Context: Surfaces (smooth projective complex)


The title is way more general than what I'd like to understand. I am trying to understand this question in the following very special situation: let $X$ be a surface and $L$ an ample, base point free and globally generated line bundle such that $\varphi=\varphi\_L$ is a birational morphism, 


$$ \varphi: X\longrightarrow Y:=\varphi(X)\subset\Bbb{P}^N $$


We wonder when is $\varphi$ an embedding. As suggested by Asal B.D. in [this SE post](https://math.stackexchange.com/questions/360446/conditions-birational-morphism-is-isomorphism), the normality of $Y$ would be enough. However I have no idea how to check normality of $Y$ in this case. Also, I would like to understand the following, possibly trivial, fact:


(Why) would it be enough to show that $H^0(Y,\mathcal{O}(r))=H^0(X,L^{\otimes r})$ for all $r$?



when a birational morphism is an isomorphism?
To say that $L$ is ample is equivalent to $X=\mathrm{Proj}\oplus H^0(X,L^{{\scriptscriptstyle\otimes} r})$, while of course $Y=\mathrm{Proj}\oplus H^0(Y,\mathcal{O}(r))$.


