Whenever Clark used heat vision, he became dazed after he close his eyes and had to shake it off. This is shown when he melts through the steel beam Zod hits him with and when he melts through the Ice. What is the significance of it. Why does it cause dizziness?
Kryptonians can harness their personal superhuman energies in directed beams of energy. It is likely not disorientation, per se, more a focusing of their personal energies since the use of his heat vision is one of the more direct applications of said stored solar power. Such a direct use of heat vision depletes a Kryptonian's energy supply faster than his other powers.
Heat Vision is a superpower possessed by Kryptonians and Daxamites when exposed to the yellow sun; see Photonucleic Effect. It is the ability to release massive amounts of solar energy stored within the body through the eyes, in focused beams of heat (presumably, as infra-red light).
In the battle with Zod, Kal-El uses his heat vision to melt a girder Zod grabs as a weapon. Melting the girder, Kal pauses for a second, making him vulnerable. It is likely he has to "pulling himself together" after emitting such a burst of energy. His heat vision can reach temperatures as hot as the surface of the sun (6,000 degrees Celsius) so it's understandable why he might need to rest for a second.
In "Man of Steel" it appeared the power required intense concentration to activate and control. So much so, it couldn't be controlled by movement of the eyes alone. It required the user to direct their entire head to manipulate the beam.
In the comics, Superman's heat vision can be used in a number of more diverse ways, from surgical laser to wide area offensive weapon. But the greater the energy output, the more difficult it is for him to recover.
- In a rare scene he is shown using his heat vision to destroy Doomsday clones attacking Wonder Woman's home of Themyscira. He is momentarily depleted at the end of this feat.