reaching diametrically opposite conclusions? I think this is not possible.

According to classical electromagnetic theory this statement is FALSE.

According to quantum theory of radiation this statement is TRUE.

Who's right?

http://cepreuni.org/?p=143


In no way does the classical theory imply that the statement is false. As said above, quantum and classical theory do not disagree on the definition of the word "intensity".
 
  • #13
orlan2r said:
According to this, if the number of photons of both radiations are equals then intensity of radiation infrared will be less than the intensidad of visible radiation.

Then, if the intensities of these radiations are equal, there must be more photons of infrared light than visible light

The above statements are true. Since the two beams do have equal intensities, there are unequal numbers of photons in the two beams.

According to quantum theory of radiation, the energy of a photon is proportional to the frequency of electromagnetic radiation, and the frequency of the infrared radiation is less than the average frequency of visible light, we conclude that the radiation energy infrared is less than that of visible light, and therefore the proposition is TRUE.

This is incorrect reasoning. You are basing this on the energy per photon of infrared vs. visible. In essence, you are assuming equal numbers of photons in the two beams to reach your conclusion. But you have already correctly reasoned that there are unequal numbers of photons in the two beams.
 

FAQ: Classical or quantum electromagnetic theory?

1. What is classical electromagnetic theory?

Classical electromagnetic theory is a set of physical laws that describe the behavior of electric and magnetic fields in space. It is based on the principles of classical physics and was developed in the 19th century by scientists such as James Clerk Maxwell and Michael Faraday.

2. What is quantum electromagnetic theory?

Quantum electromagnetic theory is a branch of quantum physics that studies the behavior of electromagnetic radiation, such as light, at the quantum level. It describes the interaction between matter and electromagnetic radiation and is based on the principles of quantum mechanics.

3. What is the difference between classical and quantum electromagnetic theory?

The main difference between classical and quantum electromagnetic theory is the level at which they describe the behavior of electromagnetic radiation. Classical theory is based on the principles of classical physics and describes radiation as continuous waves, while quantum theory is based on the principles of quantum mechanics and describes radiation as discrete particles, known as photons.

4. How are classical and quantum electromagnetic theories related?

Classical and quantum electromagnetic theories are related in that classical theory is an approximation of quantum theory in certain situations. In other words, classical theory can be seen as a simplified version of quantum theory that is valid for large-scale phenomena, while quantum theory is necessary to fully understand the behavior of electromagnetic radiation at the microscopic level.

5. What are some real-world applications of classical and quantum electromagnetic theory?

Classical electromagnetic theory is used in many practical applications, such as the design of electronic devices, communication systems, and power generation. Quantum electromagnetic theory is also used in various technologies, including lasers, solar cells, and medical imaging devices. Additionally, both theories are crucial for the development of fundamental concepts in physics, such as the understanding of atoms and molecules.

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