What's the difference between 后 and 以后?
后= after 


Examples:

吃饭后 = after eating dinner

15分钟后 = after 15 minutes 


  以后 = after 


以后 (after) emphasizes on 'time after a specific event or action'

Examples:

吃过那顿晚饭以后 = after eating that dinner (a specific dinner is emphasized)

15分钟的沉默以后 = after 15 minutes of silence (the event- 15 minutes of silence is emphasized)


  以后 = 此后 (hereafter); 其后 (thereafter) ; 以来(since)


Example: 自二战以后 = 自二战以来 = since WWII


  以后 = 今后 (from now on)


Example:

我以后不再和你说话 = I won't talk to you from now on