Robert Young in his 1862 Literal Bible renders the text :

and God saith, `Let light be;' and light is. Genesis 1:3 YLT.

This translation seems to me to allow of an absent word to be understood :

"Let light be (here)."

And if so, then a further understanding may be allowed, that light is already elsewhere. Then let light be here also.
This agrees with what is later revealed in John 1:4 of the Word from the beginning.
In him was life; and the life was the light of men. John 1:4 KJV.
If Life is in him who is the Word from the beginning, and if that uncreated Life is uncreated Light. Then let light (of a created kind) be within the creation, also.

This demonstrates the profound spirituality of the opening chapter of Genesis, that it is not a technical description of manufacture but a deeply mysterious expression of what creation is, in and of itself.

Yehi is qal imperfect 3rd person singular jussive with a apocopated ending of the being verb hayah. 
The jussive gives it the meaning "let there be."

https://hermeneutics.stackexchange.com/questions/40065/what-is-the-meaning-of-the-hebrew-phrase-yehi-or-translated-as-let-there-be