This Demonstration simulates Gaussian beam propagation and transformation by two lenses. With different placement of the lenses, the final beam waist (radius of the narrowest part) is different. In this Demonstration, the initial beam waist is 100m. The focal lengths of lens 1 and lens 2 are 50mm and 100mm, respectively. In the inset, and indicate the beam waists of the beam after lens 1 and lens 2, respectively; indicates the distance between the beam source (0 point) and the position where appears.