Try it on paper first. A hint, the key idea and the worked solution are here when you need them.
Find the shortest wavelength of the Brackett series, which ends on , and the energy of a photon of this wavelength in eV. In which part of the spectrum is it? What do you notice about the energy?
Hydrogen's lines obey , with . Lyman ends on , Balmer on , Paschen on , Brackett on . From level , different lines are possible.
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. Wavelength. .
Step 2. Energy. .
Step 3. Region. Infrared.
Step 4. Notice. , the energy to free an electron from .
Answer: , , infrared; it equals the ionisation energy from .
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