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Radiation coming from transitions n = 2 to n = 1 of hydrogen atoms fall on He + ions in n = 1 and n = 2 states. The possible transition of helium ions as they absorb energy from the radiation is -

1. n = 2 → n = 5
2. n = 2 → n = 3
3. n = 1 → n = 4
4. n = 2 → n = 4

Correct answer: n = 2 → n = 4

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