The photoelectric effect is possibly one of the most important examples of quantum physics having a noticeable physical effect. Most important because it was the experiments and the results gathered from photoelectric experiments which led to the development of quantum physics. When electromagnetic radiation is shone on a metal it will give out electrons from its surface. It was shown that it was the frequency of light, not its intensity, which triggered the release of electrons, as a threshold frequency had to be passed before any electrons were released at all. This is because all metals have a work function, a binding energy through which they hold electrons to their surface. Incoming photons have to have an energy at least matching this work function or nothing will happen.
Stopping power is the ability of a material to slow down charged particles (stopping power is only used for charged particles) passing through it by removing their energy through interactions with the particles in the material. In the early 1930s a physicist named Hans Bethe developed the Bethe formula for calculating the average energy loss over a distance within a material:
where me and e are the mass and charge of an electron; I is the average potential of the electrons in the material and n is their number density; and v and z both refer to the particle being retarded, being velocity and charge respectively (it should also be noted that this is a simplification for a non relativistic particle. Electrons and fast moving particles require corrections to the above formula). Continue reading Calculating Distance Over Which Charged Particles Pause
Axotomy is the process of severing of an axon which is normally done with a laser in today’s modern society.
Once a mammal has become an adult their ability to heal neurones and repair the nervous system becomes practically non existent. Of course there are some species of animal of different class which can repair nerve damage although these are normally simple creatures. One such example is the roundworm caenorhabditis elegans (called a nemotode in the US) which can regenerate broken neurones and is commonly studied as it is also transparent so optical observation of healing can be done. Other advantages of using this worm in studies is that they can be grown quickly and cheaply as well as being vulnerable to the laser controlled axotomy mentioned above.
Radiation pressure is a concept that is quite easy for a physics layman but becomes harder to understand once you’ve become a physics neophyte and understand some of the basic concepts. Telling someone with no physics knowledge that light produces a detectable but really small amount of pressure and they will probably believe you. Tell someone with some physics knowledge that they will point out that photons of light have no mass and so no momentum and so can’t produce any force at all. The mistake inherent here is the assumption that something with no mass has no momentum. In truth the definition of momentum as the product of mass and velocity isn’t quite accurate and so photons of light can produce a force when they bounce of a surface and this force produces a radiation pressure.
One of the most interesting questions I have been asked was on the concept of eternity (in a divergent point “eternity” is a poorly used word with people using it to both mean “constant” and “forever” even when they themselves do not agree with the situation they use it in). Time appears to have had a definite start point, that being the beginning of the Universe. Before the universe there was no time and yet despite the finite time of the past it seems like there is an infinite possible future. This thought ultimately comes back to the question of what is time? One of the most common definitions is that the arrow of time always moves in the direction of increasing total entropy, which is true. Entropy must always increase. I have also heard the interesting definition that the arrow of time must always move so that radiation travels in outgoing spheres. Spheres of radiation waves that travel to a point are impossible and so it can be seen that time and growing spherical waves are connected (although perhaps just by the entropy definition). Is it then possible that once there is no more work left to do, and entropy has reached a maximum, time no longer has any meaning. For a start it won’t have any meaning because there will no being left to experience it but also because there will be no more change. The universe will have finally become static, frozen and unmoving. In other words a universe devoid of time.
I suppose we have to hope that dark energy really does exist and that there is some force working to prevent such an end that we can harness but this really beyond the concern of anyone currently living. Until tomorrow, goodnight.
I have briefly written before about a physical theory called the electron gas where the electrons in a metal, having become separated from their ions, can be treated like an ideal gas. This involves the homogenising of the background positive charge caused by the metal ions and also ignoring the Coulomb repulsive force between the electrons. When this force is added the situation becomes known as the electron liquid model and it requires advanced computational methods to solve the many body problem that results (as all electrons modelled will exert some repulsion on each other).
In optics the refractive index of a materiel is dependent on the wavelength of light passing through it. There is no perfect equation which can theoretically predict the refractive index as a result only very tedious and computer intensive calcuations can give a definite result; but there are empirical and experimentally derived expressions which are very accurate. An example is the Sellmeier equation which is most commonly written as:
when applied to glass. The Bs and Cs are constants found for the particular material. Now when equation contains three terms it will give the refractive index to a value within 5×10−6 of the actual one. More terms can be added to improve the precision and the summation sign is used when required. For gasses the form of the Sellmeier equation is: Continue reading Producing More General Form Of Optic Predicting Equation