Fill the blanks to complete the following sentence:
Q: When light travels from a denser to a rarer medium, its speed ______

Answer

To complete the sentence: "When light travels from a denser to a rarer medium, its speed **increases**."

### Step-by-Step Solution:

1. **Understanding the Concept of Refraction**:
- Refraction is the bending of light as it passes from one medium to another. The behavior of light in different media is influenced by the refractive index of those media.

2. **Defining Denser and Rarer Mediums**:
- A denser medium has a higher refractive index compared to a rarer medium. For example, glass is denser than air.

3. **Relationship Between Speed and Refractive Index**:
- The speed of light in a medium is inversely proportional to the refractive index of that medium. This means that as the refractive index decreases, the speed of light increases.

4. **Applying the Concept**:
- When light travels from a denser medium (higher refractive index) to a rarer medium (lower refractive index), the refractive index decreases.

5. **Conclusion**:
- Since the refractive index decreases when moving from a denser to a rarer medium, the speed of light must increase.

### Final Answer:
When light travels from a denser to a rarer medium, its speed **increases**.

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