Penetration Properties of Ultraviolet Rays

http://en.wikipedia.org/wiki/Ultraviolet

It has to do with the wavelengths of the types UV. According to what I've read, UVC (shortest wave length) does not pass through glass, where UVA does, because it has the longest wave length. Most of the sun's UVC is blocked by our Ozone layer and UVA and UVB pass through it.

Ordinary glass is partially transparant to UVA but is opaque to shorter wavelengths, whereas silica or quartz glass, depending on quality, can be transparent even to vacuum UV wavelengths. Ordinary window glass passes about 90% of the light above 350 nm, but blocks over 90% of the light below 300 nm.

It depends on the thickness of the glass and the intensity of the UV radiation.

At normal levels of UV C (100 to 280 nm, which has a shorter wavelength than UV B or A) and for normal thicknesses of soda lime glass (ordinary window glass) there is effectively no UV C that passes all the way through the glass.

But glass is an attenuator, not an idealized brick wall, so some amount of UV C will get through according to the Beer-Lambert Law. If the glass is thin enough and if the UV intensity is high enough, some measureable amount of UV C will get through.

If it's something other than ordinary window glass, like quartz or borosilicate glass, the transmission of UV is significantly higher.

Even 'black glass' isn't black when it's ground and polished thin enough; likewise for any dielectric material. That's why even a black material (like obsidian) has an index of refraction. At some thin enough section, light will get through and be subject to Snell's Law and - in this case - to the Beer-Lambert Law.

That's why even a black material (like obsidian) has an index of refraction.

Interestingly enough, you can easily measure the index of refraction of a dielectric that is totally opaque. If you measure the angle that light reflected from a flat surface is totally polarized, the direction of light within the material (the refracted ray) is at right angles to the reflected ray. Snell's law gives the index of refraction as the ratio of the sines of the angles of the incident and refracted ray as measured from perpendicular to the surface.

I'm a bit antiquated, but when I finished my physics training in 1984, there was no theory why any glass would be transparent to any frequency of light. So as you underlined the physics, you must have some theory why visible light can travel through an amorphous solid. If you do, I would like you to describe this.

ignator

The fundamentals of transparency is nicely described here.

There are many non-intuitive reasons that must be considered when recognizing why glass is transparent to visible light. Visible light is a narrow notch of frequencies of the electromagnetic spectrum. The space occupied by solid matter is filled with a lot of empty space. Glass happens to be made of materials that do not absorb visible light. Light does reduce the velocity of the light traveling through it.

This is not new information discovered after 1984 but please do not feel poorly for not grasping how your schooling memory answers most of your question. Light of any wavelength will simultaneously act as a particle and a wave. It defies our intuitive understanding of our world. Physicists still are uncertain when it should be considered a particle or a wave. While each theoretical approach successfully answers many observations, they also often disagree.

IMHO light is the door to quantum mechanics. As Neils Bohr said about quantum theory... "Anybody who is not shocked by quantum theory has not understood it."

That link was a nice review for me. Thanks.

I recently read "The particle at the end of the Universe", by Sean Carroll (ISBN 978-0-525-95359-3), subtitled "How the Hunt for the Higgs Boson Leads Us to the Edge of a New World". It was given to me just minutes before a 5-hour flight, so I was able to read a majority of it in a single sitting; I haven't done that much reading in a single sitting for at least 50 years! I obviously enjoyed it!

In it I think I began to understand how everything (not just light) has both wave and particle properties, and how everything interacts with a variety of fields, not just electric, magnetic, and gravitational. I also saw how out-of-date I am in terms of modern physics!

Ignator, I'm 23 years before you! It seems utterly wrong that there is "no theory" about transmission--I just don't remember what it was. I would have to get out my old textbooks and find it, or maybe Google would get to it easier.

The transparency of an object is not so much a factor of transmission as it is of reflection and absorption. Non-transparent materials reflect or absorb most of the incident light striking them. The frequency of light reflected determines the "color" of the object. The light absorbed is absorbed by relationship to the separation and size of the materials molecules and the wavelength of the light. The light reflected is related to the electromagnetic properties of the material and the separation and size of the atom or molecule. Reflection is an electromagnetic property, which may involve return of the same photon, or may be a new photon created by state rebound when a photon strikes an atom or molecule.

"Reflection is an electromagnetic property, which may involve return of the same photon, or may be a new photon created by state rebound when a photon strikes an atom or molecule."

I beg to differ! True reflection follows Snell's law. Since the incident and reflected rays are in the same medium, there is no change in index of refraction, and the angle of incidence is always equal to the angle of reflection.

if a photon were absorbed and re-emitted, the emitted photon would not be restricted to a specific direction.

In this case we are not speaking of materials that exhibit absolute reflection, but materials in general. Ever seen a surface that looked like it has no absolute plane? They are usually difficult to look at for the very reason that at least some of the light returned from that surface is not responding in agreement with Snell's law. In addition some materials i.e. phosphorus, absolutely generate photon emission that is both out of phase, and angle of incidence with the light that originally struck it's surface. Take a close look at the spot of a laser on any object. You will see shifting moire patterns that are not incident to the surface irregularities of the target. This is also an example of re-emitted photons. Incidently, if all materials reflected light in an absolute manner, then lasers would not be able to cut any object!

Obviously rough surfaces do not have a single surface plane, so Snell's law can only be applied to microscopic portions of the surface, and the portion of light that is reflected by different microscopic portions of the surface can be reflected over widely different angles. Whatever portion of the incident light is not reflected, by definition passes into the second medium and is either transmitted through or is absorbed. Part of the energy that is absorbed may be re-emitted, but unless I am mistaken, the re-emitted energy must be of lower frequency, longer wavelength, than that of the original incoming photon.

Again, unless I'm mistaken, those Moiré patterns are the result of interference between reflections from different parts of the (not perfectly planar) surface. The Moiré patterns I'm thinking of are monochromatic, having the same color as that of the laser. If there were colored fringes, it would be a different story.

You are quite right that cutting by a laser depends on absorption of energy, which means that a laser must be chosen whose light has a frequency that can be absorbed by the material to be cut.

Your observation would be correct except for one thing. The Moire patterns we speak of are moving constantly. Variation in incident surfaces does not explain this. Also if you do the math on the state changes that occur from laser light in any atom you will find that only two states exist due to diffraction and interference. Thus the emitted light is non-coherent, while the reflected light is coherent. This results in patterns that are black, due to total absorption, or the same color as the laser, due to reflection.

The motion of the Moiré pattern is due to motion of the laser, the air, the reflecting surface, and/or the observer.

If there are black areas, they are due to destructive interference of coherent reflected waves. A significant amount of incoherent light would not produce interference, but would produce a more or less uniform background, and therefore would prevent the black areas from being black.