The surface of window glass is characterized by its hydrophilic nature, meaning it readily absorbs water, and microscopic-level unevenness that is invisible to the naked eye. When it rains, water droplets cling to the surface of the window glass, spreading slowly and forming a water film that covers and obscures the glass due to its thickness. The presence of the water film makes it difficult for light to pass through, resulting in poor visibility while driving. Applying a water-repellent coating to the window glass creates a hydrophobic film that strongly repels water, filling the surface unevenness. Even in rain, the water droplets do not spread but form round droplets that easily roll off, preventing the formation of a light-obstructing water film. Additionally, the aerodynamic pressure during driving helps blow away the water droplets, ensuring a clear and unobstructed view.