Expert: Daniel Mazur - 7/4/2006

Question
Dear sir,
We are a group of physics teachers following a professional
Course. We,d be glad if you answer our questions regarding the subject
We teach:
1. The Electromagnets spread in empty areas and  
    Is ceased by the conducting ones. Why?
2. When we put a barrier on the way of Electro magnets,
    The magnetic field will be zero. Why?
3. How does the mixing (interface) in the white light happen?
   What result from that?
Thank you

Answer
Hello,

1) I am sure you meant electromagnetic waves. The effect is called 'skin effect', saying that conductors only allow propagation of EM waves in their surface - skin. Macroscopically, this is a general result of EM theory for materials with some non-zero conductivity(see http://en.wikipedia.org/wiki/Skin_effect). Microscopically, it is caused by interaction between the electron cloud, the external magnetic field and the ionic lattice of the material. The crystal lattice wants the electron cloud distributed uniformly, while EM waves act as non-uniform excitations. Hence the crystal lattice atenuates the waves as they pass through, as much as it can.

2) Electromagnetic waves and statical magnetic fields produced by electromagnets are two different things. If you are asking about a conducting barrier in the way of advancing EM waves, the conductor will not allow the waves to proceed behind it - to an extent dependent on the shape of the barrier. If you enclose some space into a metal box then no electromagnetic waves inside the enclosure will be allowed. This is the 'Faraday Cage' effect. When you put the same box (made of, say, aluminium) into a static magnetic field, it won't have any effect. Had the box been made of steel, you WILL get an effect on static fields, because steel is ferromagnetic.

3) I do not understand this question. Did you mean splitting of white light into a rainbow of colors on an optical wedge? Or did you mean, why a mixture of colors appears white to us? Please write more details.

Have a good day!
Daniel