Physical filters are devices or materials that selectively allow certain substances to pass through while blocking others. They are commonly used in various applications, such as water purification, air filtration, and even in photography. For example, a coffee filter lets liquid coffee pass through while trapping coffee grounds. These filters can be made from different materials, including paper, cloth, or specialized membranes. The effectiveness of a physical filter depends on its design and the size of the particles it is meant to capture. In air purifiers, physical filters help remove dust, pollen, and other allergens from the air.