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A glass lens is usually used to focus visible light, but is this glass lens capable of focusing other types of electromagnetic waves? If not, are there lenses made of different materials that can be used for em waves of different wave lengths (radio waves, micro-waves, x-rays, ...)?

Logically I don't see why a glass lens could not focus em waves with different wave lengths. Visible light is bent because of the refraction that occurs between the two mediums, so would changing the wave length of this wave now stop the refraction from occurring?

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    $\begingroup$ In theory, yes, but in practice the lens's material (glass) doesn't refract the same way for all wavelengths, so if you go too far from the intended region it won't work. $\endgroup$
    – zeldredge
    Sep 23, 2016 at 4:01
  • $\begingroup$ @zeldredge Is there a way to calculate the upper/lower limits for the wavelength that will refract as intended? Also what is happening for it not to work? $\endgroup$
    – student.q
    Sep 23, 2016 at 4:07
  • $\begingroup$ At some wavelengths the glass that the lens is made of starts to absorb the radiation. So instead of being transparent as it is for visible light, the lens becomes opaque. To know what the limits are, one can simply look at the specs of the lens as provided by the supplier. Alternatively one can look at the material properties of the glass that the lens is made of. $\endgroup$
    – flippiefanus
    Sep 23, 2016 at 4:28

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This question is really about how the refractive index of glass (and other materials) varies with frequency.
Here the term refractive index being used in its broadest sense it that it has a real part which is a measure of the speed of light through the material and an imaginary part which is called the attenuation coefficient.

For light the frequency dependence of refractive index can be seen with the dispersion of white light and absorption being frequency dependent with the absorption of ultra violet by glass.

I do not think that there is a material which is transparent to electromagnetic radiation at all frequencies and so it would seem that you cannot have a "one material fits all frequencies" lens.

So that is the answer to your question but for an explanation there are two very good answers related to this question in this forum but for a fuller explanation of the topic you could refer to Feynman I-31 and Feynman I-32.

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    $\begingroup$ Besides the refractive index for it to operate as a *lens" it is also equally important that the diameter of the refracting structure be many many wavelengths, and that is quite large in microwaves. Nevertheless, especially in naval radar where size and weight considerations are less important than otherwise this technique is widely used for very broadband scanning antennas. $\endgroup$
    – hyportnex
    Sep 23, 2016 at 11:14

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