Answer:
We know that the student was measuring a spectrum, and observed double yellow lines, he claimed that it was Sodium.
There are multiple elements with double yellow lines, like Mercury or Sodium, but Sodium has two bright yellow lines, so it is usually identified by them.
So when we look at a spectrum and we see a strong doublet in the yellow range, we can easily assume that it is Sodium.
Here we assume that the student only saw the yellow doublet, this would imply that the yellow doublet is way more intense than the other lines, that can't be seen (while for other elements with double yellow lines, we should see other lines with similar intensity) then we can conclude that it is Sodium.
The student is correct.
