Some dumb questions I have been wondering about

Started Mar 28, 2013 | Discussions thread
Leonard Migliore
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In reply to mike703, Mar 28, 2013

mike703 wrote:

Barrie Davis wrote:

Sort of.  First there is the underlying neutral-density-filter effect which just costs a couple of stops of light.  That contribution affects the whole image equally so doesn't alter the DR.

The additional polarising effect selectively reduces in intensity specular refections (off surfaces such as glass, leaves, metal,

Polarising filters do NOT work on reflections from metal surfaces.

Polarising works on non-metal surfaces only...

... unless the light reflected is pre-polarised by a pola filter positioned over the light source itself.

How interesting (and slightly surprising).  Any idea why a metal surface behaves differently from the others?

It's because the electrons in metals are not attached to the atoms; the electrons migrate freely through the metal (which is why they conduct electricity).

In dielectric materials, light interacts with electrons in atoms and excites them to higher energy states. The atoms quickly decay to the ground state, emitting a photon. This is the basis of reflection. One may visualize the excited atoms as vibrating, with the vibration in the same plane as the incident wave. The emitted photon shares this polarization direction. Looking at the geometry of reflection and realizing that light is a transverse wave, you can see that no light can be reflected when the plane of vibration is parallel to the reflected ray (this really needs diagrams; I'm looking at Fundamentals of Optics by Jenkins and White, pg. 524). This causes reflections from dielectrics to be polarized since some incoming polarizations are reflected more than others.

In metals, incoming light is absorbed by the free elections. But the electrons are not coupled to the metal ions so almost all the energy is re-emitted as photons with a 180 degree phase shift. This makes metals opaque because the phase-shifted photons destructively interfere with the incoming ones. It also makes the reflectivity relatively insensitive to the polarization of the incoming light because the plane of vibration of the electrons is nearly parallel to the surface of the metal.

There is a lot of literature on this topic if a more complete explanation is required.

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Leonard Migliore

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