# Homework Help: Help Understanding Light/Atom Thoeries

1. Apr 29, 2014

### blue_lilly

1. The problem statement, all variables and given/known data
I am studying for my physics final and I had some questions about the theories we learned in class.

2. Relevant Theories
Relativity
States the speed of light is a constant (c= 3E8 m/s). This was found using the equation E=mc^2.​

Brownian Motion
Established the atomic theory which say that atoms movement is random (as the temperature increases, the atom movement increases)​

Black-body Radiation
This says that atoms have quantized energy levels (The energy levels is where the electron 'sits'). When absorbed the electron jumps up and when emitted the electron jumps down.
All atoms give off Electromagnetic Radiation-the wavelength of the radiation depends on the temperature. [Visible Light: blue is the hottest, white is in the middle, red is the coolest] ​

Photo-Electric Effect
States that light can be viewed as particles. These partials of light are known as photons. A proton is a packet of light energy E=hƒ. Photons release electrons. As photons come in, the photon gives all its energy to an electron, and the electron comes out (Photon in, electron out)​

Radioactivity
Unstable isotopes have atomic particles that randomly decay over time. Decay relates to the half-life of the isotope. The decay causes biological damage to the cell at an atomic level [Absorbed dose=Energy/mass ; Biological Equivalent Dose(rem)= absorbed dose(in rads)*RBE(Relative Biological Effectiveness)].​

Compton Scattering
Mass-less photons have momentum.
Elastic Scattering- energy is conserved before and after an electron scatters.​

deBroglie Wavelength
Particles have properties that are similar to waves.​

Rutherford Back-scattering
Established the idea that in an atom, the nucleus is a very tiny positive charge while the rest of the atoms 'space' or electron cloud is filled with electrons and a lot of empty space.​

3. The attempt at a solution
These are the questions I had relating to the theories

deBroglie Wavelength
Besides wavelength and frequency, what other wave properties are used with particles?
All I can think of is period and amplitude but I am unsure as to when I would need to use either of them when discussing particles.

Relativity/Compton Scattering/Photo-Electric Effect
Relativity says that the speed of light was found by using the equation E=mc^2. Compton Scattering says that photons (which are alpha particles[2 neutrons & 2 protons]) are mass-less. The Photo-Electric Effect says that light is made of packets of protons. So if, Energy=mass*speed of light^2, and light is made up of photons which are mass-less, does that mean the speed of light have no energy? [E=mc^2 ; E=(zero)(3E8^2) ; E=zero]​

Photo-Electric Effect
When it is talking about photons coming in and an electron coming out, that is talking a atom, right? ​

Black-body Radiation
Just to check and make sure I got this right, when a photon is absorbed, the electron jumps up an energy level (which is some energy level/orbit that is further away from the nucleus) and when a photon is emitted, the electron jumps down an energy level (which is some energy level/orbit that is closer to the nucleus). Electrons can only jump if they have the excact amount of energy to get to the orbit. ​
Just wanted to check an make sure this is correct.

Rutherford Back-scattering
Is the empty space in the electron cloud "made" of anything or is it just nothing-ness? ​
Also, in the electron cloud, does the 'empty space' carry a negative charge as well? Or have any properties? ​
I know that the electrons are negatively charged. I don't understand the concept of 'empty space' because there is a repelling force between electrons and I would have assumed that the forces is being transferred over this 'empty space'; and if there is a force repelling the electrons or attracting the electron to the nucleus, then there can't be nothing there because the forces are being transferred there. [I might be looking to far into this and confusing myself. ]

Any help/explanation would be greatly appreciated!

Last edited: Apr 29, 2014
2. Apr 29, 2014

### TheAustrian

Photons:

For photons, you have to use Einstein's Special Relativity Energy-momentum relation:

E2 = m2c4 + (pc)2

So even though the photon has no mass, it's energy and momentum is still related by E = pc.

So this equation for photons (or any massless particles) works like this:

E = pc

Last edited: Apr 29, 2014
3. Apr 29, 2014

### TheAustrian

About Rutherford stuff:

Just don't worry about any of that stuff. Trying to visualize how an atom or nucleus really looks like isn't a good idea, because it is impossible to "accurately" define what they look like. We can only construct models to predict where things are. We can't actually pinpoint for sure at any time where an electron inside an atom is. (all that we know is that it is around the nucleus somewhere)

4. Apr 30, 2014

### Staff: Mentor

I'm sorry to say, but what you wrote is full of misconceptions. I'll will try to clear up to most glaring.

The special theory of relativity is founded on the concept that observers in inertial frames will all measure the same speed of light, regardless of how they are moving with respect to one another. $E=mc^2$ is a consequence of the theory of special relativity.

What was shown with Brownian motion was that it could be explained by postulating that the motion of a big particle displaying Brownian motion was subjected to numerous random hits from smaller particles. It is an argument for the discrete nature of matter (and the existence of molecules rather than atoms, as you stated). Also very important is the fact that that Einstein worked out the size of these molecules from the Brownian motion of the big particle.

Blackbody radiation is not related to the quantization of atoms, but of the electromagnetic field. Placnk showed that the spectrum of a blackbody could be modelled by a collection of harmonic oscillators, each with a distinct frequency $f$, if one postulated that each oscillator can only be excited in discrete amounts $E = n h f$, with $n$ a positive integer and $h$ now known as Planck's constant. This results, for a given temperature, in the harmonic oscillators with a high frequency $f$ not being excited at all.

Just a comment: this links in to the previous point about blackbody radiation, as it showed that this quantization of the EM field was fundamental, that these packets of energy actually existed, and that it was not just a mathematical trick to get the right blackbody spectrum.

I don't know what "atomic particles" mean. It is the nucleus of the atom that decays. And I would say that the damage done is more at the molecular level than at the atomic level (but this is getting out of my area of knowledge).

Wrong, see above.

No no no!!! Photons are the particles making up electromagnetic radiation. Alpha particles are nuclei of helium atoms, and are definitely massive!

It is made up of photons, which can be seen as packets of energy.

"Speed of light" can't have "energy". See the post by The Austrian for the rest of the answer.

It is photons (electromagnetic radiation, can be light) hitting a solid and removing an electron from an atom.

Empty space is never completely empty (vacuum fluctuations), but lets keep it simple. Yes, you can consider that between the nucleus and the electron, there is nothing but empty space. But the lectron in an atom is not at a fixed position at any given time, but delocalized all around the nucleus. So you could also see the atom as a nucleus surrounded by a "spread" of electrons, with no real empty space at all. Empty space is electrically neutral.

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