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The transition from the state n = 4 to n = 3 in a hydrogen like atom results in ultraviolet radiation. Infrared radiation will be obtained for the transition from:

1. 3 → 2
2. 4 → 2
3. 5 → 4
4. 2 → 1

Correct answer: 5 → 4

Solution

Transition energy scales with (1/nf^2 - 1/ni^2). For 4->3 it is 0.0486 (UV). The only transition with a SMALLER gap (longer wavelength = IR) is 5->4 (1/16 - 1/25 = 0.0225). 3->2, 4->2 and 2->1 all have larger gaps, so they are more energetic, not IR.

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