Fig. 4. Path of light rays after passing through the lens-like temperature gradient under different atmospheric conditions.
On the left, the radii of curvature of the lens-like structures are larger and/or the refractive index difference smaller than optimal. On the right, the radii of curvature are substantially smaller and/ or the refractive index difference larger than optimal. Under such non-ideal conditions, variations in light intensity are not apparent to an observer on the ground and twinkling would not be observed.