Physics - Modern Physics Question with Solution | TestHub

When hydrogen atoms are excited to $n=4$, they can transition to lower energy levels. Ultraviolet lines correspond to transitions to the $n=1$ level (Lyman series).

From $n=4$, the possible transitions to $n=1$ are: $4\to 1$.

Thus, there is only 1 line in the ultraviolet region.

Therefore, the answer is 1.