Transmittance is a measure of how much light passes through a material compared to the amount of light that hits it. It is expressed as a ratio or percentage, indicating the efficiency of the material in allowing light to pass. High transmittance means that most of the light is transmitted, while low transmittance indicates that much of the light is absorbed or reflected. In scientific contexts, transmittance is important in fields like optics and materials science. For example, glass used in windows typically has high transmittance to allow natural light into buildings, while sunglasses are designed to have lower transmittance to protect the eyes from bright sunlight.