I was taught that so as to say "after" in Chinese one should use 以后, like this:
Today I came across a bare 后, like this:
What's the difference between these two words?
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'
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)
我以后不再和你说话 = I won't talk to you from now on
I think the main difference is, 后 can also mean behind, the back, the last, whereas 以后 does not take such a meaning.
*以后面 X （not a word)
房后有个车库。 Behind the house is a garage.
我坐在后排。 I'm sitting in the last (the behind) row.
后, 以后 both can mean after, afterwards, later.
后 can also mean children, as in 后生，'those born after' 九零后 'the children of the 90s'
And of course, 'the woman behind the king' i.e. the queen: 帝王的妻子：皇后，太后
后(after,behind), a single character, connotes time or position according to the context. When it refers to time sequencing, it's the same as 以后. It's like in your examples: 吃饭（以）后, 15分钟(以)后. But in 身后，房后 and etc, you can not replace 后 with 以后， because it stands for the location.
以后(after) only connotes position.