look down at sb./sth.：低头看（某人/某物）,如：She looked down at her toe and murmured something. 她低头看着自己的脚趾,小声嘟哝着.
look down on/upon：看不起,小看（某人）,如：You shouldn't look down on someone else because he is poor. 不要因为别人穷而看不起他.