Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: David on 26/04/2010 23:30:02

Title: What makes a surface reflective?
Post by: David on 26/04/2010 23:30:02

David asked the Naked Scientists:
   
Recent shows have described how wet tee shirts become transparent, but how is reflectivity explained on a molecular level, such as in a mirror?

David in Australia

What do you think?

Title: What makes a surface reflective?
Post by: jrussell on 28/04/2010 16:19:37

perhaps a less porous surface allows less light to pass through, therefore bouncing light back, as a reflection?

Title: What makes a surface reflective?
Post by: Soul Surfer on 28/04/2010 23:11:39

Two criteria are required for a surface to be fully reflective (like a mirror) to electromagnetic radiation.  Firstly the surface should be electrically conducting (probably metallic).  Secondly irregularities on the surface should be significantly smaller than the wavelength of the radiation. it follows that the lower the frequency the easier it is for a surface to be reflective and above a frequency limit in the low X ray frequencies the gaps between the atoms are large enough to prevent reflection and no surfaces made of normal materials can be reflective.

Title: What makes a surface reflective?
Post by: JP on 29/04/2010 03:43:43

Once you have a smooth conducting surface, like Soul Surfer explained, reflection occurs because the electrons in a material can be pushed around easily by the incoming electromagnetic light wave.  When electrons move around, they emit electromagnetic waves.  The wave emitted by all the electrons moving around is the reflected wave.

Title: What makes a surface reflective?
Post by: techmind on 30/04/2010 20:59:32

This is actually a very deep and interesting question.

I don't claim to know all the answers, but have given it quite some thought in the past.

I think the point about electrical conductivity can be further clarified in the case that the material is not a complete/bulk conductor. A surface can be reflective to a given frequency (wavelength) of electromagnetic radiation if that material is conductive (or contained conducting traces/threads) over lengths of the order of several wavelengths, and the gaps between the conducting traces or threads are less than about 1/4 to 1/5th of a wavelength. The material can be a near-perfect reflector even if it is full of holes... as long as the holes are a small fraction of the wavelength.
Hence why you can have holes in the 'window' of your microwave oven without the microwaves leaking out.