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"Light" vs RF

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LAN_403

Ownage

Tue Nov 13 2007, 04:23AM

Registered Member #1108 Joined: Thu Nov 08 2007, 11:16PM
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I was doing some searching on google about EMP and Faraday cages and it got me thinking. What is the difference between light and RF??

Yes I know that they are all electromagnetic radiation just at different wavelengths. But is light a special form of ER??

*Visible light carries more energy than RF or Microwaves on the ER Spectrum, but how come high intensity microwaves will burn oneself and can immediatly damage ones eyes, but visible light won't.

*If visible light is a form of ER, how come a faraday cage will not block it as it will RF or Microwave?

*Can light be formed using an RLC circut tuned to a very very high frequency. If RLC can create RF how come it can not create visible light, it is all just simple ER?? All "light" seems to be formed using diodes or bulbs.

*How come visible light can be blocked by a piece of wood (opaque) but all other forms of ER (RF, microwaves) can pass through.

Just random things I've kind of been wondering. It bugged me enough to ask people to find out, hence making this thread :).

Shaun

Tue Nov 13 2007, 04:31AM

Registered Member #690 Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616

A faraday cage will only block wavelengths significantly larger than the holes in the cage. Take the grille on the front of your microwave oven, for example. The microwaves (wavelength 12.4cm) are far too large to get thru the holes. But visible light (wavelength in the nanometers) easily gets through.

And high intensity light WILL damage your eyes easily. Looking directly at a 1000W light source (not a 1000W bulb, which may emit only 100W of light, the rest heat) will blind you. Try staring at the sun later...you'll see (or maybe not)

...

Tue Nov 13 2007, 04:58AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

They are 100% exactly the same thing.

You could, in theory, make light using a very fast tuned LC circuit, but we do not yet have switches that can switch at much over 1THz (very very far into the IR).

On that note, currently the most advanced method of making 'light' from an electrical circuit is to use essentially a very very fast photo diode with a very small helical antenna (about hundred uM across) wrapped around it. You shine 2 lasers on it that have very slightly different wavelengths on it, and because of constructive/destructive interference you get a 'beat' frequency at the difference of the frequency of the lasers, which then switches the photo diode on/off at that frequency.

On the system I was working on we could 'only' go to about 2THz (about 150uM), but that was close enough to 'light' enough that you could use lenses/mirrors to manipulate the beam.

BTW, 500nM (green light) is 600THz, and 10uM (light from a CO2 laser) is only 30THz

Dr. Slack

Tue Nov 13 2007, 08:34AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

*How come visible light can be blocked by a piece of wood (opaque) but all other forms of ER (RF, microwaves) can pass through.

When one examines what is transparent, or opaque, to what wavelength, it initially appears pretty random, and only becomes comprehensible when the scale of the object and its structures is taken into account. Is the internal structure regular like a crystal, random like a gas?

A couple of for instances - germanium is opaque to visible, but transparent to IR (just as some black plastic 35mm film containers are), window glass is transparent to visible but opaque to IR (depending on grade).

One fun demo in schools is to use prisms and lenses made of polythene or paraffin wax to do geometric optics with 3cm microwave radiation from a klystron, and diffraction and interference experiments with suitably large-scaled gratings and slits.

Compare the scale of atomic structures on a butterfly's wing, the ones that give it the irridescent blue reflection, with the copper rings and crosses on a radome "frequency selective surface".

It's all in the scale.

One recent approach to the electrical aspects of visible light is the identification of plasmons, electrical excitations one the edges of holes in an illuminated conductive film, which appear to mediate the absorption and re-radiation of photons.

We have made superheterodyne receivers for light. You may be familiar with the front end of a receiver where an RF local oscillator is mixed with the input RF signal in a shotky diode mixer to generate the difference frequency. If you shine a locally generated laser on a non-linear crystal, and also shine an incoming signal laser on the same crystal, you can recover the difference frequency electrically from the crystal. This is how you can do spectrum analysis of light back-scattered from defects in optical fibres.

What leads my inituition astray is the fact that RF "obviously" goes about in wires as electricity and jumps off antennae as continuous waves, and gamma ray photons "obviously" zoom about as particles. However, visible light is at an energy where both effects are quite apparent at the scale of human biology and engineering, so help marry the two together. However, the scale effects above would still be present, even if ER was completely classical, its quantum nature just adds another intuition buster.

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