Take the 2-minute tour ×
Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. It's 100% free, no registration required.

An electric field in a conductor causes charges to redistribute so as to cancel out the original field, bringing the field to zero. This is, I think, a common argument for why conductors are generally opaque to EM waves.

But some conductors are transparent, including various electrolytes and indium tin oxide in LCD displays. What are the mechanisms by which conductors can be transparent?

share|improve this question
add comment

2 Answers

up vote 7 down vote accepted

Your question gets at two related points: What allows materials to conduct, and what allows materials to be transparent.

Conductivity requires easy movement of charge through a material. Typically this charge is in the form of electrons but in an electrolyte the charge movement is primarily ions moving in the fluid. The quantum-mechanical nature of electrons means they can only exist in certain, discrete energy levels. The specific levels available depend on the electron configuration of the atoms or the (more complicated) electron configuration of molecules. Good conductors have many closely-spaced energy levels available to electrons and very little energy is required to move them between these levels. This is the band theory of conductivity.

The opacity of a material also depends on the electron configurations in the material and the energy levels available. The energy of a photon is proportional to its frequency which means materials tend to act as a low-pass filter. High energy (high frequency) electrons have enough energy to kick electrons into higher energy states which cause the photon to be absorbed. If the material has big gaps between the energy bands then photons won't have enough energy to force an electron energy transition so the light will pass through without being absorbed.

Generally conductivity and transparency are at odds with each other. Conductors need closely spaced energy levels and transparency needs widely spaced levels.

In the case of indium tin oxide has a mix of both properties. It has electron configurations that are closely spaced at low energies and then a large bandgap at visible light energies. This causes ITO to be opaque to low energy light (infrared) but transparent to visible light.

share|improve this answer
add comment

Your question is formulated in a strange way. It is more constructive to study the mechanisms why exactly conductors are non-transparent.

However, if you pose the question like this. Conduction means current response at zero frequency. Light absorption/reflection means current response at (quite) high frequency. There is number of ways to make a material with completely different response at different frequencies. If you speak of metals, such mechanism may be to prepare a layer of material thinner than its skin depth. If you consider semiconductors, they might be quite good conductors and at the same time have a bandgap wider than energy of photon of visible light. For electrolytes current is transferred by heavy ions which are "slow" and may be accelerated by constant electric field and feel only the averaged (i.e., zero) field of electromagnetic wave.

share|improve this answer
add comment

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.