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

[Aidyan]

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?

 

[Claude Bile]

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.

 

[Aidyan]

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.

 

[Claude Bile]

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.

 

[sardar]

Reflection and refraction of light at the microscopic scale can be understood using the principles of quantum mechanics. At this scale, light is considered to be made up of individual particles called photons. These photons interact with the atoms and molecules of the material they are passing through.

In reflection, the photons are absorbed by the atoms and molecules on the surface of the material and then re-emitted in the same direction, with the same wavelength and energy. This process is known as elastic scattering, as there is no change in the energy of the photons.

In a mirror, the smooth surface of the material allows for the photons to be reflected in an organized and coherent manner, resulting in a clear reflection. This is because the atoms and molecules on the surface are arranged in a regular pattern, allowing for a consistent and predictable reflection.

Refraction, on the other hand, occurs when the photons pass through a transparent medium, such as glass. In this case, the photons interact with the atoms and molecules of the material, causing a change in their direction and speed. This is due to the fact that the atoms and molecules in the material are not arranged in a regular pattern, causing the photons to scatter and change direction.

The change in direction and speed of the photons is caused by the interaction between the photons and the electrons in the atoms and molecules of the material. As the photons pass through the material, they are absorbed and then re-emitted by the electrons, resulting in a change in direction and speed.

This change in direction and speed is what causes the bending of light as it passes through a transparent medium. The amount of bending depends on the density and composition of the material, as well as the wavelength of the light.

To better understand these concepts at the microscopic scale, it is helpful to look at the wave-particle duality of light. This means that light can exhibit both wave-like and particle-like behavior, depending on the situation.

There are many resources available online that explain the concepts of reflection and refraction at the microscopic scale in more detail, such as Khan Academy, Physics Classroom, and HyperPhysics. Additionally, textbooks on quantum mechanics and optics may also provide a deeper understanding of these concepts.