Ah, that provided one of the more eerie moments in the film.  The visor slowly rose, retracting into the helmet, opening like the eye of a cyclopian giant.  We then saw a light through a horizontal slit that flitted back and forth, finally locking into place as an intense beam of white energy lanced out, vaporizing any object it struck.  The drama was of the sequence was conveyed mostly through music rather than overt sound effects. 

Only when Klaatu shouted an order at Gort did the monstrous robot slowly lower its visor once more, closing as through the thing were falling asleep.

Sincerely,

Bill

Tempt the Hand of Fate and it'll give you the "finger"!