Dr. Fredy Zypman, program chair of the M.A. in Physics at the Katz School, published the paper, “Permittivity from First Principles,” 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’s resting on the rim, it should look as though the straw is broken in two. That’s 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, “Permittivity from First Principles,” published in September by AIP Advances, 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 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. “Matter around us—in plastics, liquids, our skin, the air—have characteristic electrical properties,” said Dr. Zypman. “When the electrical field travels from one medium to another, light must adjust to the local environment. That’s why we see shadows and reflections—those rich sources of optical sensations—all around us.”

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’s 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—its polarizability, or tendency to separate positive and negative charges—to electric fields. Another way of looking at polarizability is the tendency of a material’s atomic electrons to become distorted when the material is placed in an electric field. The larger the electric polarization, the larger the material's permittivity—or the property of a material that measures the opposition it offers against an electric field.

“We aimed to produce a clear conceptual pathway between the atomic polarization and the familiar refraction,” he said, “The 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.”