Hi all,
I was thinking about the reflectivity of materials, for example the tiles on the nose of the Space Shuttle, are they choosen for their ability to withstand the heat or for their ability to reflect heat.
For example a mirror reflects all of the light that hit it, what other material would allow this complete reflection?
Which material does reflect Electro Magnetic radiation the best with the least absorbtion or loss?
What material would by thickness, be the best protection against a nuclear EM pulse, for example, would lead be better than gold?
The SS nose is covered with a single piece of Carbon-Carbon that can withstand enormous heat. The tiles over the body reflect some heat, but their remarkable quality is how they absorb heat and contain it internally.
When a Space Shuttle is re-entering the atmosphere it’s subject to friction with the atmosphere, so I don’t think it’s a matter of reflection.
Refractory materials such as those that line furnaces are usually good at reflecting heat, but mainly function as insulators. And like the SS nose, tiles, and furnace linings these things have a limited life when exposed to high heat.
As for an EMP pulse, that’s a different matter altogether.
Not true: When the shuttle hits the Earth’s atmosphere, most of the heating is actually because the air hasn’t got time to get out of the way, so the air gets compressed; and when you compress air, it gets hotter. It’s this compression wave that burns up meteorites and space shuttles - or would if they weren’t protected.
Electromagnetic radiation is a whole range of stuff, from long waves down at the bottom end (useful for transmitting very slow data across polar ice by submarines, but not much else), to radio waves, to light, ultraviolet, gamma rays, etc. These different regions of the electromagnetic spectrum have different properties, and different materials block different parts of the spectrum.
A chain link fence, for example, will block lower frequency radio waves (anything where the wavelength is significantly larger than the holes in the fence) but won’t block light. Your typical lead window glass doesn’t block visible light but does block ultraviolet.
A nuclear blast emits electromagnetic radiation over a very wide range. Blocking the EMP “pulse” that fries electronics is best done by a metal enclosure like a Faraday cage combined with circular conductors tied to an electrical ground. The metal cage will block the electric fields while the circular conductors (called ring grounds and halo grounds) will absorb magnetic energy and will convert it into electricity which gets shunted to ground. The gamma radiation from a nuclear blast will go right through Faraday cages and ring grounds. When you get up to the high frequency ionizing radiation part of the spectrum, your best bet is big heavy metal bricks, like lead. Gold would work, though most people can’t afford to build 8 foot thick walls of gold all around their rooms.
Electronics are going to be more damaged by the radio frequency part of the nuclear blast (what people typically refer to as the EMP). The human body is going to be more damaged by the much higher frequency ionizing radiation. The ionizing radiation can cause the memory bits to get flipped in your electronic devices, causing your smart phones and computers and other electronics to crash. Devices exposed to ionizing radiation will usually recover once they are power cycled, unless they received a relatively massive dose of radiation.
Some percentage is also transmitted, not absorbed nor reflected. This will be as near to 0% as no matter for visible light and an ordinary household mirror, but it could be significant for other portions of the spectrum (a mirror will mostly just transmit gamma rays, for instance).
Note also that the more penetrating a form of radiation is, the less dangerous it is, too. Something that passes right through your walls without interacting is also likely to pass right through you without interacting, and if it doesn’t interact with you, it can’t hurt you.
Are you still referring to EM radiation, because I think atomic radiation would not break my wall but would surely kill me in the long run…
This gamma rays you mention are they part of the original “light” or is this a conversion of sorts by the mirror, would not gamma rays pass through a mirror?
Iron is ferro magnetic - and this gets us into a whole new area. The simple answer is that the field will penetrate the foil. However the field won’t be undisturbed. Ferro-magnetic materials concentrate the field through themselves.
In general you can’t shield against a magnetic field. Not in the same sense as you can shield against say alpha or beta radiation. The magnetic field always continues. What you can do is divert it around you. Here you can use a ferro magnetic material to wrap the field around a body. This isn’t easy. It must completely enclose the shielded space, it isn’t something that you just interpose between a magnet and the thing you want to shield. The usual material used is mu-metal - and it is horrendously finicky is how it is handled. Also, ferro magnetic materials saturate. Once the field goes above a given strength the material ceases to accept more, and suddenly starts to behave like a normal material. So it only works up to a certain point.
A superconductor will reject changes in the magnetic field, and so similarly you could shield a volume against an imposed field with a superconducting case. However, again, superconductors fail when the field strength reaches a critical strength.
You also get para-magnetic and anti-magnetic materials. These also disturb a magnetic field, but not by much.
All the above is really talking about a field that is essentially static. You could consider it as an EM field with a frequency of essentially zero. (Often colloquially termed DC) This is a limiting case of your question. As indicated by the replies above, as the frequency climbs different mechanisms come into play. The wavelength get shorter and shorter ad you need to consider how long it is, and how this relates to the structures it is interacting with. Radio frequencies have wavelengths from kilometres to millimetres and as the wavelength changes what will reflect it changes in scale. As a rule, you need structures that are of similar or smaller size to interact. This nice picture of the Parkes radio-telescope illustrates this well. Note how the inner and outer parts of the dish have different density mesh. The outer mesh adds area to the dish for longer wavelengths only. The shorter wavelengths the middle part can reflect go straight through the outer ring. The outer ring can be lighter, and need not have the same tight tolerances in shape as the inner part, but adds useful sensitivity and directivity for longer wavelength studies.
Once your wavelengths get down to microscopic sizes the quantum effects start to dominate and you have to start thinking in terms of photons and the rules of Quantum Electro-Dynamics to work out what it going to happen.
And to close the loop. You QED to work out how the hell magnets work in the first place. In the end it is all QED, but for a wide range of EM, classical magnetic and electrostatic fields work better (easier) in describing what is going on.
Any highly-penetrating radiation, no matter its form, will be likely to pass through you without harm. Note, however, the key word “likely”. Some small fraction of gamma rays will be absorbed by your body, and that small fraction will, with time or a large enough exposure, add up to significant damage. It’ll just take a lot more exposure than it would with a less-penetrating form of radiation.
And mirrors don’t produce gamma rays. I’m just saying that if you put something which does produce gamma rays on one side of a mirror, most of them are going to go through the mirror instead of being absorbed or reflected by it.
Atomic radiation is an ill defined thing. It encompasses a lot of different things. The EM radiation is gamma rays. But you need to include alpha, beta (not especially worrysome) and neutron radiation (possibly seriously worrying, and what will likely kill you).
Gamma radiation is light - just light with a very very short wavelength (and thus made of photons with very high energy.) Mirrors can reflect light if the structure the mirror is made of is of a similar or smaller scale than the wavelength of the EM radiation. Once you get to light that has a wavelength that is going beyond far ultra-violet, atomic structures are too big. This EM radiation gets called by a new name - X-Rays. It passes through stuff, and doesn’t reflect of anything we know how to make. As the wavelengths get even shorter you run from soft x-rays (they can be adsorbed pretty easily) to hard x-rays. One the way you pass though X-rays that are used to image your body for medical purposes, to higher energy x-rays that can be used to image metal parts (for airplane and other critical uses) and x-rays used to blast cancers. Shorter still and the light gets called gamma radiation. Gamma radiation will pass through anything we can make. If you put a lot of matter in the way you can get lucky and a photon will hit something, and you can stop most of it. but you need a lot of stuff. Lead is a good start.
Actually, we can reflect X-rays, but it’s really hard. The usual technique is to put your mirror at a glancing angle of only a degree or two. With gammas, though, even that doesn’t work, which makes it very difficult to make a gamma-ray telescope.
Yeah, I know. But it is so hard that it hardly counts as a simple mirror. More a sort of Fresnel mirror. It isn’t as if some ultra high energy alien being could comb their hair in one.
