Optical reflection by metals

[harambe]

https://eng.libretexts.org/Core/Materials_Science/Optical_Properties/Metallic_Reflection

According to this article,metals are good reflectors at low frequencies.What about at high frequencies like visible or UV spectrum?Will this show deviation

 

[sophiecentaur]

At higher frequencies, the behaviour of a metal surface becomes different as the energies are higher. X rays will tend to travel through, rather than being reflected. However, the reflectivity is higher at oblique angles. An X Ray telescope can be made by using the sides of a parabolic reflector to focus an image onto a sensor, using to oblique reflections. https://imagine.gsfc.nasa.gov/science/toolbox/xray_telescopes1.html

 

[harambe]

So for frequencies,the light will simply pass through the metals rather than being reflected so there will be deviations for UV,X rays,gamma rays,etc,right?

 

[sophiecentaur]

So for frequencies,the light will simply pass through the metals rather than being reflected so there will be deviations for UV,X rays,gamma rays,etc,right?

It's not possible to generalise because there are such a variety of material structures. You will have heard of Refractive Index which is often used to characterise the way (visible) Light is affected by transparent materials. RI in that case is a Real Number. To characterise many substances, you need the Complex Refractive Index (a complex number) and the predominant part of RI for metals is Imaginary; EM waves do not propagate through metals (i.e. mostly reflected) at optical frequencies. By the time you get to X rays, both the real and imaginary parts of the RI are significant and metals stop being easy to describe.
X rays and gamma rays will not interact with the charges that affect light and radio waves because the photon energies are too high. They tend to plough on through the surface but (as for light on glass at oblique incidence) they will also be partly reflected (a fraction will pass through and be absorbed) and reflected more with oblique incidence.

 

[DrDu]

Often, metals can can be described in terms of the so called plasma frequency. Below, they are reflecting, above, they are transparent.

 

[Henryk]

Actually, at very low frequencies, metals don't reflect EM radiation, they conduct it.
Only when the size of the metal piece is much larger than the wavelength of the EM wave and the thickness larger than the skin-depth, you get reflection of the electromagnetic wave. (note, metal does not have to be solid, a metal mesh of the size smaller than the wavelength acts pretty much the same as a foil).
Once you get to the point of a metal being larger than the wavelength, the next limit is the plasma frequency. Below plasma frequency, metals are good reflectors, above it, they transmit EM wave. The plasma frequency depends on mass an density of electrons in metals. In most cases it falls in the near UV range and that means that materials reflect visible light of all the wavelengths (giving it the 'metallic' colour). Notable exceptions are copper and gold. Their plasma frequency falls within the visible range, that means they will not reflect shorter wavelength light (blue, blue-green) and that give them their characteristic colours.
To sum up, metallic conductivity gives good reflection of light up to plasma frequency.
Above that, free carries contribution to the optical properties drops off very quickly and they are determined by possible transition from inner atomic shells to higher level. But that is the same for semiconductors and insulators.

 

[sophiecentaur]

Actually, at very low frequencies, metals don't reflect EM radiation, they conduct it.

How low a frequency are you talking of? Radio antennae for pretty well all frequencies have been made, incorporating 'reflecting' parts. Of course, you don't get a specular reflection from a small metal object but where does reflection stop and scattering start?

 

[DrDu]

notable exceptions are copper and gold. Their plasma frequency falls within the visible range, that means they will not reflect shorter wavelength light (blue, blue-green) and that give them their characteristic colours.
/QUOTE]
No, their plasma frequency is in the uv, too. Their colour is due to d to conduction band transitions.