| Assertion (A): | The absorption line observed in the spectra of an element is never completely dark. | 
| Reason (R): | The sample used for absorption is thin, so that all photons corresponding to a transition may not be absorbed. | 
| 1. | Both (A) and (R) are true and (R) is the correct explanation of (A). | 
| 2. | Both (A) and (R) are true but (R) is not the correct explanation of (A). | 
| 3. | (A) is true but (R) is false. | 
| 4. | Both (A) and (R) are false. | 
| 1. | \(13.6~\text{eV}\) | 
| 2. | \(\dfrac{13.6} {2}~\text{eV}\) | 
| 3. | \(13.6×\left(\dfrac{3}{4}\right)~\text{eV}\) | 
| 4. | \(13.6×\left(\dfrac14-\dfrac19\right)~\text{eV}\) | 
| 1. | the same phase. | 
| 2. | the same energy. | 
| 3. | the same direction. | 
| 4. | the same phase, energy, and direction. | 
A set of atoms in an excited state decays:
| 1. | in general to any of the states with lower energy. | 
| 2. | into a lower state only when excited by an external electric field. | 
| 3. | all together simultaneously into a lower state. | 
| 4. | to emit photons only when they collide. |