# Emission Spectrum and Absorption Spectra, Absorption Spectrum, Question (For CBSE, ICSE, IAS, NET, NRA 2022)

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## Questions

### I Have an Unknown Gas in a Glass Container. I Shine a Bright White Light through One Side of the Container and Measure the Spectrum of Transmitted Light. I Notice That There is a Black Line (Absorption Line) in the Middle of the Visible Red Band at 642 Nm. I Have a Hunch That the Gas Might be Hydrogen. If I Am Correct, between Which 2 Energy Levels Does this Transition Occur? (Hint: Look at Figure and the Transitions Which Are in the Visible Part of the Spectrum.)

Solution:

#### Step 1: What is Given and What Needs to be Done?

We have an absorption line at . This means that the substance in the glass container absorbed photons with a wavelength of . We need to calculate which 2 energy levels of hydrogen this transition would correspond to. Therefore, we need to know what energy the absorbed photons had.

#### Step 2: Calculate the Energy of the Absorbed Photons

The absorbed photons had an energy of

#### Step 3: Find the Energy of the Transitions Resulting in Radiation at Visible Wavelengths

• Figure shows various energy level transitions. The transitions related to visible wavelengths are marked as the transitions beginning or ending on Energy Level 2. Let us find the energy of those transitions and compare with the energy of the absorbed photons we have just calculated.
• Energy of transition (absorption) from Energy Level 2 to Energy Level 3:

• Therefore, the energy of the photon that an electron must absorb to jump from Energy Level 2 to Energy Level 3 is
• This is the same energy as the photons which were absorbed by the gas in the container! Therefore, since the transitions of all elements are unique, we can say that the gas in the container is hydrogen. The transition is absorption of a photon between Energy Level 2 and Energy Level 3.

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