A mirage is created by refraction of light near the surface of the Earth. On a still

Question:

A mirage is created by refraction of light near the surface of the Earth. On a still day (no wind), absorption of sunlight by the Earth makes the air temperature decrease as one moves upward, away from the surface. The speed of light in air depends on the air temperature, and this variation of the speed of light causes light to refract near the surface. As a result, an observer looking along the Earth’s surface (Fig. P12.74A) will see light that comes from the sky (Fig. P12.74B). Using the ray diagram in Figure P12.74A, does the existence of a mirage mean that the speed of light increases or decreases as the temperature increases?

Figure P12.74