Fluoroscence differs from phosphorescence in being

Options

A. Delayed emission
B. Instant emission
C. Bioluminescence
D. Chemiluminescence.

Answer

To answer the question "Fluorescence differs from phosphorescence in being," we can break down the concepts of fluorescence and phosphorescence step by step.

### Step-by-Step Solution:

1. **Define Fluorescence**:
- Fluorescence is a process where a molecule or atom absorbs energy and gets excited to a higher energy level. This absorption occurs almost instantaneously.

2. **Explain the Emission in Fluorescence**:
- After the absorption of energy, the excited molecule or atom will quickly (almost instantaneously) release that energy in the form of light (photon). This emission happens very rapidly, typically within nanoseconds.

3. **Define Phosphorescence**:
- Phosphorescence also involves the absorption of energy, but the key difference is that the excited state of the molecule or atom is maintained for a longer duration before it emits light.

4. **Explain the Emission in Phosphorescence**:
- In phosphorescence, the emission of light occurs over a longer time frame, which can range from microseconds to several hours after the initial energy absorption. This delayed emission is due to the transition of electrons to a triplet state, which is a forbidden transition, making it take longer to return to the ground state.

5. **Compare the Two Processes**:
- The main difference between fluorescence and phosphorescence is the time it takes for the emitted light to occur. Fluorescence emits light almost instantly after excitation, while phosphorescence has a delayed emission.

6. **Conclusion**:
- Therefore, the correct answer to the question is that fluorescence differs from phosphorescence in being an "instant emission" of light.

### Final Answer:
Fluorescence differs from phosphorescence in being **instant emission** (Option 2).