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1. I remember the theme music by Vangelis.
2. (Original post by Stonebridge)
I remember the theme music by Vangelis.
I remembered it when I heard it.

Now appears that they were running a one off night of Brian cox's picks and won't be repeating the whole series...
3. Hello, long time no here!

Is there a "special table" for radioactive elements (and isotopes) where the half periods can be recognised? or a list which contains the half periods of all kinds of radioactive elements? I have a big interest in that at the moment.
4. Hello

Can someone please describe and explain the differences between the interference pattern from double-slit and diffraction grating?

5. (Original post by TaraStilton)
Hello

Can someone please describe and explain the differences between the interference pattern from double-slit and diffraction grating?

Differences:

- the interference in a diffraction grating causes more minimums in comparison to double slit, see here
- the diffraction grating has many different maximums, the one which have small amplitudes and the one which have great amplitudes (see the link).
- the double slit has three maximums with equal amplitudes (see the link).
- That is why the diffraction grating has differences in brightness, while the double slit has three equal brightnesses (see the link).
6. (Original post by Kallisto)
Differences:

- the interference in a diffraction grating causes more minimums in comparison to double slit, see here
- the diffraction grating has many different maximums, the one which have small amplitudes and the one which have great amplitudes (see the link).
- the double slit has three maximums with equal amplitudes (see the link).
- That is why the diffraction grating has differences in brightness, while the double slit has three equal brightnesses (see the link).

Thanks!

The thing I don't get is why double slit has equal amplitudes whilst single slit has varying maxima and minima?
7. (Original post by TaraStilton)
Thanks!

The thing I don't get is why double slit has equal amplitudes whilst single slit has varying maxima and minima?
Perhaps this picture helps you.

As you can see the highest maximum is in the middle, while the maxima on the left and right side are getting fainter. That has something to do with distribution of intensity. Its most affected in the middle, but the more the light waves are away from the middle, the fainter the intensities are. The same applies to minima.
8. Aspiring theoretical physicists http://physicsandphysicists.blogspot...l-physics.html
9. Hi! It's so nice to find a group of other physics nerds. My own field is chemical physics, pretty much everything on the border of the two disciplines, but I'm happy to talk about anything in any field of physics - all of it fascinates me.
10. (Original post by Kallisto)
Hello, long time no here!

Is there a "special table" for radioactive elements (and isotopes) where the half periods can be recognised? or a list which contains the half periods of all kinds of radioactive elements? I have a big interest in that at the moment.
I don't think there is unfortunately, it'd be massive if it was and that's without including intermediate nuclei. Also I'm pretty certain that you can make any element radioactive - you just need to force enough neutrons into its nucleus to destabilize it. Since you're interested, a really cool element to read about is 114. Its got a ridiculously long half life compared to its neighbors and has been dubbed "the island of stability." It's pretty nifty. Those magic nuclei sure are something.

I remember seeing some article about it on a conspiracy theorist's website too. The guy thought you could stabilize it and that the government is using it to make some kind of space-time bender by "magnifying the strong force from its nucleus." It was pretty hilarious, you should definitely check it out. I don't remember the guy's name but it should come up if you google element 114.
11. (Original post by Peroxidation)
I don't think there is unfortunately, it'd be massive if it was and that's without including intermediate nuclei. Also I'm pretty certain that you can make any element radioactive - you just need to force enough neutrons into its nucleus to destabilize it. Since you're interested, a really cool element to read about is 114. Its got a ridiculously long half life compared to its neighbors and has been dubbed "the island of stability." It's pretty nifty. Those magic nuclei sure are something.

I remember seeing some article about it on a conspiracy theorist's website too. The guy thought you could stabilize it and that the government is using it to make some kind of space-time bender by "magnifying the strong force from its nucleus." It was pretty hilarious, you should definitely check it out. I don't remember the guy's name but it should come up if you google element 114.
Yeah, that is right. After 'shooting' by a neutron, it comes to a decay of the nucleus, so particles are emitted. That is the radioactive ray. From this perspective, even a hydrogen nucleus can be destabilized by hitting it.

Element 114 sounds interesting. Maybe I have a closer look at it next time.
12. (Original post by Peroxidation)
x
Hello and merry christmas to you for first!

You said not that long ago that you are very interest in particles and the characteristics of it. What about the physical aspects of them?

Would love to know the principle of photoionisation and how it works. That is why I have so many questions: what is the difference between photoionisation and stimulation of an electron by a photon in general? which circumstances have to come true that photoionisation happens? are there huge differences between 'normal' ionisation and photoionisation? thanks!
13. Merry Christmas to you too, though I guess now it's more accurate to say merry 3 days after Boxing Day

Electrons inside atoms behave in some really wacky ways. They can only have certain energies because of the wave functions that are associated with them (that's a whole different topic though). They can also jump from one energy level to the next, assuming that there's an unfilled orbital there. This is what enables atoms/molecules/whatever to absorb photons. Photons with energies equal to these energy gaps are absorbed by the electrons which causes them to be promoted to higher energy levels. This is the normal stimulation of the electrons which doesn't result in ionisation. Naturally this high energy state isn't very stable so the electrons decay back to their original energy levels quite quickly. They can do this by remitting the original photon, scattering the photons in all directions and causing colour, or by making smaller jumps down and releasing infra red photons, causing the material to heat up slightly. The route taken depends on the availability of unfilled orbitals in lower energy levels.

Photo ionisation works in almost the same way. However, the photon has so much energy that it gives the electron enough energy to leave the atom. You can think of it like the escape velocity from a gravitational field.

Most photons don't have enough energy to do this, it's only the shorter uv wavelengths and shorter from there that can do it. I don't know for sure but I'd wager that all uv wavelengths can do is kick out valence electrons. The real fun starts with X-rays (sorry Mr. Uv you're just not beefy enough). X-rays are capable of knocking the inner electrons loose (or at least some are)! This results in bremstrausslung radiation being emitted. That's just a fancy German word for X-rays being emitted as the electrons move down to fill the gaps in lower energy levels. You get one going down, then that leaves a gap so another moves down and you get this big chain of X-ray emissions. It's pretty nifty. Gamma rays do pretty much the same thing as X-rays but more often. I suppose part of the bremstrausslung could be longer wavelength photons, but I've only heard the term used with X-ray emissions.

Photo ionisation isn't particularly common though. The photons are really weakly ionising so you need a big dose of them to get the same damage as an alpha particle or beta plus or minus etc. Having said that, it's still pretty dangerous. There was this one guy (this is a well known story but I can't remember his name) back when the imaging use of X-rays was discovered who worked in a photography shop. They had an X-ray producing thing (most likely something like a cathode ray tube) and used it to amaze customers by getting the poor sod I mentioned above to show off the bones in his hands on photographic film (I can't remember the name of the film they use but it's 3am so meh). The guy had to quit the job 4 days in because his hands had become swollen bright red blobs of dermatitis and the skin was peeling off them (if you're eating while reading this I'm sorry!).

The poor shlub's troubles didn't end there, the long term effects of his extreme over exposure were skin cancer in both his hands. It spread up both his arms and in the end he had to have them amputated. It was done too late though and he soon died of his cancer R.I.P poor photography shop shlub. The moral of the story is to always respect the X-rays!

Sorry if I've scared you, it's 3am and I'm too tired to consider the fear factor of these stories

If we don't converse again before new year have a really good new year!
14. Ahh, it's a while since I studied Physic's now
used to love it as a subject, but don't study it now. I did AS Level (and failed, it's a long and complicated story though) then went onto a Diploma and now at uni both In Outdoor Activities (not much Physics nowdays )

However, as I only did AS level/GCSE that is completely beyond me!

Something simpler would be nice, but either way Hello Physics my old friend...
15. (Original post by Peroxidation)
(...)
Photo ionisation works in almost the same way. However, the photon has so much energy that it gives the electron enough energy to leave the atom. You can think of it like the escape velocity from a gravitational field.
That is to say only photons in ultraviolet range are energy-rich enough to ionise electrons, so to overcome the attactrion of the nucleus. I see.

(...)I don't know for sure but I'd wager that all uv wavelengths can do is kick out valence electrons. (...)
Well. So photoionisation works only, if electrons are on the outermost electron paths?

(...)Photo ionisation isn't particularly common though. The photons are really weakly ionising so you need a big dose of them to get the same damage as an alpha particle or beta plus or minus etc. Having said that, it's still pretty dangerous. There was this one guy (this is a well known story but I can't remember his name) back when the imaging use of X-rays was discovered who worked in a photography shop. They had an X-ray producing thing (most likely something like a cathode ray tube) and used it to amaze customers by getting the poor sod I mentioned above to show off the bones in his hands on photographic film (I can't remember the name of the film they use but it's 3am so meh). The guy had to quit the job 4 days in because his hands had become swollen bright red blobs of dermatitis and the skin was peeling off them (if you're eating while reading this I'm sorry!).(...)
I see. Have never thought that photons are able to gain such a high energy that it is the same like X-ray particles (in energy levels at least). It stands to reason why this method of ionisation is not often used in physics.

To sum up the essentials of photoionisation:
1. It need high energy levels to work anyway.
2. This energy level is equal to the one of X-rays, that is why it is very dangerous.
3. Even if this required energy was achieved by photons, electrons have to be on the valence path to be ionised, otherwise the attraction of the nucleus is too strong.

Am I right with my thoughts?
16. Mostly correct

Firstly I need to correct myself though. I got the term bremsstrahlung and double photoionisation mixed up. Bremsstrahlung is actually the radiation released when a beta particle slows down due to the attraction from atomic nuclei, this happens mainly in heavy elements. This picture sums it up nicely http://physics.uwb.edu.pl/ptf/fizyka...es/braking.jpg

Photons which can cause photoionisation tend to be UV wavelengths and shorter. As far as I'm aware, photoionisation by UV photons only takes place with the valence electrons, because these require the least amount of energy to remove from the atom. Shorter wavelength photons like Xrays can knock out the inner electrons as well as valence electrons, because they have much more energy than UV photons. This increased energy allows them to give the inner electrons enough energy to escape the nucleus' attraction. This leaves a "hole" in (one of) the inner shell(s), where an orbital is now only half filled. That causes more radiation to be released as infra-red or visible light as the electrons move down energy levels. here's a good diagram, careful not to confuse the bottom and top ones though. The bottom one is double photoionisation - where 1 photon knocks out 2 electrons. http://www.esrf.eu/files/live/sites/...RS/Figure5.jpg
17. (Original post by Peroxidation)
(...)
Photons which can cause photoionisation tend to be UV wavelengths and shorter. As far as I'm aware, photoionisation by UV photons only takes place with the valence electrons, because these require the least amount of energy to remove from the atom. Shorter wavelength photons like Xrays can knock out the inner electrons as well as valence electrons, because they have much more energy than UV photons. This increased energy allows them to give the inner electrons enough energy to escape the nucleus' attraction. This leaves a "hole" in (one of) the inner shell(s), where an orbital is now only half filled. That causes more radiation to be released as infra-red or visible light as the electrons move down energy levels. here's a good diagram, careful not to confuse the bottom and top ones though. The bottom one is double photoionisation - where 1 photon knocks out 2 electrons. http://www.esrf.eu/files/live/sites/...RS/Figure5.jpg
I see. If photons have the wavelengths of X-rays, the photons are even able to ionize electrons which are not on valence electrons and so on inner electron paths. Have taken a closer look on the picture of photoionisation. Can more than one electron be ionised on inner shells, if the wavelength of a photon is greater than the one of an x-ray? would interpret b.) so. After ionizing electrons from higher paths are going into the "holes" and an emission of light comes into being, right?
18. (Original post by Peroxidation)
x
Hello, me after a while!

I have heard that physicists have discovered gravitational waves. The one which were supposed by Einstein, but would never be discovered after his estimation. Do you know more about that?
19. (Original post by Kallisto)
Hello, me after a while!

I have heard that physicists have discovered gravitational waves. The one which were supposed by Einstein, but would never be discovered after his estimation. Do you know more about that?
The full paper is up online if you want to see the graphs - I expect it's not behind a paywall because of government funding.

https://journals.aps.org/prl/abstrac...ett.116.061102

the list of authors is crazy... and afaik they have to pick just 3 of them for the Nobel
20. (Original post by Joinedup)
The full paper is up online if you want to see the graphs - I expect it's not behind a paywall because of government funding.

https://journals.aps.org/prl/abstrac...ett.116.061102

the list of authors is crazy... and afaik they have to pick just 3 of them for the Nobel
Have taken a closer look in the last week to the gravitational waves. Just to understand the meaning of these ones:

If two neutron stars or two black holes are circling each other, a distortion of space-time comes into being what emits gravitational waves which are causing a length variation of objects in the range of those waves. That is to say the objects are getting shorter or longer, even if its a tiny bit. And this length of variation was proved by high sensitive lasers which were really changed in their length a little tiny bit.

Yeah, that is right. three people at the maximum are just able to win a nobel prize in physics in a year, not only in physics, but also in another categories.

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