What is the missing word?
Substances that continue to emit light even after the source of light illuminating them is withdrawn are known as.______ substances.

Explanation

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks for a specific term that describes substances that continue to emit light after the external light source is removed.

2. **Identifying the Key Concept**: We need to focus on the definition of substances that can emit light even in the absence of a light source. This is a characteristic of certain materials.

3. **Recall Relevant Terms**: In the study of light, there are terms like "fluorescent" and "phosphorescent" that describe how materials interact with light. Fluorescent materials emit light only while being illuminated, whereas phosphorescent materials can continue to emit light after the light source is removed.

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