Scattering

Scattering is an important concept in physics that describes how moving particles or waves, like light or sound, change direction when they hit something uneven in the material they are passing through. These uneven areas, called scatterers, can be tiny particles, bubbles, droplets, or even imperfections in solids. When waves or particles hit these scatterers, they spread out in many directions instead of continuing in a straight line.

Historically, the idea of scattering began with Isaac Newton in the 1600s when he studied how light behaves. Over time, scientists like William Herschel and John Tyndall expanded this idea to other types of waves and rays. Today, scattering is used in many technologies, from medical ultrasound to radar systems and computer graphics. It helps scientists understand how particles interact in fields like particle physics, astrophysics, and more.

Single and multiple scattering

When radiation, like light or sound, hits one small spot, it's called single scattering. More often, there are many spots close together, and the radiation may bounce around many times. This is called multiple scattering.

Single scattering looks random because we usually don't know exactly where the spot is. For example, when an electron hits an atom, we can't predict its exact path after the hit.

With multiple scattering, many small bounces average out, making the path look more predictable. Think of a flashlight shining through fog—the light spreads out evenly. This spreading is very similar to how things spread out in diffusion, so multiple scattering is often called diffusion. Tools that spread out light this way are called diffusers.

In some cases, like with a strong laser aimed at a tiny particle, the scattering can be very exact. This also happens in radar when it hits big objects like planes.

Sometimes, even with multiple scattering, the light can show random patterns called speckles. These are tricky to predict and model. The ideas of single and multiple scattering are linked to how waves and particles behave in the world around us.

Theory

Scattering theory helps us understand how waves and particles behave when they hit something else. For example, when sunlight hits raindrops, it scatters and makes a rainbow. Other examples include billiard balls hitting each other or alpha particles bouncing off gold atoms.

There are two main types of scattering problems. The direct scattering problem looks at how much of a wave or particle spreads out after hitting an object. The inverse scattering problem tries to figure out what an object looks like by measuring how waves or particles scatter off it.

Scattering can also affect how strong a beam of particles or light becomes. When many small objects scatter the beam, its strength decreases over distance. This idea is used in many areas, from studying light to understanding how particles move in materials.

In elastic scattering, the particles don’t change their internal state after scattering. In inelastic scattering, they do change, like getting excited or breaking apart. For example, when two hydrogen atoms scatter, they might change energy levels or even break apart.

In mathematics, scattering theory looks at how solutions to equations change when they move toward each other, interact, and then move away again. This helps describe how particles behave in quantum mechanics and other areas of physics.

Theoretical physics

In mathematical physics, scattering theory helps us understand how different things, like sound waves or light, change direction when they hit objects or move through uneven areas. For example, in acoustics, we study how sound waves scatter from solid objects or move through places like sea water where things aren't the same everywhere.

In particle physics, scientists use special equations to study how tiny particles like electrons and protons move and interact. When these particles come together from far away, they can bounce off each other, react, or even create new particles. The theory helps predict where these particles will go after they interact and how likely different outcomes are. Two main methods used are partial wave analysis and the Born approximation.

Electromagnetics

Electromagnetic waves are well-known types of energy that spread out and bounce off many things. We see this when light or radio waves, like those used in radar, change direction when they hit objects. Two important ways this happens with light are Rayleigh scattering and Mie scattering. Other types include Brillouin scattering, Raman scattering, and Compton scattering.

When we look at objects, light scattering helps us see their colors and shapes. For example, white objects look white because light bounces off them many times. The smoothness or roughness of a surface also affects how it looks. Some surfaces look shiny because light reflects off them in a straight way, while others look dull because the light scatters in many directions.

The way light scatters depends on the size of the object it hits compared to the wavelength of the light. Small particles cause Rayleigh scattering, while larger ones cause Mie scattering. For very large particles, the rules of geometric optics help explain what happens. Scientists use special computer programs to study how light scatters from irregularly shaped objects.

Electrophoresis is a process where tiny particles move in a liquid when an electric field is applied. By studying how light scatters during this process, scientists can learn about the particles’ properties.