describe of light and explain how it help determine the arrangement of atom and molecules in solids. what type of light ( electromagnetic wave) is used when probing solids?
"Describe of light" : isn't clear what that means.
For probing solids, usually you're trying to figure out where the individual atoms sit, within their molecules, and how the molecules are stacked up within the solid (if, indeed, they are regularly stacked).
If the molecules are regularly stacked, we call this a crystalline solid. This kind (which is usually what's looked at) is easy to probe with X-rays of appropriate wavelength, using the technique of X-ray diffraction.
You can best take a single crystal, put it in the path of the X-rays, and look for the directions in space in which X-rays are emitted from the crystal (they're being diffracted = sent in various directions different from their original direction of travel). These directions is space are related to the exact wavelength of the X-rays used and to the positions of identical atoms within molecules within stacks in the crystal. If this seems weird to you (which it might), consider as an analogy trying to push a baby stroller over a cobblestone street. The crevices between the cobblestones are continually trying to catch the wheels of the stroller and change the direction of the stroller! Inside the crystal of the solid, the same thing is happening, except that the atoms (the tops of the cobblestones, rather than the ruts between them) are doing the deflecting, and the stroller equivalent (the X-ray particle) is like a long horizontal row of wheels. When all the wheels catch cobblestones at the same time, look out, the stroller is going to be thrown! In which direction it is thrown, for the X-rays, depends on the spacings of the X-rays and the atoms; a condition called "constructive interference" is satisfied, somewhat like happens with sunlight shining on thin oil slicks on a wet, dark-colored street (certain colors are reflected from different places in the oil slick, depending on the spacing of the light (called the wavelength) and the spacing of the oil layer (its thickness)).
You can also do X-ray diffraction using a finely ground powder of a crystal; the result here is emission of rings of x-rays (centered on the original beam).
Also, there are other techniques involving electromagnetic radiation (= light in its many forms) that furnish information about arrangements of atoms within a single molecule, such as NMR (nuclear magnetic resonance); I don't think your question is asking for them, though.
By the way, the description of X-rays would be, light of a very short wavelength (shorter than visible and ultraviolet light, but longer than gamma rays); you can look up a definition and more information on the web.
Hope this helps you a little,