# What is reflection and refraction of light at the microscopic scale?

I'm not asking for what reflection and refraction are or the usual law governing it, but I would like to understand what they represent at the quantum atomic, molecular level? In a mirror is it about photons absorbed and emitted with the same wavelength and same direction through atomic electron transitions? How can that be? And what is microscopically refraction? Why should a photon traveling between atoms of a trasparent medium change not only its speed but also its direction? I'm a bit confused... Can someone indicate some nice links explaining all that?

## Answers and Replies

Claude Bile
Science Advisor
Reflection and refraction are linked to a single property of a solid; the refractive index. The refractive index is commonly defined as the ratio of the speed of light in the solid compared to the speed of light in a vacuum, however this definition has two main drawbacks.

1. The term "speed of light" is ambiguous when multiple frequencies are concerned, as we can define multiple velocities (group velocity, phase velocity etc).

2. It says nothing about the properties of the solid.

A better definition of refractive index is $1/\sqrt{\epsilon \mu}$ where $\epsilon$ is the electric permittivity and $\mu$ is the magnetic permeability. Permittivity and permeability are defined as the dipoles generated per unit volume, per unit of electric and magnetic field respectively. These are the atomic properties that affect reflection and refraction on an atomic scale.

Claude.

Reflection and refraction are linked to a single property of a solid; the refractive index. The refractive index is commonly defined as the ratio of the speed of light in the solid compared to the speed of light in a vacuum, however this definition has two main drawbacks.

1. The term "speed of light" is ambiguous when multiple frequencies are concerned, as we can define multiple velocities (group velocity, phase velocity etc).

2. It says nothing about the properties of the solid.

A better definition of refractive index is $1/\sqrt{\epsilon \mu}$ where $\epsilon$ is the electric permittivity and $\mu$ is the magnetic permeability. Permittivity and permeability are defined as the dipoles generated per unit volume, per unit of electric and magnetic field respectively. These are the atomic properties that affect reflection and refraction on an atomic scale.

Claude.

But why does dipole generation in an amorphous material emit photons always in the same direction? I would have expected an isotropic re-emission of photons in every direction. And how do we have to distinguish dipole generation in the case of reflection from refraction? I think things are a bit more complicate than this.

Last edited:
Claude Bile
Science Advisor
But why does dipole generation in an amorphous material emit photons always in the same direction? I would have expected an isotropic re-emission of photons in every direction.

Photons are not being absorbed and re-emitted. Only photons that lie in specific frequency bands are absorbed by any given material. Photons that do not lie in these absorption bands (i.e. those that are transmitted) instead cause the positive and negative charges in nearby atoms to separate (by virtue of the E-field that the photon is a quantum of).

And how do we have to distinguish dipole generation in the case of reflection from refraction? I think things are a bit more complicate than this.

The difference in the laws of reflection and refraction is due to the boundary conditions that apply at an interface between two media with different refractive indices.

Claude.