What is primarily used to reflect photons back and forth through atoms?

Mirrors are primarily used to reflect photons back and forth through atoms because of their ability to provide a highly reflective surface that directs light efficiently. When laser light is generated, it consists of photons that need to be contained and directed within the laser cavity to stimulate further emissions. Mirrors are designed to reflect a high percentage of incoming light, allowing photons to bounce back and forth within the laser medium. This process increases the intensity of the light and contributes to the amplification necessary for the laser operation.

In contrast, lenses are primarily used to focus or disperse light rather than reflect it. Prisms are effective for refracting light and separating it into its constituent colors but do not serve the purpose of reflecting photons. Filters selectively allow certain wavelengths of light to pass while blocking others, which does not facilitate the back-and-forth movement of photons in the same way as mirrors do. Thus, mirrors are essential components in the design and function of laser systems, making them the correct choice for this question.