The expression $\sqrt{-16}$ involves taking the square root of a negative number, which requires using imaginary numbers. We know that:

$\sqrt{-1} = i$

Thus, $\sqrt{-16}$ can be rewritten as:

$\sqrt{-16} = \sqrt{16 \cdot (-1)} = \sqrt{16} \cdot \sqrt{-1} = 4i$

So, the result in the form $a + bi$ is:

$0 + 4i$

Would you like further explanation or details?

Here are 5 related questions for practice:

1. What is $\sqrt{-9}$ in the form $a + bi$?
2. How do you simplify $\sqrt{-25}$?
3. What is $\sqrt{-1}$ known as?
4. Convert $\sqrt{-36}$ to the form $a + bi$.
5. How do you handle square roots of negative numbers in general?

Tip: When dealing with square roots of negative numbers, always factor out $\sqrt{-1} = i$ first!