Huygen's principle allows us to determine,

Options

A. Shape of wavefront at any instant, if the shape of wavefront at earlier time is not known.
B. Shape of wavefront at any instant, if the shape of wavefront at earlier time is known
C. Shape of wavefront at any time, if the shape of wavefront at next time interval is known
D. Both (1) & (3)

Answer

To solve the question regarding Huygen's principle, we need to understand what the principle states and how it can be applied to determine the shape of wave fronts over time.

### Step-by-Step Solution:

1. **Understanding Huygen's Principle**:
Huygen's principle states that every point on a wave front can be considered as a source of secondary wavelets. These wavelets spread out in all directions at the speed of light.

2. **Formation of New Wave Front**:
The new wave front at a later time can be determined by the envelope of these secondary wavelets. This means that if we know the shape of the wave front at an earlier time, we can predict the shape of the wave front at a later time.

3. **Analyzing the Options**:
- **Option A**: "Shape of wave front at any time instant if the shape of wave front at earlier time is not known."
- This is incorrect because we need to know the previous wave front to determine the new one.

- **Option B**: "Shape of wave front at any instant if the shape of wave front at earlier time is known."
- This is correct because knowing the earlier wave front allows us to apply Huygen's principle to find the new wave front.

- **Option C**: "Shape of wave front at any time if the shape of wave front at the next interval is known."
- This is incorrect because we cannot determine the current wave front based on the future wave front.

4. **Conclusion**:
The only correct option is **Option B**. Huygen's principle allows us to determine the shape of the wave front at any instant if the shape of the wave front at an earlier time is known.

### Final Answer:
**Option B**: Shape of wave front at any instant if the shape of wave front at earlier time is known.

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