Dr. Fredy Zypman, program chair of the M.A. in Physics at the Katz School, published the paper, 鈥Permittivity from First Principles,鈥 in AIP Advances in September.By Dave DeFusco
If you took a clear drinking glass, filled it two-thirds of the way with water and placed a straw inside the glass on an angle so it鈥檚 resting on the rim, it should look as though the straw is broken in two. That鈥檚 a consequence of light slowing as it crosses the boundary from air to water, causing the light to bend, or refract.
The root of this simple phenomenon is the focus of a research paper recently published by Dr. Fredy Zypman, director of the M.A. in Physics at the Katz School. In his paper, 鈥Permittivity from First Principles,鈥 published in September by , Dr. Zypman considers the explicit connection between refraction and its atomic origin and proposes a mathematical model to gain physical insight on permittivity, a property related to refraction.
Refraction occurs when light moves from one substance to another, changing鈥speed and direction. The index of refraction is the relationship between the speed of light in a vacuum and the speed of light in a substance. 鈥淢atter around us鈥攊n plastics, liquids, our skin, the air鈥攈ave characteristic electrical properties,鈥 said Dr. Zypman. 鈥淲hen the electrical field travels from one medium to another, light must adjust to the local environment. That鈥檚 why we see shadows and reflections鈥攖hose rich sources of optical sensations鈥攁ll around us.鈥
Transparent materials, like glass, water and diamonds, refract more than air so that when light enters from air, its path bends toward an imaginary line perpendicular to the water鈥檚 surface. Because the index is uniform throughout a material, the bending occurs only as the light crosses the boundary.
This commonplace refractive index, according to Dr. Zypman, depends on the atomic electric response of a material鈥攊ts polarizability, or tendency to separate positive and negative charges鈥攖o electric fields. Another way of looking at polarizability is the tendency of a material鈥檚 atomic electrons to become distorted when the material is placed in an electric field. The larger the electric鈥polarization, the larger the material's permittivity鈥攐r the property of a material that measures the opposition it offers against an electric field.
鈥淲e aimed to produce a clear conceptual pathway between the atomic polarization and the familiar refraction,鈥 he said, 鈥淭he main focus of the study is to produce exact expressions for the permittivity to all orders of an electric field that is valid for any intensity of light, and to compute the permittivity for typical materials.鈥
