Photonic crystals are materials that have a periodic structure, which affects the movement of light in a similar way that regular crystals affect the movement of electrons. These structures can manipulate light by creating band gaps, preventing certain wavelengths from passing through while allowing others to be transmitted or reflected. This property makes them useful in various applications, including optical filters and sensors. The design of photonic crystals can be tailored to specific wavelengths, making them valuable in technologies like fiber optics and LEDs. By controlling how light interacts with these materials, researchers can enhance performance in devices such as lasers and solar cells.