In a hillingar mirage, the Earth’s surface appears flat, because nearly horizontal light rays have the same curvature as the Earth. A linear temperature profile is traditionally inferred; its gradient is calculated to give this curvature to the exact horizontal ray. To see an image, however, a bundle of rays is required. To ensure that each ray in the bundle have the same curvature, the temperature profile must contain a small positive quadratic term, the coefficient of which is derived. © 2001 American Association of Physics Teachers.