Planar waveguides are structures that guide light or other electromagnetic waves along a flat surface. They consist of a thin layer of material, often made of glass or semiconductor, sandwiched between two other materials with different refractive indices. This design allows the waveguide to confine and direct the light, minimizing loss and enabling efficient transmission. These waveguides are commonly used in various applications, including telecommunications and integrated optics. They play a crucial role in devices like optical fibers, laser diodes, and photonic circuits, facilitating the manipulation and routing of light signals in compact formats.