Atmospheric refraction is the bending of light as it passes through different layers of the Earth's atmosphere. This bending occurs because the atmosphere has varying densities, which affect the speed of light. When light travels from a less dense medium, like space, into a denser medium, like air, it slows down and changes direction. This phenomenon is responsible for various optical effects, such as the apparent position of celestial objects. For example, the Sun appears slightly higher in the sky than it actually is during sunrise and sunset due to atmospheric refraction, allowing us to see it even when it is just below the horizon.