Lenses and Mirrors for Other Wavelengths

I know we have lenses and mirrors for visible light made of glass or plastic.

What other wavelength do we have lenses/mirrors for? I mean, I guess active lensing such as an electromagnetic field that acts like a lens would apply as well.

Can we focus/reflect X-Rays? Gamma rays? UV? IR? Microwave? Radio waves?

Regards,

Groman

Yes. That’s why satellite dishes work. :slight_smile:

Astronomers build different telescopes to focus light from all different parts of the spectrum.

For most of the IR and UV part of the spectrum, you can use the same glass lenses and metal-coated mirrors that we use for visible light.

Radio waves are reflected by a sheet of metal, or, for longer wavelengths, by a screen. Astronomers, of course, don’t have the monopoly on this techonology, but we do get testy when people refer to the VLA as “all them satellite dishes.”

X-rays and gamma rays are focused using grazing-incidence optics, where the light barely skims the mirror. This lets the rays be deflected, kind of the same way a bullet can bounce off a wall if it hits at a shallow angle, though it would punch right through the wall if it hit head-on.

X-ray telescopes exist, thought they’re a rather extreme case. Currently in orbit are Chandra and Swift. The latter page has a brief explanation of how they work: basically the “mirrors” are designed so that the photons only glance off them, rather than be reflected back as in more conventional telescope/antenna designs.

You can divide the technology of electromagnetic waves into roughly three categories based on wavelength: Long (radio, microwave), Medium (infrared to ultraviolet), and Short (x-ray, gamma).

Long waves I don’t know how well lensing and mirroring work because wave effects like diffraction predominate for all but the largest objectives. You’ll read about microwave “lenses”, but I’m not sure if they really work by diffraction or not. The famous radio telescope at Arecibo uses a parabolic dish to reflect radio waves to the detector.

Medium waves behave more or less like visible light. The main thing is to use substances that are shiny (for mirrors) and transparent (for lenses) for the wavelengths of interest. The Hubble telescope uses a special coating on it’s main mirror in order to be able to work in the infrared, visible and ultraviolet ranges.

Computer chip manufacturers need the shortest wavelengths they can use and still be able to use lenses to focus an imaging mask onto the chips. Currently the best material available is calcium fluoride, which can be used with UV light with wavelengths as small as 170 nanometers.

Short wavelengths present a problem because of the way they interact with matter. Most materials will absorb short wavelengths rather than reflect or refract them. The very shortest wavelengths start to be on a par with the size of atoms and the spacing between them. One way around this is to use mirrors that consist of multiple layers each about as thick as the wavelength that is to be reflected. Diffraction effects cause a backscatter between the layers that end up reflecting some of the short wavelength light back. Parabolic mirrors made this way can be used as imaging elements for so-called Extreme Ultra-Violet light.

      • Long-wave IR lasers (used for cutting purposes) often use copper mirrors for optics, as copper is highly reflective at such wavelengths. There is probably some material that would work as a conventional lens, but it’s not commonly used.
        ~

The obvious answeers have been covered, but I’ll note that , as the refractive index varies with wavelength, lenses that work at one wavelength won’t work as well (or will focus in a different place) for other wavelengths. This is already an issue in the visuible, where you’d very often like all the wavelengths to focus at the same place (This isn’t an issue for mirrors, where all wavelengths reflected focus in the same place). One solution has been the achromat, where you use lenses made of two or more different materials with different refractive index variations, so that the differences at different wavelengths cancel out. In practice, it’s impossible to correct perfectly at all wavelengths, but you can keep the errors downacross the visible range.

This variation with wavelength can get to be a problem when you move out of the ranges people normally use. If you’re trying to build a system for focussing infrared LED light, for instance, the focal distances will all be slightly off. Work at two different IR wavelengths, and you might really have a problem – the achromat princiopl still works, of course, but everyone’s building achromats in the visible, not at your wavelengths. If you absolutely needed an achromat in the IR, you’d have to design and build it yourself, or get someone else to.

One nice thing is that, as you move out of the visible, you get more materials you can work with. You can’t see through silicon and germanium, but they’re transparent in the infrared. What’s more, their refractive indices are enormous compared to the indices of visible materials.
In a heavily-used range there are often a lot of pre-made items you can use. This is good, because they’re cheap. If you go into a new regime to exploit some unusual fluorescence or laser line, you may be on your own. As mentioned, x-ray and gamma-rays are often focused using grazing-incidence reflections (since they go therough most things without much bending0, but there are other options, like using diffracting apertures and Fresnel Zone Plates and Photon Sieves to focus these wavelengths.

My lab has a set of lenses and prisms made of paraffin wax that work quite well with 12 cm microwaves.