whose solution is known as Beer–Lambert law and has the form I = I 0 e − − x / ℓ ℓ , where x is the distance traveled by the beam through the target, and I 0 is the beam intensity before it entered the target; ℓ is called the mean free path because it equals the mean distance traveled by a beam particle before being stopped. To see this, note that the probability that a particle is absorbed between x and x + dx is given by