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

Answer

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

### Step-by-Step Solution:

1. **Understanding the Mediums**:
- Identify the types of mediums involved: rarer and denser. A rarer medium has a lower refractive index, while a denser medium has a higher refractive index.

2. **Refractive Index Concept**:
- Recall that the refractive index (n) is a measure of how much light slows down in a medium compared to its speed in a vacuum. The formula is given by:
\[ n = \frac{c}{v} \]
where \(c\) is the speed of light in a vacuum and \(v\) is the speed of light in the medium.

3. **Relationship Between Speed and Refractive Index**:
- Understand that speed of light in a medium is inversely proportional to the refractive index. This means that as the refractive index increases, the speed of light decreases:
\[ v \propto \frac{1}{n} \]

4. **Comparing Rarer and Denser Mediums**:
- Since the refractive index of the rarer medium is less than that of the denser medium, we can conclude that:
\[ n_{\text{rarer}} < n_{\text{denser}} \]
- Therefore, when light moves from the rarer medium to the denser medium, the refractive index increases.

5. **Conclusion on Speed**:
- Since the refractive index increases when moving from a rarer to a denser medium, the speed of light must decrease. Thus, we fill in the blank with "decreases".

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