Penetration depth is a measure of how deep light or any electromagnetic radiation can penetrate into a material. When electromagnetic radiation is incident on the surface of a material, it may be partly reflected from that surface and there will be a field containing energy transmitted into the material. This electromagnetic field interacts with the atoms and electrons inside the material. Depending on the nature of the material, the electromagnetic field might travel very far into the material, or may die out very quickly. For a given material, penetration depth will generally be a function of wavelength.
Under the hood: the physics of projectile ballistics
The physicist Sir Isaac Newton first developed this idea to get rough approximations for the impact depth for projectiles traveling at high velocities. Newton's approximation for the impact depth for projectiles at high velocities is based only on momentum considerations. Nothing is said about where the impactor's kinetic energy goes, nor what happens to the momentum after the projectile is stopped. The basic idea is simple: The impactor carries a given momentum. To stop the impactor, this momentum must be transferred onto another mass.
The penetration depth is calculated according to equation 2 , which shows how it depends on the dielectric properties of the material. The power density will decrease exponentially from the surface to the core region. Figure 1.