On a hot summer night, the refractive index of air is smallest near the ground and increase with height from the ground. When a light beam is directed horizontally, the Huygens principle leads us to conclude that as it travels, the light beam

As per Huygen’s principle, each point on wave behaves as a source. So, from such hypothetical source, rays would emerge in all directions. The rays, inclined upwards, are travelling from rarer to denser medium and hence, would bend towards normal i.e. become vertical. While, rays inclined downwards, are travelling from denser to rarer. Hence, these rays would bend away from normal i.e. would become horizontol.

 

So, in totality, beam bends upwards.