Help understanding light intensity

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Discussion Overview

The discussion centers around the concept of light intensity, particularly in the context of monochromatic light and its relationship to energy and photon emission. Participants explore the definitions and implications of intensity, energy, and photon behavior, with a focus on how different sources can emit varying intensities despite having the same wavelength.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions how two sources emitting the same monochromatic wave can have different intensities, given that the energy is defined as h*f and the area is constant.
  • Another participant suggests that the discussion may relate to the energy of a single photon, prompting clarification on whether a single photon is being considered.
  • A participant outlines a series of claims regarding electromagnetic radiation, including the dual wave-particle nature, the energy of photons, and the number of photons passing through a surface per second.
  • One participant challenges the notion of a photon being described as a "crest" of a wave, indicating that this model is not widely accepted.
  • Another participant questions the validity of equating wavelength with speed, expressing frustration over the method of inquiry being used.
  • A participant introduces the concept of "flux" as a more common term for intensity, explaining that the energy per unit time is dependent on the number of photons, which can vary.
  • A later reply corrects earlier definitions, stating that the energy of a wave is related to the number of photons and clarifying the distinction between intensity and fluence.

Areas of Agreement / Disagreement

Participants exhibit a range of views, with some agreeing on the definitions of energy and intensity while others contest the models and assumptions presented. The discussion remains unresolved with multiple competing perspectives on the nature of light intensity and photon behavior.

Contextual Notes

There are limitations in the assumptions made regarding the definitions of intensity, energy, and the behavior of photons. The discussion also reflects varying levels of understanding and acceptance of models related to light and electromagnetic radiation.

Ippolitos
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I have a problem understanding the intensity of light. Let's assume a monochromatic light of a single wavelength. The energy of the wave is constant and equal to h* f where h is the Planck's constant and f the frequency of the wave. The intensity of this wave is its energy divided by a given area.
What I can't understand is how two sources emitting the same monochromatic wave to the same area can have different intensity. The area is the same, h is stable and if we change f then we talk about a different wave.

What do I miss?
 
Science news on Phys.org
Are you trying to relate the answer to the energy of a single photon? Would there just be a single photon?
 
I will explain my way of thinking and tell me where I am wrong.

1) The electromagnetic radiation can be either presented as a wave or a particle. An electromagnetic wave of 600nm, for example, means that there is a wave with each creast abstain 600nm from adjacent crests or a series of photons abstain 600nm from adjacent photons. Correct?

2) Each photon (or crest) has energy equal to (its frequency)*(Plunck's constant). Correct?

3) This wave travels with the speed of light. Which means that from a surface, each second, the amount of photons that fit in 299792458 meters are passing through. So, if we have a source emitting 1 meter wavelength photons then 299792458 photons will pass through this surface in a seccond. Correct?

4)if 3 is correct this means that the intensity of every 1 meter wavelength radiation is stable for a given surface but this is wrong. There might be millions of sources emitting this radiation in different intensities. What am I missing?
 
A photon is not a "crest" of a wave. This is not an accepted model. Read further about it.
 
Is a photon an EM wave (constant wavelength) with length equal to the speed of light?
Can we model a photon like this?
 
Although the photon is an entity and it cannot be divided, in contrast to the wavelength.
 
I think you need to pause and then 'find out' rather than making up your own possible models. For a start, how can a waveLENGTH equal a SPEED? I have no idea how to put you right on this by using the Q and A method. It's so inefficient and far too demanding on my time (and that of others).
It's not that I don't want to help but it's just impractical. I'm sure it didn't (/doesn't) work that way in your School.
Just look at the wiki pages if you want to make a start.
 
The intensity that you mention is actually more usually called the "flux" (at least that's what I'm used to as an astronomer). The energy of a SINGLE photon is hf, however, there can be a variable number of photons impinging on a certain area during a given unit time. The energy impinging on that area per unit time is then Nhf where N is the number of photons. N is the number that changes to give you variable fluxes.
 
Matterwave nails it.

Ippolitos said:
I have a problem understanding the intensity of light. Let's assume a monochromatic light of a single wavelength. The energy of the wave is constant and equal to h* f where h is the Planck's constant and f the frequency of the wave. The intensity of this wave is its energy divided by a given area.
What I can't understand is how two sources emitting the same monochromatic wave to the same area can have different intensity. The area is the same, h is stable and if we change f then we talk about a different wave.

What do I miss?

Your definition of intensity is erroneous;

1. The energy of the wave is not h*f; the energy of a photon is h*f. The total energy of a wave is h*f times the number of photons.

2. Intensity (or more correctly, irradiance) is power/area not energy/area. Energy/area is called the fluence.

Two sources with the same wavelength over the same area can have different irradiances by emitting a different number of photons per second.

Claude.
 

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