## Wavelength vs. Photons [ENDORSED]

Armani Dodson 1A
Posts: 24
Joined: Fri Jul 22, 2016 3:00 am

### Wavelength vs. Photons

I understand the visual of a wavelength in terms of light, but I don't get the concept of a photon. Is it a particular property of light or is it just like a measurement of the energy of light. Basically, I would like to know the definition of a photon? Also Dr. Lavelle said that they stopped measuring light in wavelength and began measuring it in photons. how does this make a difference?

Amber_Carlton_3D
Posts: 17
Joined: Fri Jul 15, 2016 3:00 am

### Re: Wavelength vs. Photons  [ENDORSED]

-photons are basically particles of light/a quantum of light that travel at the speed of light in empty space too (because their mass is nonexistent - when they're emitted from an atom their mass is 1/2 positive from proton and 1/2 negative from electron --> mass of zero)
-photons carry energy that is proportional to radiation frequency
-photons make up an electromagnetic wave, although I'm not sure why we switched from measuring light in wavelengths to photons (photons have very small wavelengths so I'm not sure why we would measure in photons rather than in wavelengths :/)

Jessica_Nakahira_1G
Posts: 16
Joined: Wed Sep 21, 2016 2:57 pm

### Re: Wavelength vs. Photons

To continue your thought Amber, a light ray is a stream of energy made up of photons, which basically resemble little packets of energy. For the purpose of the experiment, I think visualizing the light this way made it easier to understand because now they realized that the light had to be a certain shorter wavelength. So, they decided to measure the photons, the individual entities of energy, when observing the effects of their impact on the metal. I think it's more effective than measuring the energy as a wavelength, especially because seeing the correlation between just a change in wavelength to the amount of electrons being removed is a little more difficult.

Joanne_Park_4C
Posts: 18
Joined: Wed Sep 21, 2016 2:56 pm

### Re: Wavelength vs. Photons

How does the photoelectric effect exactly prove that light doesn't act as waves but as particles?

Wenqian_Deng_1L
Posts: 23
Joined: Wed Sep 21, 2016 3:00 pm

### Re: Wavelength vs. Photons

@Joanne

I think we have to take into account that it's the ENERGY of the photons that are ejecting the electrons from the metal atoms in the photoelectric effect. So in that case, light is acting as photon particles, not as waves. Also, the equation that is related to the photoelectric effect measures the amount of kinetic energy of the ejected electrons (in Joules). If we used the equations that measures light as a wavelength, our units would be in meters instead of joules.

And light acts as both a wave AND as particles, depending on how you look at it. Remember when Prof Lavelle was using water to explain the difference between classical mechanics and quantum mechanics? He was saying how at the macroscopic level, or when using classical mechanics, water looks like a continuous stream, just like how light acts as wavelengths to produce different types of electromagnetic radiation. HOWEVER, when zoomed in to the microscopic level, or when using quantum mechanics, water is reduced to single H2O molecules (quantized/you can count the number of water droplets), just like how light can be measured as photon particles.

Hope that helps!
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Alex Dib 4H
Posts: 34
Joined: Wed Sep 21, 2016 2:57 pm

### Re: Wavelength vs. Photons

Joanne_Park_4C wrote:How does the photoelectric effect exactly prove that light doesn't act as waves but as particles?

I think its because when they were attempting to eject electrons with light, they knew that the light needed to carry with it enough energy to eject the electron. So when they weren't getting the electrons ejected with the light source they were using, they decided to increase the energy by, so they thought, increasing the intensity (amplitude), much like a more intense ocean wave will have lot more energy than a small ocean wave. They then realized that increasing the intensity does nothing when electrons were previously not being ejected, but increasing the frequency of the light does cause electrons to eject. Not completely sure how they arrived to this, and not completely sure I'm correct, but this told them that light was not acting like a wave in this experiment, but like a particle, and to eject the electron we need to increase the energy of the particles (photons) themselves. Then Einstein discovered, through these observations, that E=hv, stating that the energy of a photon is proportional to the frequency of the light for that photon. So increasing the frequency was actually increasing the energy, showing that light was acting as a particle in this experiment.

They also found that increasing the intensity is only increasing the amount of photons, not the energy. So, if the light actually did have a high enough frequency to eject an electron, increasing the intensity would just cause more electrons to be ejected, based off of the 1 photon per 1 electron ratio.

Hope this helps, sorry if I'm not fully correct!