To let in light or to let light in ?

[gtmm168]

The primary function of a window is to let in light.
The sentence is from The American Heritage dictionary for Learners of English.The word "in" is an adverb? Can wu put it after the word "light"?
The primary function of a window is to let light in. Is it right? If it is right,which one is more common?

 

[5jj]

Is the word "in" is an adverb?

Grammarians do not agree on this. It was traditionally considered an adverb, but some writers now consider it a preposition.
You can put it in either position you mention.

 

[gtmm168]

If it is considered as a prepsition, as there is no object in the sentence, can the verb "let" be used as an intransitive verb in this condition?
But I can't find the sentences or usage in English dictionary. In the entry of the word "let" in Macmillan English dictionary for Advanced Learners, we can find it as follow,
"to allow something to happen: let sb/sth do sth; let sth in/out/through etc".

 

[5jj]

Grammarians can argue about the appropriate label for as long as they wish. It doesn't really need to worry learners who wish simply to communicate in the language. I think of 'let' as a transitive verb there.

 

[jutfrank]

Grammarians can argue about the appropriate label for as long as they wish. It doesn't really need to worry learners who wish simply to communicate in the language. I think of 'let' as a transitive verb there.

I completely agree with this. From my point of view as a teacher, it's often the case that the terminology of grammarians gets in the way. Whether it's right or wrong, I think it's very reasonable even to say that let in itself is a transitive verb, in that it takes an object: light. I mean, I know that's not correct but if it helps the learner to understand and use the language, then it's valid, in my view.