Case : Read the passage and answer the question(s).

Hydrogen spectrum consists of discrete bright lines in a dark background and it is specifically known as hydrogen emission spectrum. There is one more type of hydrogen spectrum that exists where we get dark lines on the bright background, it is known as absorption spectrum.

Balmer found an empirical formula by the observation of a small part of this spectrum and it is given by : \(\frac{1}{λ}\) = R(\(\frac{1}{2^2}\) - \(\frac{1}{n^2}\)) , where n = 3,4,5,...

For Lyman series, the emission is from first state to nth state, for Paschen series, it is from third state to nth state, for Brackett series, it is from fourth state to nth state and for Pfund series, it is from fifth state to nth state.

Which of the following lies in infra-red region?

Both 2 and 3

The correct answer is option 4: Both 2 and 3

Both the Paschen series (Option 2) and the Brackett series (Option 3) lie in the infrared (IR) region of the electromagnetic spectrum.

• Paschen series (n=3 → higher levels):

  • Wavelength range: ~820–1875 nm (near-infrared).

• Brackett series (n=4 → higher levels):

  • Wavelength range: ~1.46–4.05 μm (mid-infrared).

Since the question asks which series lies in the IR region, and both Paschen and Brackett qualify, the correct choice is Option 4: Both 2 and 3.